QRP QCX-mini 5W CW Transceiver kit Instruction Manual

: October 30, 2023
: QRP

Table of Contents

QRP QCX-mini 5W CW Transceiver kit
Product Information: QCX-mini CW Transceiver
Features
Product Usage Instructions: QCX-mini CW Transceiver
Introduction
Parts list
Assembly general guidelines
PRECISION
Install LCD module
References
Read User Manual Online (PDF format)
Download This Manual (PDF format)

QRP QCX-mini 5W CW Transceiver kit

QRP-QCX-mini-5W-CW-Transceiver-kit

Product Information: QCX-mini CW Transceiver

The QCX-mini is a high-performance 5W CW transceiver kit designed and produced by QRP Labs from 2017-2020. This single-band transceiver comes with built-in alignment and test equipment, iambic keyer, WSPR beacon mode, and more. The kit includes a voltmeter, RF power meter, frequency counter, and signal generator which can aid in debugging and fault-finding.

Features

High-performance single-band 5W CW transceiver kit
Built-in alignment and test equipment
Iambic keyer
WSPR beacon mode
Portable-friendly features
No test equipment is required to build, align, and operate the transceiver
Self-alignment and self-test features to help and guide in setting up the transceiver in a few easy steps

Resources:

Measurement section for typical performance measurements
Operation section for transceiver, alignment, and test
equipment operation in detail
Single page reference cheat sheet
Troubleshooting resources at http://qrp-labs.com/qcxmini
QRP Labs discussion forum on groups.io for further help

Product Usage Instructions: QCX-mini CW Transceiver

Please read the assembly manual carefully and follow the instructions step by step in the recommended order. The circuit design is described in detail later in the manual, and it is recommended to read and understand this section as well to get the maximum enjoyment and education from your new radio.
Before starting the assembly, check the QCX-mini web page http://qrp- labs.com/qcxmini for any updates, tips, etc. If you have any problems, please make use of the troubleshooting resources at
http://qrp-labs.com/qcxmini. If you need further help, join the QRP Labs discussion forum on groups.io and post a message about your problem.

QCX-mini CW Transceiver
QCX-mini 5W CW Transceiver kit assembly instructions
PCB Rev 3
The “QCX-mini”: a single band, high performance 5W CW Transceiver with built- in alignment and test
equipment, iambic keyer, WSPR beacon mode, and more…
Designed and produced by QRP Labs, 2017-2020

pictured with:
Palm Radio pico paddle http://palm-radio.de and the XYL’s old iPhone’s earbuds

QCX-mini assembly Rev 1.12

Introduction

Thank you for purchasing this high performance single-band 5W CW transceiver kit, the QCX-mini (for QRP Labs CW Xcvr mini). This kit has a long list of features!
Special portable-friendly features:
· Small size: 95 x 63 x 25mm enclosure (plus protrusions) · Sunlight view- able, 16 x 2 yellow/green LCD screen, back-light can be switched
on/off · Low current consumption (for example 58mA receive current, with 12V supply and
display back-light off) · Low weight, 202 grams · Sturdy extruded aluminium enclosure · All-metal BNC short connector, bolted to enclosure
Standard QCX-series features
· Easy to build, two-board design, board with main circuit and connectors, display panel board with LCD; all-controls board-mounted on a press-out sub- board. No wiring, all controls and connectors are board-mounted
· Professional quality double-sided, through-hole plated, silk-screen printed PCBs · Choice of single band, 160, 80, 60, 40, 30, 20 or 17m · Approximately 3-5W CW output (depending on supply voltage) · 7-16V recommended supply voltage · Class E power amplifier, transistors run cool… · 7-element Low Pass Filter ensures regulatory compliance · CW envelope shaping to remove key clicks · High performance receiver with at least 50dB of unwanted sideband cancellation · 200Hz CW filter with no ringing · Si5351A Synthesized VFO with rotary encoder tuning · Iambic keyer or straight key option included in the firmware · Simple Digital Signal Processing assisted CW decoder, displayed real-time on-screen · On-screen S-meter · On-screen real time clock (not battery backed up) · Full or semi QSK operation using fast solid-state transmit/receive switching · Frequency presets, VFO A/B Split operation, RIT, configurable CW Offset · Configurable sidetone frequency and volume · Connectors: 2.1mm power barrel connector, 3.5mm keyer jack, 3.5mm stereo earphone
jack, 3.5mm stereo jack for PTT, 3.5mm stereo jack for CAT control, BNC RF output · Built-in test signal generator and alignment tools to complete simple set-up adjustments · Built-in test equipment: voltmeter, RF power meter, frequency counter, signal generator · Beacon mode, supporting automatic CW, FSKCW or WSPR operation · GPS interface for reference frequency calibration and time-keeping (for WSPR beacon) · CAT control interface · Optional 50W PA kit · Optional aluminium extruded cut/drilled/laser-etched black anodized enclosure

No test equipment is required to build, align and operate this CW transceiver. Its innovative self-alignment and self-test features will help and guide you in setting up the transceiver in a few easy steps. The kit also includes a voltmeter, RF power meter, frequency counter and signal generator which can aid in debugging and fault-finding.

QCX-mini assembly Rev 1.12

We hope you enjoy building and operating this kit! Please read this assembly manual carefully, and follow the instructions step by step in the recommended order. Later in the manual the circuit design is described in detail and we recommend reading and understanding this section too, to get the maximum enjoyment and education from your new radio.
Typical performance measurements are shown in the measurements section. The operation section of the manual describes transceiver, alignment and test equipment operation in detail.
There is a single page reference “cheat sheet” near the end of the manual.
Please check the QCX-mini web page http://qrp-labs.com/qcxmini for any updates tips, etc., before starting the assembly.
Please make use of troubleshooting resources at http://qrp-labs.com/qcxmini if you have any problems. If you need further help, join the QRP Labs discussion forum on groups.io and post a message about your problem.

Parts list

Many components are SMD, pre-soldered to the PCB in the factory. Only through- hole components need to be installed by the constructor. SMD components in the parts list are identified in the Description column and by the text colour being purple.

Resistors

Qty Value

Description

Component numbers

4 100-ohms SMD

1 150-ohms SMD

1 270-ohms SMD

1 560-ohms SMD

1K SMD

1.2K SMD

13 3.3K SMD

3.9K SMD

4.3K SMD

5.1K SMD

18 10K SMD

33K SMD

36K SMD

47K SMD

4 120K SMD

1 750K SMD

1 500-ohm Multi-turn trimmer potentiometer

50K Multi-turn trimmer

R5, 6, 8, 9 R41 R50 R48 (on display board) R3, 4, 19, 26, 37, 45, 49, 54, 55, 62, 63 (R45 on display board) R42 R12, 13, 15, 16, 20, 22, 23, 25, 44, 53, 56, 59, 65 (R44 on controls board, R65 on display board) R61 R18 R11 R1,2,7,10,14,21,34,36,39,40,46,51,52,57,58,64 (R102, 103 on controls board) R28, 29 R32, 33 R30, 31 R38, 43, 60, 100 (R100 on display board) R35 R27
R17, 24

QCX-mini assembly Rev 1.12

Qty Value

Description

Component numbers

potentiometer

5K Linear potentiometer R36 (on controls board)

22K Trimmer

potentiometer

R47 (on display board)

Capacitors (50V, Multi-layer Ceramic capacitors)

Qty Value

Description

Component numbers

7 1nF SMD
2 2.2nF SMD 1 3.3nF SMD 6 10nF SMD 1 33nF SMD 1 39nF SMD 1 47nF Label “473” 1 47nF SMD 2 0.1uF Label “104” 14 0.1uF SMD 5 0.47uF Label “474” 2 1uF Label “105” 2 2.2uF SMD 3 10uF SMD 2 470uF Electrolytic 1 30pF Ceramic trimmer
capacitor

C14, 16, 18, 23, 33, 54, 55 C19, 20 C53 C4, 7, 10, 42 (C101, 102 on controls board) C15 C17 C9 13 C12, 29 C2, 3, 6, 32, 34-36, 39-41, 48-50, 52 C11, 43-46 C21, 22 C31, 100 (C100 on display board) C37, 38, 51 C24, 47 C1

Band-specific capacitors (50V, 5% capacitors which must be C0G/NP0 type)

Note: depending on band, some capacitors may be left over at the end. This is normal!

160m

Qty Value

Description

1 22pF Label “220” 1 100pF Label “101” 1 820pF Label “821” 2 820pF Label “821” 2 2200pF Label “222”

C5 C8 C30 C27, 28 C25, 26

Component numbers

QCX-mini assembly Rev 1.12

80m

Qty Value

Description

1 39pF Label “390” 1 22pF Label “220” 1 180pF Label “181” 2 470pF Label “471” 2 1200pF Label “122”

C5 C8 C30 C27, 28 C25, 26

Component numbers

60m (C30 is two capacitors in parallel)

Qty Value

Description

Component numbers

1 39pF Label “390”
1 22pF Label “220” 1 30pF or Label “300” or “330”
33pF 1 56pF Label “560” or “56J” 2 680pF Label “681” 2 1200pF Label “122”

C5 C8 C30 (C30 is two capacitors in parallel)
C30 (C30 is two capacitors in parallel) C27, 28 C25, 26

40m (no C8 capacitor)

Qty Value

Description

1 39pF Label “390” 1 56pF Label “560” or “56J” 2 270pF Label “271” 2 680pF Label “681”

C5 C30 C27, 28 C25, 26

Component numbers

30m (no C8 capacitor)

Qty Value

Description

1 22pF Label “220”
1 30pF or Label “300” or “330” 33pF
2 270pF Label “271” 2 560pF Label “561”

C5 C30
C27, 28 C25, 26

Component numbers

20m (no C5 or C8 capacitors)

Qty Value

Description

1 30pF or Label “300” or “330” 33pF
2 180pF Label “181” 2 390pF Label “391”

C30
C27, 28 C25, 26

Component numbers

QCX-mini assembly Rev 1.12

17m (no C5 or C8 capacitors)

Qty Value

Description

1 30pF or Label “300” or “330” 33pF
2 100pF Label “101” 2 270pF Label “271”

C30
C27, 28 C25, 26

Component numbers

Semiconductors

Qty

Description

Component numbers

SMD

D1, 2, 4, 5, 6

1N5819 diode

D33 (was previously D3)

SMD: Si5351A, 10-pin MSOP IC1

ATmega328, microcontroller IC2

SMD: 74ACT00N

IC3

SMD: FST3253

IC4

SMD: LM4562 dual op-amp IC5

SMD: TLC2262 dual op-amp IC6-9

SMD: OPA2277 dual op-amp IC10 (NE5532 may be used)

SMD: AMS1117-5.0, 5V

IC11

SMD: BSS123 MOSFET

Q4, 5, 7, 100 (Q100 on display board)

BS170: TO92 MOSFET

Q1-3

MPS751 TO92 transistor

Inductors

Qty

Description

SMD: 47uH inductor

L5, 6

Component numbers

Band-specific inductors

160m

Qty Value

Description

1 4.0uH 31 turns on T37-2 core (red) L1

1 6.4uH 40 turns on T37-2 core (red) L2

1 3.9uH 30 turns on T37-2 core (red) L3

1 1.4uH 19 turns on T37-2 core (red) L4

3+3+3+10 turns, FT50-43 (black) T1

Component numbers

QCX-mini assembly Rev 1.12

80m

Qty Value

Description

2 2.4uH 25 turns on T37-2 core (red)

1 3.0uH 27 turns on T37-2 core (red)

1 2.3uH 24 turns on T37-2 core (red)

5+5+5+68 turns, T50-2 core

(red)

L1, L3 L2 L4 T1

Component numbers

60m

Qty Value

Description

2 2.1uH 23 turns on T37-2 core (red)

1 2.3uH 24 turns on T37-2 core (red)

5+5+5+46 turns, T50-2 core

(red)

L1, L3 L2 L4 T1

Component numbers

40m

Qty Value

Description

2 1.4uH 21 turns on T37-6 core (yellow) L1, L3

1 1.7uH 24 turns on T37-6 core (yellow) L2

1 1.0uH 16 turns on T37-2 core (red) L4

5+5+5+38 turns, T50-2 core

(red)

Component numbers

30m

Qty Value

Description

2 1.1uH 19 turns on T37-6 core (yellow) L1, L3

1 1.3uH 20 turns on T37-6 core (yellow) L2

1 0.78uH 14 turns on T37-2 core (red) L4

4+4+4+30 turns, T50-2 core

(red)

Component numbers

20m

Qty Value

Description

2 0.77uH 16 turns on T37-6 core (yellow) L1, L3

1 0.90uH 17 turns on T37-6 core (yellow) L2

1 0.40uH 10 turns on T37-2 core (red) L4

3+3+3+30 turns, T50-2 core

(red)

Component numbers

QCX-mini assembly Rev 1.12

17m

Qty Value

Description

2 0.55uH 13 turns on T37-6 core (yellow) L1, L3

1 0.67uH 15 turns on T37-6 core (yellow) L2

1 0.32uH 9 turns on T37-2 core (red)

3+3+3+22 turns, T50-2 core

(red)

Component numbers

Miscellaneous

Qty Value

Description

Component numbers

1 2×3-pin Male pin header

2.1mm 2.1mm DC Power barrel connector

3.5mm 3.5mm stereo jack socket

BNC BNC connector socket

1 2×5-pin Female pin header socket

1 2×5-pin Male pin header socket

1 2×4-pin Female pin header socket

1 2×4-pin Male pin header socket

6x6x8 6x6x8mm tactile switch button

S2, 3

Rotary encoder with shaft button

SW1

1602 HD44780 LCD 1602, yellow/green back-light

20MHz HC49/4H quartz crystal

XTAL1

27MHz HC49/4H quartz crystal

XTAL2

PCB Main PCB

PCB Display PCB panel

Knob Knob to fit rotary encoder and R36

200cm 0.33mm diameter wire (AWG #28)

1 M3 10mm Steel 10mm long M3 screw

Steel M3 nut

1 M3 12mm Steel 12mm diameter M3 washer

11mm Nylon M3 hex spacer

6mm Nylon M3 6mm screw

6mm Nylon or steel M3 6mm screw

Nylon or steel M3 nut

QCX-mini assembly Rev 1.12

Assembly general guidelines

Assembly of this kit is quite straightforward. But there are quite a lot of components. So please keep them methodically in trays or some convenient storage boxes, and be careful not to misplace any. The usual kit-building recommendations apply: work in a well-lit area, with peace and quiet to concentrate. The IC (chips) and some of the other semiconductors in the kit are sensitive to static discharge. Therefore, observe Electrostatic discharge (ESD) precautions. And I say it again: FOLLOW THE INSTRUCTIONS!! Don’t try to be a hero and do it without instructions!
A jeweler’s loupe is really useful for inspecting small components and soldered joints. You’ll need a finetipped soldering iron too. It is good to get into the habit of inspecting every joint with the magnifying glass or jeweler’s loupe (like this one I use), right after soldering. This way you can easily identify any dry joints or solder bridges, before they become a problem later on when you are trying to test the project.
You could also take photos with a mobile phone, and use the phone’s zoom features to view the board in detail.
Triple check every component value and location BEFORE soldering the component! It is easy to put component leads into the wrong holes, so check, check and check again! It is difficult to de-solder and replace components, so it is much better to get them correctly installed the first time. In the event of a mistake, it is always best to detect and correct any errors as early as possible (immediately after soldering the incorrect component). Again, a reminder: removing a component and re-installing it later is often very difficult!
Please refer to the layout diagram and PCB tracks diagrams below, and follow the steps very carefully.
Assembly steps will be in the order of smallest to largest components. I generally follow the order semiconductors, capacitors, resistors and finally all other (generally larger) components. It is probably unnecessarily thorough and complex to build the radio one stage at a time and test each stage one by one… I recommend just install everything then power up.
As per standard QRP Labs practice, the ATmega328P microcontroller has a 28-pin DIP socket in case you may wish to subsequently replace it for firmware upgrades etc. Many of the components in this kit are in surface mount packages (SMD) and these are already soldered to the PCB for you, at the factory. All other components used are all leaded through-hole packages, and all are installed on the top side of the PCB except where otherwise noted.
Use of a good quality soldering iron and solder is highly recommended for best results!

QCX-mini assembly Rev 1.12

SPECIAL CARE tips by Hans G0UPL
The QCX-mini kit is very compact. It requires a higher degree of precision than many other QRP Labs kits, if you are intending to install it in the optional enclosure. It’s not more difficult. It just requires a bit more care. Pay careful attention to the following points, throughout the assembly.
1. Board inspection
Even the SMD assembly line is not perfect and mistakes can occur. I recommend a careful visual inspection before commencing assembly, with a jeweler’s loupe or other optical magnification aid. Look for any solder bridges between IC pins, or solder splashes which may cause short-circuits.
2. Soldering
In the QCX-mini kit, there are SMD components installed on BOTH sides of the PCB. When soldering through-hole component leads, you will often be soldering a joint that is very close to nearby SMD components. In this case, angle the PCB and soldering iron so that you are approaching the joint, and touching and heating the joint, from the opposite direction to the SMD component, to avoid heating it and removing it.
In the photograph below, SMD capacitor C4 is very close to the component leads being soldered; I approached the joint from the right-hand side, away from the SMD component, and successfully soldered the joint without coming anywhere near C4. I successfully assembled this PCB even though my soldering iron tip is an enormous 3mm chisel-bit.

3. Mistakes
If you do make a mistake and end up with a solder bridge somewhere, desoldering braid (a.k.a. solder wick) can really help clean this up, it is well worth having some of this in the workshop. If you have no braid to hand, you can even use the braid from a piece of old coax or shielded cable, perhaps soaked in flux first if you have some.

QCX-mini assembly Rev 1.12

Component lead offcuts
When cutting wire-offcuts, take great care not to damage any nearby SMD components with the wire cutter!
At the same time, you need to cut quite close to the PCB to keep the remaining wire lengths short, to avoid any wires short-circuiting with the base of the aluminium enclosure.
In this kit, you must KEEP all the component lead off-cuts! Yes, I know, it’s fun when you cut them with the wire-cutter to hear them pinging around the room somewhere, and even more fun when your XYL finds them embedded in the carpet somewhere else in the house, and nothing quite beats the entertainment value of when she steps on one and it is embedded in the sole of her foot… ah yes, oh, the baby keeps waking up all night, EVEN more often than he would normally anyway. What? Who put that 2cm piece of wire offcut in his nappy (a.k.a. diaper), that can’t have been very comfortable. Ooops.
Yes, fun fun! BUT, in this assembly you actually have to KEEP the component wire offcuts because you are going to use them later in the assembly, for connecting the LCD module to the display board, and for connecting some controls to the controls board.
So please read this and keep those off-cuts safely.

Fuses and current-limited supplies
Generally an in-line fuse and/or a current limited supply is a good thing. A QCX-mini should not draw more than 600-700mA and if it does, you may have a problem somewhere and a fuse or other current limiting could save something from burning up.

PRECISION

I can’t emphasize this enough: this board is compact, and the fit in the optional enclosure is precise with low tolerance for error. The following is essential:
· In general, the holes in the PCB are larger than the corresponding component leads/ pins. Try to align each component centrally and squarely, where there is movement range.
· All connectors and controls need to be seated flush on the PCB, orientated square to the PCB, and precisely aligned as described in the corresponding assembly manual steps.
· All components need to be installed seated fully on the PCB, such as electrolytic capacitors and the trimmer resistors, because in many cases even an extra fraction of a mm height can cause the boards to not fit together properly.
· Follow all assembly steps carefully and precisely. Pay particular attention to any description involving component orientation, such as cutting off the feet of the trimmer potentiometers,

QCX-mini assembly Rev 1.12

The following diagrams show the PCB layout and track diagrams of the display and main PCBs of the QCX-mini kit (red track = top side; blue track = bottom side; there are no hidden layers).

QCX-mini assembly Rev 1.12

Tracks shown in BLUE are on the bottom layer. Tracks shown in RED are on the top layer. There are only two layers (nothing is hidden in the middle). Not shown in this diagram are the extensive ground-planes, on both sides of the board. Practically everything on both layers that isn’t a RED or BLUE track, is ground-plane! The two ground-planes are connected at frequent intervals (not more than 0.1-inches) by vias. This is the kind of layout I have done previously for a quad-band GSM device operating at up to 1900MHz… it is probably overkill in an HF transceiver… but if you can, why not! I used to say often that you can never have too much supply line filtering and decoupling, and never have too much shielding. Both these statements don’t apply so conveniently to kits as they do to homebrew projects. In a kit every decoupling capacitor has a cost in both money and PCB area (which also means more money). Shielding is even more difficult and expensive. So shielding and decoupling should be applied where needed only! But ground-plane well that’s another story. It’s free, and without drawbacks so why not, let’s just put it everywhere.
All components on the main PCB are installed on the top (component side) of the PCB and soldered on the bottom (solder side) of the PCB. On the display PCB the 2×5-pin header connector is installed on the reverse side so refer to the assembly manual steps very carefully.
Take care when installing integrated circuits. All through-hole integrated circuits are supplied by the manufacturers with their pins bent a little wide. You need to carefully bend the rows of pins of the ATmega328 microcontroller together a little, in order to fit it into the 28-pin IC socket.
The band-specific Low Pass Filter (LPF) parts are supplied in a separate LPF kit bag.
In the construction for some bands, not all of the capacitors supplied in the kit are used. Do not be alarmed if you have a few components left over at the end!
Wind the L1-3 inductors with the enameled copper wire supplied in the LPF kit bag. Wind the other inductors (L4 and transformer T1) using the wire supplied in the main kit bag.
The component colour coding of the layout diagram at every step of the assembly instructions is as follows (kind of: components past, present and future):
Components shaded grey have already been installed Components shaded red are the ones being installed in the current assembly step Components shaded white are the ones which have not yet been installed
The following photographs show the final assembly. You can keep these photographs in mind when assembling the kit, they will give you some idea of how the kit fits together and help avoid assembly errors.

QCX-mini assembly Rev 1.12

Main board: Display board:

Controls board:

QCX-mini assembly Rev 1.12

There are two PCB panels supplied in the kit, each is packaged in a separate static-proof zip-lock bag. The main PCB has SMD components soldered to both sides; handle it with care to avoid dislodging any components. The display PCB also has several SMD components installed.
The display PCB panel has cut-out areas, which are used to create additional PCBs used in the design. These additional PCBs will be broken off the display PCB along lines of drillholes.

The “Controls PCB” connects to the main PCB using a 2×4-pin header connector, and is fixed with a hex spacer and screws. This board holds the gain control and rotary encoder, as well as the two push-button switches.
The controls PCB needs to be slightly higher relative to the main board than the display PCB, this is accomplished using two small spacer PCBs, one which fits on the 2×4-pin header, and one which fits on the hex spacer; together these elevate the controls PCB by 1.6mm (one standard FR4 PCB thickness) above the height of the display PCB.
The uSDX PCB is a small daughtercard that can be installed on the main PCB and provides an easy conversion of the QCX-mini to the uSDX SSB SDR transceiver. Components are not supplied to populate this PCB. The conversion is supported by the uSDX group https://groups.io/g/ucx and there is no technical support from QRP Labs. The daughtercard is supplied as-is, to assist this conversion for experimenters who wish to try uSDX.

QCX-mini assembly Rev 1.12

The PCBs may come with a surplus strip of material along one or more edges; this is used during the manufacturing processes to panelize PCBs on a larger panel of blank FR4. If this is the case, just gently snap off the excess strip of board.

It’s also important to gently file away any rough edges of the PCB from where it has been snapped out of the larger panel.

QCX-mini assembly Rev 1.12

3.1 Inventory parts
Refer to parts list in section 2. The following photographs are to aid component identification. Several components are missing from this photo. So for a complete inventory refer to the parts list, remembering to check the band-specific components for your band.
Remember that the main kit bag contains several smaller clear plastic zip-lock bags, and components are spread through the different bags with no apparent rules or reason.

QCX-mini assembly Rev 1.12

3.2 Wind and install transformer T1
This is the only really tricky piece of the assembly: the receiver input transformer T1. Follow these instructions carefully, it is tricky but quite feasible if you go step by step.
This is the FIRST step because it is easiest to do the installation when there are no other components nearby.
In the end, you are going to end up with an installed transformer which hopefully looks something like the photo (right, shows 40m version).
This transformer has FOUR windings. Three identical short windings, and one long winding. There are therefore eight wire ends, which must all be soldered into the correct holes on the PCB, and with the enamel properly removed.
Here are two diagrams which enumerate each of the windings, wire endings and holes on the PCB, on both the layout diagram and on the circuit (schematic) diagram. It should help to explain diagrammatically which wires must go where.

On the following page is a diagram hand-drawn by Ed WA4MZS (thanks Ed!) which may also clarify the construction and installation of T1.
The four windings on T1 must all be in the same “sense”. There are two ways to wind toroids. You might call them left-handed and right-handed; clockwise and counterclockwise; whether the wire goes through the toroid from top to bottom, or from bottom to top. Whatever you call it, all the four windings have to be the same, to be sure to get the phasing to the quadrature sampling detector correct.
(Technically, only the secondary 1 and secondary 2 have to be in the same sense; but it is simpler to just wind everything the same way).

QCX-mini assembly Rev 1.12

The number of turns in each winding depends on the band you are building the kit for. Refer to the following table. For convenience, the remainder of the instructions in this section refer to the 40m version (38 + 5 + 5 + 5 turns).

But MAKE SURE you wind the correct number of turns and toroid for your band!

Band
160m 80m 60m 40m 30m 20m 17m

Toroid
FT50-43 T50-2 T50-2 T50-2 T50-2 T50-2 T50-2

Primary
3 5 5 5 4 3 3

Secondary 1 3 5 5 5 4 3 3

Secondary 2 3 5 5 5 4 3 3

Secondary 3 10 68 46 38 30 30 22

Note for 80m and 60m versions: the toroid ring is not large enough to neatly hold all of those turns in a single flat tidy winding. The large secondary winding WILL end up looking messy, with overlaps in some places. You should try to ensure any overlaps turns are evenly spaced throughout the winding! Do not try to wind one neat layer, then wind the remaining turns as an additional neat layer on top of that. Just go with the messy overlapping turns and don’t worry about it: everything will work fine, regardless.
There is a suggested modified way of winding the turns for 60/80m versions, which may make it easier; see http://www.qrp-labs.com/qcx/qcxmods.html#80m

QCX-mini assembly Rev 1.12

In all cases, there is one long secondary winding, and three other short identical windings. To make things easier, we will wind all windings together in one go, this will guarantee that the “sense” of each winding will be the same. At the intended breaks between the windings, we will leave large loops of wire, that we can later cut one by one to make sure the wires go in the correct holes. There are a lot of steps listed below but in reality, it is easier DOING it than it is writing instructions how to do it. Take it patiently, step by step.

Hold the toroid between thumb and finger, and thread the first turn of the wire through from top to bottom. Leave about 3cm of wire at the free end.
Apply tension to the wire after each pass through the center hole, to try to keep the windings tight and even. The wire turns should sit neatly side-by- side on the toroid, without overlapping.
Grip the toroid between thumb and finger as you wind.
When you have completed 38 turns, pass the wire through the hole for the 39th turn but leave a large loop of wire between the 38th and 39th, without pulling it tight.
Grip the toroid and wound turns tightly between thumb and finger, and with the other hand apply some twists to the wire loop, tight next to the toroid.
The result is a wire loop after 38 turns, as shown.

QCX-mini assembly Rev 1.12

Do the same thing for two more loops, which are between the 43rd and 44th turns, and 48th and 49th turns respectively. It is easy to lose count. An easy way is after step 6, pass the wire through the hole 5 more times, and on the 5th one, make a loop. Similarly count five more and make a loop on the fifth. Then finally five more turns to complete the toroid’s 53 windings. Count the windings to make sure you have 53. When you are sure everything is fine, cut the wire leaving about 3cm of wire free at the end.
Thread the original start of your winding (from step 1) into hole 2 in the diagram. Thread the final end of your winding (from step 7) into hole 6
Twist these two wires under the board to keep the toroid in place while you deal with the remaining wires.
10)Now cut the 3rd loop (the one nearest the end of the winding work), and un- twist the twisted section you made near the toroid body.
11)When you cut the loop, you therefore created two wires. One of these came over the top of the toroid, and you can easily verify that this is the one which has five turns through the toroid then passes through hole 6. So, insert this wire into hole 5. Once again to be clear: you should now have a winding of five turns (which is labeled “secondary 1”, above), with one end inserted into hole 6 and one end inserted into hole 5. The other wire came from UNDER the toroid. You must push this wire towards the toroid and pull it out through the center hole of the toroid. Now thread it through hole 4. Under the board, twist the two new ends of wire together to keep them in place.

QCX-mini assembly Rev 1.12

12)Next cut the 2nd loop, and un-twist the twisted section near the toroid body. 13)Similar to step 11; the wire which came over the toroid body has five turns then goes
into hole 4, which you did in step 11. Insert this wire-end into hole 3. The other wire end that came from under the toroid body should be gently pushed toward the toroid, and pulled up through the center hole of the toroid. Insert that wire end into hole 8. So now you have the “primary” winding consisting of five turns, between holes 3 and 4. Twist the two new wires under the board again, to keep everything in place.
14)Finally, cut and un-twist the loop which you created first, which was between the 38th and 39th turns of the toroid winding. Push the wire which came over the toroid body, into hole 7. Now you have five turns of wire, which make up “secondary 2”, between holes 7 and 8.
15)The last wire came from under the toroid body when you cut the loop; this wire is the other end of the 38 turns “secondary 3” winding, so insert it into hole 1. It is already near to hole 1 and you don’t need to push it under the toroid body as you did in previous steps. Take a moment now to review the situation. You should be able to identify the four windings of T1, and squeeze them together neatly as in the photo, to verify that each end of the 5-turn windings goes into the correct holes.
16)Under the PCB, you should have three pairs of twisted wires, and one pair (that you installed last), un-twisted.

QCX-mini assembly Rev 1.12

17)Now you can solder the eight connections under the PCB. I recommend doing one pair of wires at a time; this way, the other wires will hold the toroid in place and stop it falling out. Start with the two un-twisted wires. Pull each wire tight, bend it over at about 45-degrees, and cut it 1-2mm away from the PCB surface. Having bent the wire prevents it from falling out. Now solder the wire. Remember to hold the soldering iron to the joint for 10 seconds or so, to allow the enamel insulation to burn off. Repeat for all the other wires, one pair at a time, until all eight are soldered. If you have a DVM, check for DC continuity (zero ohms resistance) across each winding. If you do not get the expected continuity, then it means either a) you have not managed to scrape or burn away the enamel insulation properly so there is no electrical connection AND/OR b) you put the wires into incorrect holes AND/OR c) your expectation is wrong because you have not identified which pad on the PCB is labeled 1-8 in the diagram.
18)The final picture shows the toroid installation completed.

QCX-mini assembly Rev 1.12

3.3 Install IC2 socket
PUSH IT UP TOWARDS THE TOP OF THE BOARD, TO MAKE MORE SPACE FOR THE TCXO PCB
Install the 28-pin IC socket for IC2. Take care to match the dimple on the socket, with the dimple on the PCB silkscreen. It is critical to insert the microcontroller with the correct orientation. Lining up the dimple on the PCB silkscreen, socket and actual IC is the best way to avoid confusion and potential error.
There is some leeway in where exactly the socket is placed, because the pins are smaller than the diameter of the PCB holes. Try to position the socket as far towards the top edge of the PCB as possible (oriented as the diagram). This is necessary, to create sufficient space for the Paddle and Earphones connectors; and also to create enough space for the TCXO daughterboard PCB and the uSDX daughterboard PCBs if fitted.
I recommend soldering one pin at each diagonal, for example pins 1 and 15. You can then check that the IC socket is seated firmly on the PCB, and correct any issues easily. Once all the pins are soldered it will be difficult, if not impossible, to change anything. When you are happy with the position of the socket, proceed with soldering the other 26 pins.

QCX-mini assembly Rev 1.12

3.4 Install 100nF (0.1uF, “104”) capacitors
There are two 100nF (0.1uF) capacitors, these have the code “104” written on them. Be sure to identify the correct capacitors, using a magnifying glass or jeweler’s loupe. These capacitors are C12 and C29. Place each in the correct position on the board, and slightly bend the legs outwards at about 30-degrees angle so that they stay in place. Then solder the wires, and trim the excess wire length with wire cutters.
It does not matter which way round the capacitors are installed. However, it is very good practice to install them all with the capacitor label facing in the same direction. For example, ensure all the “104” labels face to the front, or to the right (depending which way the capacitor is orientated on the PCB). This makes it much easier to inspect the PCB assembly later.
An exception to this “rule” is where a component label cannot be read, if it is facing the common direction; an example in this kit is C12 (in this section), which should be installed with its label facing away from R27 so that it can be easily checked later.
Take particular care soldering wires which connect to the ground plane. Despite the “thermals” (a ground pad is connected to the ground plane by four thin traces, not a continuous ground plane, to make soldering easier), the heat dissipation is still more, and it can be harder to make a good joint.

QCX-mini assembly Rev 1.12

3.5 Install all 470nF, “474” capacitors
The five 470nF capacitors are labeled “474”, and are capacitors C11, C43, C44, C45 and C46.

3.6 Install 47nF, “473” capacitor
The 47nF (0.047uF) capacitor is labeled “473” and is capacitor C9.

QCX-mini assembly Rev 1.12

3.7 Install capacitors C25 and C26 from Low Pass Filter kit

The value of these capacitors depends on your chosen band. The capacitors are located inside the separate Low Pass Filter bag in your main kit bag. Refer to the following table to find the correct capacitor value for your band:

Band 160m 80m 60m 40m 30m 20m 17m

Value 2200pF 1200pF 1200pF 680pF 560pF 390pF 270pF

Label “222” “122” “122” “681” “561” “391” “271”

QCX-mini assembly Rev 1.12

3.8 Install capacitors C27 and C28 from Low Pass Filter kit

Band 160m 80m 60m 40m 30m 20m 17m

Value 820pF 470pF 680pF 270pF 270pF 180pF 100pF

Label “821” “471” “681” “271” “271” “181” “101”

QCX-mini assembly Rev 1.12

3.9 Install capacitor C30

This capacitor is band dependent. The kit contains all required capacitor values for all bands. Install the one appropriate to your band. Refer to the following table to find the correct capacitor value for your band

Band 160m 80m 60m
40m 30m 20m 17m

Value 820pF 180pF 30pF or 33pF 56pF 56pF 30pF or 33pF 30pF or 33pF 30pF or 33pF

Label “821” “181” “300” or “330” “560” or “56J” “560” or “56J” “300” or “330” “300” or “330” “300” or “330”

Note that for 40m and 80m versions, the 56pF and 180pF capacitors respectively, may be packed in a separate small bag with the T37-2 and T50-2 toroids.
Note that your kit might contain a 30pF or 33pF capacitor, as shown in the table. It is not critical which one you have, either is OK.
60m important note: for the 60m band, the capacitor requires both the 30pF and 56pF capacitors to be soldered in parallel, but there is only one component position on the PCB. For 60m, you will need to install one of the capacitors (e.g. 56pF) in the component holes provided, and solder the other one (e.g. 30pF) to the same pads under the PCB. Take care to keep the component wires short and not accidentally touching any other nearby components or soldered pads.
80m important note: the 180pF capacitor supplied has a 0.2-inch (5.08mm) pin spacing. But the holes on the PCB are spaced 0.1-inch (2.5mm). It is necessary to squeeze the capacitors wires carefully closer together to fit the PCB holes.

QCX-mini assembly Rev 1.12

3.10 Install capacitors C5 and C8

These capacitors are band dependent. They add parallel capacitance to trimmer capacitor C1 to bring it to the required value. The kit contains all required capacitor values. Install the capacitor(s) appropriate to your band. Refer to the following table to find the correct capacitor value(s) for your band. Where “none” is indicated in the table, do not install the corresponding capacitor.

Band 160m 80m 60m 40m 30m 20m 17m

C5 Value 100pF 39pF 39pF 39pF 22pF none none

C5 Label “101” “390” “390” “390” “220”

C8 value 22pF 22pF 22pF none none none none

C8 Label “220” “220” “220”

QCX-mini assembly Rev 1.12

3.11 Install 1uF, “105” capacitors C21, and C22
There are two 1uF capacitors labeled “105”, which are C21 and C22.

QCX-mini assembly Rev 1.12

3.12 Install 1N5819 diode
This diode D33 is the diode with a black body, and a white stripe. It is installed vertically. It must be orientated correctly, with the white stripe on the diode matching the white stripe on the PCB.
This diode protects the radio against reverse polarity. So that if you connect the power to the board the wrong way around, you do not destroy it. A Schottky diode is used because the forward conducting voltage drop is less than an ordinary diode. However, on transmit, the voltage drop across this diode can still be up to 400mV. This reduction in voltage does slightly decrease the output power.
IF you want to squeeze every last milliwatt of output power out of the radio, and IF you trust yourself NEVER to connect the power supply in reverse by mistake, then if you just install a jumper wire instead of D33, it would give you a little higher RF power output. It is very strongly recommended to install D33.

QCX-mini assembly Rev 1.12

3.13 Install 20MHz crystal XTAL1
The engraving on this crystal is “20.000” or “CE20.000”.

QCX-mini assembly Rev 1.12

3.14 Install 27MHz crystal XTAL2 or TCXO option
Install the 27MHz crystal or 25MHz TCXO. The engraving on the crystal is “27.000”. If you are going to install the 25MHz TCXO, skip this diagram and refer to the following instructions.
Menu entry 8.5 must be changed to 25,000,000 if using the TCXO

If installing the TCXO module: this is installed in the same position as the 27MHz crystal would have been. There is a rectangle on the PCB indicating the position of the TCXO:

Install the TCXO module as follows: QCX-mini assembly Rev 1.12

Find a suitable file.

File the rough edges of the TCXO module flat

Solder wire off-cuts into the holes shown, Thread the TCXO module carefully onto the

and cut any excess wire on the underside of wires. Make sure module is pushed as far

the board.

towards the 28-pin IC socket as possible.

Solder the wires to the pads, holding the Cut off excess wire; be careful because the

TCXO module firmly tight against the 28-pin TCXO board is single sided without through-

IC socket.

hole plating, so the pads are a bit delicate.

Note that when using the TCXO module, the reference frequency setting must be 25,000,000 MHz, not the default which is 27,004,000 MHz. This is further described in the initial set-up instructions, below. This is because the QCX- mini can operate either with 25MHz reference (the TCXO module) or with 27MHz (the supplied crystal), BUT, you have to tell it which one you have installed!

3.15 Install 500-ohm multi-turn trimmer potentiometer
This 24-turn trimmer resistor is the small blue box component with label “501”. It is R27.
The trimmer resistor has to be prepared carefully before installation. The resistor has little ledges on either side which make it slightly too high to fit in the QCX-mini PCB assembly. Therefore simply cut off these protrusions with a wire-cutter.

QCX-mini assembly Rev 1.12

Note the ledges

Cut ledge off with wire-cutter Enjoy less high trimmer pot!

The screw on the resistor must match the screw on the PCB silkscreen and layout diagram, facing towards the lower edge of the board as drawn. Ensure it is firmly seated.

3.16 Install 50K multi-turn trimmer potentiometers
There are two 50K multiturn trimmer potentiometers, R17 and R24. They are the small blue box components with label “‘503”.
The trimmer potentiometers must be prepared before

QCX-mini assembly Rev 1.12

installation by cutting off the small plastic ledges with a wire-cutter, as described in the preceding section.
The screws on the resistors must match the screws on the PCB silkscreen and layout diagram.

QCX-mini assembly Rev 1.12

3.17 Install 470uF capacitors
There are two 470uF capacitors in the kit: C24, C47. These are polarized electrolytic capacitors and MUST be installed with the correct orientation! The capacitor NEGATIVE wire must be installed in the hole indicated on the PCB silkscreen and the layout diagram by the solid black bar; the POSITIVE wire must be installed in the hole indicated on the PCB silkscreen and the layout diagram by the hollow bar and the + symbol.
Electrolytic capacitors are also supplied with one wire longer than the other. The + wire is the longer wire, the wire is the shorter one (see photo, right).
These capacitors must be installed on the PCB so that they are sitting flush with the PCB, with no gap between the PCB and the bottom of the capacitor.

QCX-mini assembly Rev 1.12

3.18 Install 30pF trimmer capacitor C1
Insert the component pins carefully and with the correct orientation which matches the PCB.
Cut the small pin stubs on the underside (solder side) of the PCB, they only protrude a few mm but when installed in the enclosure could be rather close to the aluminium floor.

QCX-mini assembly Rev 1.12

3.19 Install MPS751 transistor Q6
Be careful to correctly identify this transistor by its markings, as the package style is similar to the other transistors. Carefully bend and insert the wires so that the transistor’s flat side is flat flush against the PCB surface, and the body of the transistor is aligned with the square on the layout diagram (which is not visible on the PCB silkscreen). The corner of the transistor should not overlap the PCB hole.

QCX-mini assembly Rev 1.12

3.20 Install three BS170 transistors
The remaining three transistors in the kit are BS170 MOSFETs: Q1, Q2 and Q3
For Q1, Q2 and Q3, carefully follow the same installation procedure as the previous section, making sure that the transistors are neatly aligned in the correct positions near the hole in the PCB.
After installation, use the supplied 10mm M3 steel screw, 12mm steel washer and M3 nut to bolt the transistors’ flat sides firmly flat on the PCB surface, as shown (photo, right).
The kit may contain both a 10mm screw AND a 12mm screw. It is essential to use the 10mm screw, not the 12mm screw (which is too tall). 10mm is the desired length of the threaded section.

QCX-mini assembly Rev 1.12

3.21 Install 2×3-pin in-circuit programming header
This male pin header can be used to connect an AVR Programmer to apply firmware updates if desired. Insert the SHORT end of the pins into the PCB. Solder one pin in place first and check that the header is nicely seated on the PCB before soldering the other 5.

QCX-mini assembly Rev 1.12

3.22 Install 2×5 LCD header
This female pin header (socket) connects the main QCX-mini PCB to the display board above it. Solder one pin in place first and check that the header is nicely seated on the PCB before soldering the other 9.
Be sure to install the 2×5-pin FEMALE pin header connector (see right), not the male header connector which is installed on the LCD board.
Try to hold the socket as far towards the top side of the board (in the diagram below) as possible, that is, as far away from the Paddle connector as possible. This is because installing the earphones and paddle connectors is a tight fit.

QCX-mini assembly Rev 1.12

3.23 Install 2×4 UI header
This female pin header (socket) connects the main QCX-mini PCB to the controls board above it. Solder one pin in place first and check that the header is nicely seated on the PCB before soldering the other 8.

3.24 Wind and install toroid L4

L4 is type T37-2. It is a small ring with red paint on one side. Each time the wire passes through the hole in the middle of the toroid, this counts as one turn. The number of turns depends on the band of your kit, refer to the following table. Inductance values are approximate and will depend on variations in the core, and how tightly you wind the turns. Do not worry about these variations which are not critical in this case.

Band 160m 80m 60m 40m 30m 20m 17m

Value 1.4uH 2.3uH 2.3uH 1.0uH 0.78uH 0.40uH 0.32uH

Turns 19 24 24 16 14 10 9

Try to keep the wire quite tight (but not so tight that you break the wire). Try to spread the turns evenly around the toroid. Leave about 2cm or 3cm of wire at the ends.
The wire is coated with an enamel insulating paint and it is CRITICAL to remove this enamel at the soldered joints otherwise there will be no electrical connection through

QCX-mini assembly Rev 1.12

the toroid! This is the number 1 cause of problems with QRP Labs kit construction: failure to remove the wire enamel.
One method of removing the wire enamel is to scrape it off at the ends, either with sandpaper, or carefully scratching with a knife or wire cutters. However, my favourite method is just to burn off the enamel. For quite a number of years, the type of enamel used on copper wire can be burnt off using the temperature available from an ordinary soldering iron, immersing the wire in a blob of solder. (This was not the case, with much older wire found in vintage valve/tube equipment). You could also use a cigarette lighter to burn off the enamel.
Insert the ends of the wires into the correct holes of the PCB and pull it through tight, so that the toroid sits secure and flat on the PCB.
Bend the wires over so that the toroid does not fall out while you are trying to solder the wires. Cut off the excess wire using wire cutters, leaving only about 2mm protruding through the PCB on the lower side. Now apply solder quite generously from the soldering iron. Hold the soldering iron to the joint for a few seconds I usually count to 10 slowly and the solder will surround the wire, which will become hot enough to burn off the enamel. You can sometimes see a small puff of smoke when the enamel burns off.
Carefully inspect the soldered joints with a magnifying glass to make sure that the wire is correctly soldered. If it looks like the solder hasn’t flowed nicely and adhered to the wire, then this is usually a sign that the enamel probably hasn’t been burnt off.
If you have a DVM it is a good idea to check for DC electrical continuity (zero ohms resistance) between the two ends of the wire. If you do NOT have a DVM and if the radio doesn’t work, then we can use the built-in test equipment later, to trace the fault.

QCX-mini assembly Rev 1.12

3.25 Wind and install toroid L2

L2 is a small toroid ring painted yellow or red on one side. It is part of the supplied Low Pass Filter kit bag. Installation of the inductor is similar to the previous section. Remember to remove the wire enamel and check!

In the QCX-mini kit, there is some advantage to winding the toroid with the turns tightly squeezed together, then installing it, and only then spreading them out evenly. This is because you will probably later want to try squeezing and expanding the turns of the toroids to optimize output power. The way the toroids are installed laying flat on the PCB in the QCX-mini, it’s much easier to spread out the turns if they were initially bunched together, than it would be to bunch them up if they were initially spread out. In the latter case the wire will be rather tight to try and bunch up the turns.

The number of turns is band-dependent, refer to the following table. Inductance values are approximate and will depend on variations in the core, and how tightly you wind the turns. Do not worry about these variations which are not critical in this case.

Band 160m 80m 60m 40m 30m 20m 17m

Toroid T37-2 T37-2 T37-2 T37-6 T37-6 T37-6 T37-6

Colour Red Red Red

Yellow Yellow Yellow Yellow

Value 6.4uH 3.0uH 2.3uH 1.7uH 1.3uH 0.90uH 0.67uH

Turns 40 27 24 24 20 17 15

QCX-mini assembly Rev 1.12

3.26 Wind and install toroids L1 and L3

L1 and L3 are small toroid rings painted yellow or red on one side. They are part of the supplied Low Pass Filter kit bag. Installation of the inductors is similar to the previous section. Remember to remove the wire enamel and check!

Band 160m 80m 60m 40m 30m 20m 17m

Toroid T37-2 T37-2 T37-2 T37-6 T37-6 T37-6 T37-6

Colour Red Red Red
Yellow Yellow Yellow Yellow

Value 3.9uH 2.4uH 2.1uH 1.4uH 1.1uH 0.77uH 0.55uH

Turns 30 25 23 21 19 16 13

QCX-mini assembly Rev 1.12

3.27 Install 2.1mm power connector
Install the 2.1mm power connector, orientated to match the PCB silkscreen. It is important to install this accurately so that if you install the QCX-mini in the optional aluminium enclosure, the connector is correctly aligned with the associated panel hole.

QCX-mini assembly Rev 1.12

3.28 Install RF output BNC connector
An alternative is installation of a right-angled SMA connector here (not supplied).
Solder the center pin first, to check that the alignment is correct; when happy, proceed to solder the remaining pins. It is easier to ensure good alignment if the washer and nut are removed prior to installation.
Accurate alignment is very important, to ensure that the connector fits into the hole of the optional QRP Labs QCX-mini enclosure (if used). The rim on the connector body should not overlap the edge of the PCB. The BNC connector must sit squarely on the board with the metal body extending at 90-degrees to the PCB edge. Cut the excess wire length of the center pin after soldering.
The metal body of the BNC connector is directly cast to the four ground pins of the connector. Soldering these pins requires a lot of heat! Using a 60W soldering iron during prototype assembly, the iron was held to each pin of the connector for at least 20 seconds to heat it up fully and ensure a good electrical and mechanical bond.
It needs a lot of heat, some time, and a lot of solder… Do not worry if you create solder bridges to the ground pads which are available optionally for the SMA connector; but do make sure that there are no short-circuits to the center pin!

QCX-mini assembly Rev 1.12

3.29 Install 3.5mm stereo jack connectors
This is the final step in the assembly of the main board. There are four stereo 3.5mm jack connectors, used for the audio output (earphones), optional connection of a paddle, optional CAT connection, and optional PTT output to 50W PA kit (optional QRP Labs kit).
Accurate alignment is very important, to ensure that the connectors fit into the holes of the optional QRP Labs QCX-mini enclosure (if used). The connectors have a tendency to sit at a crooked angle. This must be corrected. One way to ensure precise positioning is to angle some solder a few cm above the workbench, then hold the board in one hand, pushing firmly to straighten the connector, then solder the ground tab (the tab nearest the PCB edge). Next, solder a tab near the inside of the PCB area; then check the alignment again before continuing with the remaining three pins (photo 1 shows PTT and CAT; Photo 2 Audio and Paddle connectors).

A tendency to sit at a crooked angle…

…push firmly during soldering to correct this

QCX-mini assembly Rev 1.12

3.30 Break apart inner PCBs of display board
Now that the main PCB assembly is complete, the display board and controls board are assembled next.
The display board panel contains several smaller PCBs which are joined to the display board via thin PCB bridges which have a line of holes, designed to be easily snapped out.
· Controls PCB: this will hold the gain control, rotary encoder (tuning) control and the two buttons.
· Controls board spacers: two tiny PCBs which are fit on the mounting points of the control PCB to increase its height above the main QCX-mini PCB by 1.6mm
· uSDX daughtercard: the PCB for the SSB modification supported by the uSDX group https://groups.io/g/ucx

The small PCBs can be snapped out of the panel using needle-nosed pliers or wire cutters or some such similar tool. Breaking out the controls board is easiest, just a twisting action while gripping the thin PCB bridge is enough to break it out. The uSDX daughtercard and control board spacers require a bit more care; particularly the uSDX daughtercard, be careful not to break the main center spar of the display board while snapping it off.
Once removed, it is essential to file the rough edges of all the PCBs (except the uSDX daughtercard it is only necessary to file this one if you are intending to use it).
The Controls board has to fit through the hole in the lower part of the display board and will not do so unless the rough edges have been smoothed off!

QCX-mini assembly Rev 1.12

Location of five small PCBs to snap out of Squeeze and twist the PCB bridge to snap

the display board panel

out using pliers or wire cutter

Gently grip and bend to remove but be File rough edges of the cut-outs in the panel careful of the central spar of the panel PCB PCB and the broken-out pieces

All five parts after careful filing QCX-mini assembly Rev 1.12

Install LCD module

Precision assembly is essential follow the guide below carefully.

Identify the pairs of M3 6mm screws and nuts. They may be nylon or metal.

Fit the LCD module from behind the PCB, with its body through the rectangular cut-out

Bolt the LCD module, ensuring equal gaps Drop component off-cuts through the 16

at top and bottom; tighten screws firmly.

holes, their bottom ends sitting on the bench

Solder the component off-cut wires to the Turn over the PCB. Ensure the LCD sits flat top of the PCB and trim (cut) the excess wire on the PCB before soldering; trim excess.
Cut these wires VERY short to avoid touching components on the main PCB later.

QCX-mini assembly Rev 1.12

3.32 Install 2×5-pin male pin header connector
Install the 2×5-pin male header from below the display PCB; the short-end of the pins should be inserted through the PCB from the bottom side, as shown.
Solder one pin first, and check alignment before continuing with the other nine. Try to ensure that the pin header sits squarely and centrally in its allocated position.

3.33 Install four 11mm nylon spacers
Install four 11mm nylon hex spacers on the underside of the LCD PCB using four 6mm nylon screws as shown.
Ensure the hex spacers are positioned such that a flat side is parallel to the nearby PCB edge, so that no corners overhang the edge of the PCB, which would prevent the enclosure end panels fitting.

QCX-mini assembly Rev 1.12

3.34 Install 20K trimmer potentiometer R47
The 20K single-turn trimmer potentiometer allows adjustment of the LCD contrast.
This potentiometer, similar to the 24-turn trimmer potentiometers installed on the main QCX-mini PCB, has four little feet, one in each corner. Unfortunately these make the trimmer too high and it may prevent the PCB from sliding into the QCX-mini enclosure later. Therefore it is necessary to cut off the protruding feet using a wire cutter, so that the potentiometer can sit flat on the PCB.
Follow the steps below to install this part.

Cut each little plastic foot using wire-cutters. It doesn’t matter if the corner of the potentiometer body is damaged slightly.

In the end it could look like this.

Install and solder, with the potentiometer body sitting flat on the PCB as shown. Trim excess pin length from the bottom side.

QCX-mini assembly Rev 1.12

3.35 Install 2×4-pin male header on controls PCB
Next comes the assembly of the controls PCB, which holds the gain control, rotary encoder (frequency tuning), and the two tactile switch buttons.
If the header pins are LONG, as in the photo, right (around 16-17mm), then please use the steps on the following page, not this page.
The 2×4-pin header must be installed with the small spacer PCB sandwiched between the connector body and the underside of the PCB. Carefully follow the steps below to install this part.

Thread the small spacer PCB having 8 matching holes, onto the short-pin side of the 2×4-pin header.

Insert the remaining length of the short pins into the PCB from the under- side, as shown.

The pins don’t protrude from the top side of the holes. However the holes are throughhole plated. Be generous with the solder, and apply heat to the hole for at least 5 seconds to ensure the solder flows down inside the hole and makes a good connection to the pin.

Likewise, solder the remaining seven pins. Don’t worry, this method really does work reliably; just ensure the soldering iron is poked into the hole if possible and apply heat for at least 5 seconds on each joint, and plenty of solder.

QCX-mini assembly Rev 1.12

On newer QCX-mini kits, the supplied 2×4-pin header has longer pins (about 17mm). This change was made, because the shorter pin headers were a little difficult to install with reliable connections, due to their short length; this was a cause of several construction issues.
The 2×4-pin header must be installed with the small spacer PCB (see photo, right) sandwiched between the connector body and the underside of the PCB. Carefully follow the steps below to install this part.

Insert the long pins into the 2×4-pin header Use a screwdriver to firmly push down on

socket (that will already be installed on the the plastic former of the 2×4-pin header as

main QCX-mini PCB, in reality, but is shown shown, to slide it down the long pins until it

separately here for clarity).

sits flush on the 2×4-pin socket body.

Thread the small spacer PCB onto the top Use wire cutters to cut off the excess length

side of the pins, and insert them into the

of the long pin headers. They need to be

controls PCB from below (silkscreen up, as trimmed otherwise they will risk touching the

shown), then solder carefully on the top side, metal of the enclosure when the QCX-mini is

ensuring no shorts. The spacer PCB is

installed in its enclosure.

important! Don’t forget it to install it!

QCX-mini assembly Rev 1.12

3.36 Install rotary encoder
The rotary encoder is installed in the large hole labeled SW1 on the controls PCB.

First cut off two large PCB-mounting lugs as Position a flat-headed screwdriver above shown using wire-cutters. Do NOT cut pins. one of the five switch pins.

Bend over the pin through 180-degrees so Repeat the same procedure for the four

that it points to the front of the control.

remaining pins.

Install the rotary encoder, first line up pins so Make sure the nut is on the “top” (silkscreen-

that they fit in corresponding PCB holes. The printed) side of the PCB.

rotary encoder has a locating tab which fits Do not use the washer.

into a matching hole on the PCB.

Tighten the nut.

QCX-mini assembly Rev 1.12

Solder each of the five switch pins, on the underside of the PCB (the side with the blue body of the rotary encoder). The center pin of the three may benefit from a piece of wire off-cut to extend it to reach the PCB hole.
3.37 Install tactile switch buttons
The two buttons should be installed on the control PCB as shown. These have four pins on a rectangular footprint that can only fit into the PCB one way. The only special precaution to observe here, is to make sure that the switch button is seated squarely on the PCB, so that the shaft is perpendicular to the PCB. Solder two diagonally opposite pins first then check the alignment and make any adjustments necessary; when all is well, solder the two remaining pins.
3.38 Install gain control potentiometer R36
Remove the nut and washer from the potentiometer shaft. Install the potentiometer into the position labeled R36. Align the potentiometer squarely with its locator tab in the provided hole in the PCB. Secure the potentiometer in place with the nut (do not use the washer), and tighten.
The pins of the potentiometer will not reach the PCB however much you bend them. Therefore it is necessary to bridge the gap between the pins and the PCB holes using component off-cut wires. Just keep the pins as they are, don’t attempt to bend them.

QCX-mini assembly Rev 1.12

Install component off-cut wires, soldering first the PCB end then the potentiometer pin end; make the connection as close to the green potentiometer body as possible.

Trim any excess length of both the pins, and the off-cut component wires. It is important to trim these as close to the joint as possible so that there are no shorts when the boards are fitted into the enclosure.

3.39 Install 11mm nylon hex spacer
The final 11mm nylon hex spacer is bolted to the controls PCB using an M3 6mm nylon screw. Push the screw through the hole from the front side of the PCB. Thread the small square spacer PCB that was broken out from the Display PCB panel, onto the screw. Then screw on the 11mm nylon spacer.

This completes the Controls PCB assembly. QCX-mini assembly Rev 1.12

3.40 Fit Controls PCB to main PCB
Now fit the Controls PCB to the main PCB by plugging together the two 2×4-pin header connectors.
Fit an M3 6mm screw from the underside of the main PCB, screwed into the 11mm nylon hex spacer pillar that is fixed to the Controls PCB, as shown in the following photograph.

QCX-mini assembly Rev 1.12

3.41 Install microcontroller
Install IC2, the programmed ATmega328P microcontroller, in the 28-pin DIP socket on the main QCX-mini PCB.
Be very careful to ensure that the dimple on the chip is aligned with the dimple on the 28pin DIP socket, which itself should already be aligned with the dimple on the PCB silkscreen.
Note that IC pins are always slightly splayed outward and will not fit into the socket. It is necessary to gently apply pressure on each row of pins on a flat surface such as the workbench, to be able to fit the IC into the socket.

QCX-mini assembly Rev 1.12

3.42 Plug together the two boards
Now you can carefully plug together the two circuit boards. The best way to do this is to concentrate on getting the 5-pin headers at the top left of the PCBs, to mate accurately with each other. The rest should fall into place by itself.
If you have taken care particularly with filing off the rough edges of the PCBs when the Display board panel was broken out into the sub-PCBs, then you should find that the Controls PCB will fit perfectly (though snugly) through the gap in the Display board, and it will be elevated 1.6mm (one PCB’s thickness) above the Display board.
Check that C24 (470uF electrolytic capacitor) isn’t blocking the way for the bottom left (as viewed from the front) black metal retaining tab of the LCD body; if it is, you can gently bend the metal tab flatter onto the LCD module PCB to avoid the conflict.
Construction is now complete! Alignment steps must be done BEFORE installation in the optional enclosure.

3.43 Connections for basic operation
The following connections are required for basic transceiver operation.

Power supply
A power supply is required, which needs to be able to supply up to 0.5A or a bit more, on transmit. The supply voltage may be from 7 to 14V, and the RF power output will depend on the supply voltage (higher output power is produced at higher supply voltages). Operation much above 5W output is not recommended and could lead to overheating and destruction of the final amplifier.
A 2.1mm DC connector plug is required; the center pin is + and the barrel is ground (negative).
Earphones
The earphones can be any stereo earphones such as commonly used with audio equipment, mobile phones and so on, with a 3.5mm stereo jack plug. These commonly have a 32-ohm impedance. Some people have noted unstable audio operation when low impedances are connected such as 4 or 8-ohm speakers; this is because the output opamp IC (IC10) is not able to supply the required power output.

QCX-mini assembly Rev 1.12

If you want to use a small loudspeaker you will need to ensure this is an “amplified speaker” because the audio output will not be strong enough to drive a speaker directly.
3) Antenna system
The RF output is a filtered 50-ohm BNC output for connection to a usual antenna system (antenna, and matching unit if applicable).
4) Straight key or paddle
To operate the QCX-mini transceiver a straight key or paddle should be connected to the appropriate jack, having a 3.5mm stereo jack plug. The shield (or main body) is ground. It does not really matter which way around the tip and ring connections are (to dit or dah of the paddle) since if they are the wrong way, there is a menu configuration item to swap them around. Similarly if using a straight key, you can select in the firmware either tip, ring or both for the connection; this allows use of a 3.5mm mono plug when using a straight key.

3.44 Notes on fault-finding for the QCX-mini

If your QCX-mini doesn’t work at all, or doesn’t work properly, don’t panic. This is a KIT and as such, often things may not go perfectly as planned and some fault-finding is necessary. The following are some tips to help.

On first power up, the QCX-mini may look dead, totally dead. No backlight on the LCD module, and no text. This is probably OK! The backlight is controlled by a menu item (in menu 7, the “Other” settings menu) and if that is OFF, then this explains why the backlight is off. Seeing no text on the display is also normal until you have adjusted the LCD contrast potentiometer R47.
You can remove the Controls board to do certain fault-finding. Since R46 (10K resistor) was thoughtfully placed on the main board, not on the plug-in controls board, this means that if you remove the Controls board, the processor will not register any phantom button presses. The radio will remain in whatever state you put it in, until you plug back in the Controls board. Removing and inserting the Controls board can be done while QCX-mini is switched on.
If you remove the Controls board however, you are also necessarily interrupting the audio signal path because you have removed R36, the gain control potentiometer. Therefore no audio signal will reach IC10 and there will be no audio output.
You can remove the Display board and the QCX-mini will continue to work just fine only you won’t be able to see anything on the screen, of course. This can be useful for accessing the main board to check signals at various points. If you plug in the LCD module again while the QCX-mini is powered, you will NOT see any sensible display until you cycle the power, because the display module requires a certain initialization sequence.
Always connect a 50-ohm dummy load during testing or investigation!
There is a thorough trouble-shooting guide here: http://qrp- labs.com/qcxmini/trouble.html it is written originally for the QCX transceiver but the circuit is the same, so it is equally applicable to the QCX-mini.
There is a trouble-shooting YouTube video see http://qrp- labs.com/qcxmini/troublevideo.html, which explains how to use low cost

QCX-mini assembly Rev 1.12

test equipment to investigate the signal path of the QCX. Again, this guide was filmed for the QCX transceiver, but since QCX and QCX-mini share the same circuit, it is equally applicable to the QCX-mini.
8) POWER OUTPUT: Often, the power output is less than you had hoped. If you have power output at all, this is a good indication that the transmitter is working properly. There is a YouTube video about tuning your QCX+ and again, this applies equally to the QCX-mini since the circuit is the same, just a different physical layout. The video explains how to adjust the toroid inductances to achieve at least 4W output with 12V supply (normally closer to 5W) on any band QCX+ 80 to 17m, and at least 5W with 13.8V supply (normally nearer to 6W).
9) https://groups.io/g/qrplabs is the QRP Labs discussion forum and is full of very helpful people who will be able to advise. Be sure to describe your problem as fully as possible since without this, any faults will be impossible to diagnose.
3.45 Adjustment and alignment
The first thing that you will notice when you apply power to the radio, is that there is probably nothing at all shown on the display. Perhaps the LCD back-light will not even be on (it depends on what configuration is currently saved), and there will be no visible signs of life at all. Don’t worry (yet).
This is because you need to adjust the contrast trimmer potentiometer R47 at the top left of the display board! Adjust it with a screwdriver until the display text looks right to you.
You should now see the following text on the display:
Select band: 160m

Turn the rotary encoder knob to select the band you have built the kit for. Then press the left button to make your selection.
If you do NOT see the “Select band” message, but instead the unit appears to be already on a particular band, then please refer to the operation manual and execute a “factory reset” then try again.
There are four more adjustments which now need to be made as part of the alignment procedure. The adjustments are:
Band-pass trimmer capacitor peaking, C1 I-Q amplitude balance, R27 Audio phase shift adjustments, R17 and R24
Until these are adjusted, the sensitivity of the radio will be very low. So, do this first, before going any further!
The location of these four adjustments is very easy to see and use, right above the pushbuttons, as shown in the following picture:

QCX-mini assembly Rev 1.12

In summary: the alignment tools built into the radio consist of a signal generator which injects a signal into the RF front end, and digital signal processing which adds a 250-Hz digital filter to the existing 200Hz analogue filter, and calculates the amplitude of the signal detected in that bandwidth. During alignment, the amplitude is displayed on-screen as an intuitive bar across the bottom row of the display. Using a screwdriver, you adjust the trimmer component in order to maximize or minimize the displayed amplitude.
Unplug the antenna during alignment of the radio! Connect a 50-ohm dummy load such as the QRP Labs dummy load kit http://qrp-labs.com/dummy
When using the TCXO module option, change menu item 8.5 Ref frq to 25,000,000. It is critical to this before any further alignment. Enter the menu system as described below, and turn to menu 8.5, edit it to 25,000,000.

QCX-mini assembly Rev 1.12

First adjust the band-pass trimmer capacitor C1. To do this, give one long press to the “Select” (left) button. The screen now displays the first menu category:
1 Preset
Turn the rotary encoder until you see the alignment menu:
8 Alignment
Now press the “Select” button, to enter the alignment menu. For example, for 17m operation, the alignment frequency menu item should already be set to a frequency in the CW section of 17m, as follows:
8.1 Align frq 18,120,020
Now turn the rotary encoder until you see:
8.7 Peak BPF Press Select!
Do as it says! But if you have earphones plugged in, please take them out of your ears first. The tone will be very loud. Press the “Select” button to switch on the signal generator so you can adjust the C1 trimmer potentiometer. Now if you have earphones plugged in (and lying hopefully, on your bench), you will hear a loud tone at 700Hz. The display will look like this photograph (QCX/QCX+ i.e. blue display; on the QCX-mini, the display is yellow):

Adjustment of the C1 trimmer capacitor should change the size of the amplitude bar. You need to adjust the C1 trimmer for MAXIMUM amplitude. There will be TWO peaks per rotation, since the capacitor has no stops; either peak is fine. When this is done, the peak of the band pass filter will be centered on the CW section of the band.

It is very important to understand the number at the top right of the LCD, here shown as 09. This is an amplitude scaling factor, expressed as a power of 2. In this example, the actual amplitude is divided by a factor of 512 (2 to the power of 9) then displayed on the screen. In

QCX-mini assembly Rev 1.12

this photograph, 27 little vertical bars are shown, which means the actual measured amplitude value is 13,824.
If the displayed bar drops below one third of the width of the LCD, then the division factor is reduced by one and the bar is re-displayed. On the other hand, if the displayed bar overflows the right edge of the screen, the division factor is increased by one. This simple method creates an auto- scaling display of the amplitude.
Therefore, to peak the band pass filter trimmer, first adjust the trimmer capacitor while looking at the division factor in the top right of the LCD. Then carry out the fine adjustment using the displayed amplitude bar. The peak is quite sharp.
Be wary because there can be more than one peak (more than one response of this simple band pass filter). So, tune the trimmer capacitor through its whole range, and determine the maximum scaling factor that you see. In my case here, it is 09. You may see 07, 08 etc, no problem. Then make the very fine adjustment necessary to peak the amplitude bar.
When you have peaked the response, check carefully that the C1 trimmer capacitor is not at either end of its range. If it is, this means the resonant circuit is NOT correctly peaked. You need to adjust the number of turns on the long secondary winding of the T1 transformer. Visual inspection will show clearly whether or not the trimmer capacitor is at the end of its range.
Perfect!
OK! The “solder blob” on the top plate is somewhere between the 4 o’clock and 8 o’clock position. The plates of the trimmer capacitor are somewhere nicely in the their range, not at the minimum or maximum capacitance. You have found the peak response of the BPF and all is well.
Plates completely closed
Here the fixed and movable sets of capacitor plates are completely meshed, resulting in the highest capacitance. The “blob” is to the right. It means MORE inductance is needed. So carefully unsolder one end of the secondary 3 winding of T1, join a piece of wire, and wind it 5 more turns through the toroid. Then try again. Don’t worry if it looks a bit messy.
Plates completely open
Here the fixed and movable sets of capacitor plates are completely unmeshed, resulting in the lowest capacitance. The “blob” is to the left at the 9 o’clock position. It means LESS inductance is needed. So carefully unsolder one end of the secondary 3 winding of T1, remove 5 more turns through the toroid, and resolder. Then try again.

QCX-mini assembly Rev 1.12

Adjustment of I-Q balance
Now turn the rotary encoder “one click” clockwise to measure I-Q balance.
Note that the previous alignment used the audio signal BEFORE the final amplification stage, so the gain control had no effect on the signal level. In contrast, the I-Q balance and audio phase shift adjustments use the audio signal AFTER the audio amplification stage. This is necessary because these alignment adjustments inject a signal into the opposite (unwanted) sideband, and the signal level is much lower, therefore it needs to be amplified for the microcontroller to be able to measure it accurately. In this case therefore, the gain control DOES now have an effect. I suggest adjusting the gain control approximately to mid-way to start with. This will provide enough gain, yet not so much gain that the amplifiers are driven into overload, which distorts the signal and measurements.
You should try to carry out the adjustments with the measured audio values in the range 5 to 10 (as indicated by the division ratio in the top right of the display). If it reaches 12, the operational amplifiers are limiting, clipping the signal which will make it difficult or impossible to make the adjustment accurately. If the displayed value (top right of the display) is only 2 or 3, that indicates the gain is too low. Therefore, adjust the volume control so that the display is something like 9.
The I-Q trimmer potentiometer is R27. It is a multi-turn trimmer potentiometer so it may need to be turned quite a few times to get to the optimum value! For this adjustment, you are looking for the MINIMUM amplitude, not the maximum we adjusted the BPF trimmer to. We adjust for minimum because now the injected signal is measuring the unwanted sideband. We want to MINIMIZE the unwanted sideband level.
Adjustment of 90-degrees audio phase shift
Similarly turn the rotary encoder one more “click” clockwise, which automatically sets the unwanted sideband audio signal to appear at 600Hz, and adjust the “low audio phase shift” trimmer potentiometer, R24. Again, adjust it for MINIMUM signal.
Turn the rotary encoder clockwise one more “click” and adjust the “high audio phase shift” trimmer potentiometer R17, again for minimum signal.
Now it is necessary to go back and forth between these three menu items for minimum unwanted sideband:
8.8 I-Q Bal (adjust R27) 8.9 Phase Lo (adjust R24) 8.10 Phase Hi (adjust R17)
This is because to some extent, these adjustments influence each other. Obtaining the optimum set of adjustments is an iterative process. So, turn the rotary encoder a click at a time anti-clockwise or clockwise, back and forth through these three menu items. Each time make small further adjustments to the appropriate trimmer potentiometer and observe the lower amplitude. Keep doing this until you see that you cannot really manage to get the unwanted sideband any lower in any of those adjustments. Pressing the “Exit” button twice leaves the menu system and returns the radio to normal operation.
Other items in the alignment menu relate to the calibration of the 27MHz reference oscillator of the synthesizer, and the 20MHz system clock oscillator of the microcontroller. These adjustments can be made manually, or by connection of a GPS module such as the QRP Labs QLG1 GPS receiver kit. However, since this calibration is a lot less urgent than

QCX-mini assembly Rev 1.12

the Band Pass Filter peaking and unwanted sideband cancellation, they are left until the description of these menu items in the operating manual.
Following the adjustment of these alignment trimmers, the radio is ready to use. A lot of settings are available in the configuration menu, and you should read the operation manual to understand and make use of all the features!
You may now bolt the two PCBs together by screwing the four M3 6mm nylon screws into the 11mm spacers which hold the two PCBs at the correct separation; and you may fit the knobs to the two rotary controls. However, if you are going to install the QCX-mini in its optional enclosure, please follow the steps in the following section.

QCX-mini assembly Rev 1.12

3.46 Installation in the optional QCX-mini enclosure
Installation in the QCX-mini enclosure is simple and requires no wiring. It is important to do the assembly in the correct sequence, as follows.

Start with the Display board…

… and the main board, with the controls board bolted in position.

Peel off the plastic protective coating from Here’s the top half of the enclosure. Note the

the LCD module.

PCB guide rails in the extrusion walls.

Slide the display board into position along the PCB guide rails in the enclosure.

Bolt the right-hand side panel to the main PCB using the supplied BNC washer and nut

QCX-mini assembly Rev 1.12

Place the front of the enclosure face down on the bench as shown, and prepare to attach the main board.

Align the 2×5-pin header connector between the main and display boards; some wriggling will be needed to get the controls to fit through the holes in the front panel.

Fit four M3 6mm nylon screws in the positions shown

This is how it looks from the DC connector end

Determine correct orientation of the bottom half of the enclosure; note the tongue-andgroove arrangement which means that the bottom half only fits one way round! Make sure you have the correct way.

Now bolt the left-hand side panel to the enclosure extruded top and bottom halves using four of the supplied small black countersunk screws in the panel corners. The screws need to be carefully aligned and should screw in easily (if properly aligned).

QCX-mini assembly Rev 1.12

Screw in the other four black screws in the corners of the right-hand side panel.

Apply the supplied four self-adhesive feet to suitable positions on the base of the enclosure if desired (optional).

Install the supplied knobs by tightening their grub screws. Leave a small gap between each knob and the front panel, to allow the knob to turn easily. The grub screw of the volume control knob should be at the position of the flat on the shaft. The grub screw position on the rotary encoder is not important.

Plug in the power… SUCCESS! (hopefully)

Disassembly of the QCX-mini should follow similar steps, in reverse.

QCX-mini assembly Rev 1.12

3.47 QCX-mini GPS interface and PTT output
The picture below shows the connectors on the main QCX-mini PCB.

GPS interface
The QCX-mini has a GPS interface which can be used to:
· Calibrate the reference oscillator (27MHz crystal or 25MHz TCXO module option) and the 20MHz system oscillator
· Keep the oscillators disciplined and drift-free (frequency and time) during WSPR beacon operation
· Set the internal Real Time Clock, which is critical for WSPR options and may be displayed on-screen if you have configured it.
The GPS produces two output signals, PPS (Pulse-per-second) and RxD (Serial data), in addition to ground. Optionally +5V may also be connected, in order to power the GPS module. A GPS such as the QRP Labs QLG1 is perfect for this see http://qrp-labs.com/qlg1
The GPS signals (PPS and RxD) use the same microcontroller pins as the paddle Dah and Dit respectively. Therefore you cannot use the Paddle and GPS at the same time. In fact, you should only connect the GPS during the calibration of the synthesizer reference frequency, system clock, and while operating the QCX-mini as a beacon (CW, FSKCW or WSPR).

QCX-mini assembly Rev 1.12

The following diagram shows the connections.

PTT output
The PTT output is at the ring of the 5V/PTT connector jack. This signal is 0V when the QCX-mini is in Receive mode, and +5V when the QCX-mini is in Transmit mode. If you are connecting the QCX-mini to the companion 50W PA kit, then this signal has to be connected to the 50W PA kit to cause it to switch to Transmit mode. A standard 3.5mm stereo audio cable can be used (having a 3.5mm stereo jack plug on each end).
Note that the pinout of the 5V/PTT connector is not the same as the Rev1/2 QCX+ PCBs. In the QCX+, ring is +5V and tip is PTT. This has the disadvantage that plugging in a cable while the QCX+ is powered, can short the +5V to ground. Another disadvantage is that standard 3.5mm stereo audio cables can’t be used. For this reason, complete compatibility was broken in this instance. If you wish to use a 50W PA kit with either QCX+ or QCX-mini, the QCX+ manual contains details of the very simple modification needed to swap the tip and ring connections in the QCX+.
3.48 QCX-mini CAT port
The QCX-mini CAT port allows a PC or other CAT-enabled host to control every aspect of the QCX-mini. Operation of this feature is detailed in the operating manual. The connection diagram below shows the connections to the 3.5mm stereo jack socket connector on the rear panel of the QCX-mini.

QCX-mini assembly Rev 1.12

4. Circuit design of the QCX-mini 4.1 Block diagram and summary

This CW transceiver is a high performance, yet simple and low cost, analogue design. The transmitter uses a high efficiency Class-E amplifier which results in low current draw on transmit, and inexpensive transistors with little or no heatsinks.
The receiver is a direct conversion type utilizing the famous high performance Quadrature Sampling Detector, also known sometimes as the “Tayloe Detector” or even “I-Q Mixer”. This receiver front end architecture has been used in the early Flex Software Defined Radios, Softrock series, Norcal NC2030 and many other SDR’s and other high performance front ends. The detector has very high third order intercept (IP3) and dynamic range, as well as low loss.
The resulting I & Q outputs are at audio baseband and go through a 90-degree phase shift network which cancels the unwanted sideband. A 200Hz bandwidth CW filter is followed by more amplification and drives common earphones.
The oscillators in the transceiver are provided by the modern Si5351A digital phase locked loop IC controlled by the microcontroller.
Permeating the entire design is microprocessor control by the ATmega328P microcontroller. This allows implementation of a large number of functions normally only found in radios costing 10-100x the price!
A really nice feature of the design is the built-in alignment and test equipment, which make it possible to build, align and even debug the assembly of the radio, all with NO additional test equipment.
4.2 Circuit diagram
A bit small to read but a larger resolution version is available on the web page http://qrplabs.com/qcxmini, and anyway the explanation will be in smaller circuit blocks.

QCX-mini assembly Rev 1.12

The circuit diagram (schematic) of the main PCB is shown on the previous page. The circuit diagram of the display PCB is shown below.

The circuit diagram of the uSDX daughterboard is shown below. Refer to the uSDX groups.io forum https://groups.io/g/ucx for details of this modification.

QCX-mini assembly Rev 1.12

4.3 Synthesized oscillator

I always start with building the VFO of a radio. It was the hardest thing to get right. How to get that analogue LC-tuned VFO accurate, free of drift, free of chirp, tuning over the required range, and with mechanical gearing to be able to make fine frequency adjustments? A real challenge. Not anymore! Now we have Direct Digital Synthesis (DDS) ICs and Digital Phase Locked Loop (PLL) ICs, inexpensive and easy to use, that solve all the problems.

The Si5351A Synthesizer chip used in this design provides three separate frequency outputs, with a frequency range spanning 3.5kHz to 200MHz. The frequency stability is governed by the 27MHz crystal reference. Pretty stable, in other words.

The block diagram (right) is taken from the SiLabs Si5351A datasheet. Briefly, the 27MHz reference oscillator is multiplied up to an internal Voltage Controlled Oscillator in the range 600-900MHz (the PLL), then divided down to produce the final output frequency. The multiplication up and the division down are both fractional and so the frequency resolution is extremely finely controlled. The chip has two PLLs and three output divider units.

For best jitter performance, the Si5351A datasheet recommends the use of even integer dividers (no fractional component) in the MultiSynth dividers and in this CW transceiver design, this recommendation is followed.

The synthesizer section of the circuit diagram is shown here (right). The Si5351A datasheet dictates the use of a 25 or 27MHz crystal. QRP Labs has always used the 27MHz crystal in our designs because it allowed us to obtain precise 1.46Hz tone spacing for WSPR transmissions all the way up to the 2m amateur band (145MHz). Those calculations don’t work out with the 25MHz crystal. This requirement doesn’t apply to this CW transceiver design but economies of scale means there are advantages to sticking with the same component values, all other things being equal!

The Si5351A has a large number of internal 8-bit registers to control the synthesizer behaviour, and these are programmed by the microcontroller using the I2C serial protocol. 1K resistors R3 and R4 are pull-ups required for the operation of the bus at 400kHz.

The Si5351A chip requires a 3.0 to 3.6V supply (nominally 3.3V) but the rest of this transceiver’s digital circuits operate with a 5V supply. For the reduction of complexity and costs, two 1N4148 diodes in series are used here to drop the 5V to a suitable voltage for the Si5351A. It works well.

There are three outputs of the Si5351A synthesizer and these are all used to good advantage. The Clk2 output is used to feed the transmit power amplifier, and the Clk0/1 outputs are used to drive the Quadrature Sampling Detector (QSD) during receive. These

QCX-mini assembly Rev 1.12

outputs can be switched on and off under the command of the microcontroller. This provides an opportunity for some simplification because the Clk0/1 outputs can be simply switched off entirely during transmit. This relieves pressure on the transmit/receive switch. There just cannot be any reception during transmit because there is no oscillator input to the receive mixer. Conversely, the Clk2 output is switched off during receive.
A feature of the Quadrature Sampling Detector is that either the RF input, or the LO input, must provide two paths in 90-degree quadrature. This is normally applied at the Local Oscillator where it can be easily controlled for best performance. So, two oscillator signals are required, with the same frequencies but a precise 90-degree phase offset. Generating this quadrature Local Oscillator signal is always difficult. Analogue phase shift circuits have limited accuracy. Often a divide-by-4 circuit is used, to produce quadrature oscillator outputs from an oscillator input at 4x the reception frequency. This also creates challenges particularly as you try to increase the reception frequency to cover higher bands. For example, on 10m e.g. 30MHz, a local oscillator at 120MHz is required and the divide-by-4 circuit must be able to operate at such a high frequency. Devices such as the 74AC74 can do so, but pushing it higher into the 6m band cannot be done with the 74AC74.
The Si5351A has a phase offset feature, which is not really very clearly described in the SiLabs documentation. However, QRP Labs has perfected the technique to put two of the Si5351A outputs into precise 90-degree quadrature, which is maintained without tuning glitches as the frequency is altered. It’s a nice development because it eliminates one more circuit block (the 74AC74 divide-by-4 circuit), again reducing complexity and cost. To the best of my knowledge this is the first time the Si5351A has been implemented in a product directly driving a QSD with two outputs in quadrature (no divide-by-4 circuit).
4.4 Transmit/Receive switch
Since the receiver is entirely disabled during transmit, because of the absence of any local oscillator signals to the Quadrature Sampling Detector, the demands on the transmit/receive switch are considerably reduced. Now the circuit does not have to provide the massive amount of attenuation necessary to prevent the transmitter from overloading the receive circuits. All it has to do is provide a reasonable amount of attenuation, enough to stop the 5W signal (45V peak-peak) from damaging the receiver input mixer.
The transmit/receive switch is implemented by a single BSS123 MOSFET. The source is at DC ground (via the primary of input transformer T1). The control signal from the microcontroller switches the MOSFET on or off. Interestingly, capacitor C34 close to the MOSFET gate is found to be necessary to prevent inductive pickup of the 5W RF from partially switching on the MOSFET.
The switch wouldn’t provide enough attenuation to mute an operating receiver; but during transmit, our receiver isn’t operating; all the switch has to do is protect the Quadrature Sampling Detector from seeing 45V peak-peak which would destroy it.

QCX-mini assembly Rev 1.12

4.5 Band Pass, Phase Splitter, QSD and pre-amps
Since the band-pass filter, Phase splitter, Quadrature Sampling Detector and pre-amp circuits are so tied up together, I am going to consider them all together in this section.

This circuit implements an input band-pass filter and double-balanced Quadrature Sampling Detector with low-noise pre-amps. Yet it does this with a low parts count, and resulting low complexity and cost. The FST3253 is a dual 1:4 multiplexer which is often seen in QSD circuits. It has fast switching times and very low on resistance of only a few ohms. The input signal is switched by the quadrature LO to each of the four integrating capacitors C43-C46 in turn, for 90-degrees of the RF cycle each. The result is that the audio difference (beat) between the RF input and LO input appears across each of the four integrating capacitors, with four phases at 0, 90, 180 and 270 degrees.

The operational amplifier IC5a takes the difference of the 0 and 180-degree outputs and amplifies it, resulting in the I output of the QSD. Similarly, IC5b differences the 90 and 270degree outputs to produce the Q output.

The combination of the relatively large 470nF capacitors and the low source resistance results in a fast roll-off of the audio response. This is effectively a very narrow band pass filter since any incoming RF more than a few kHz away from the LO frequency is greatly attenuated. The QSD is therefore inherently a very high performance mixer design with high third order intercept and dynamic range, and low loss (0.9dB).

The FST3253 dual switch is often connected with the two switches simply paralleled together (which does half the switch ON resistance). But I prefer the double-balanced mixer configuration which provides higher performance. The double-balanced configuration requires two RF inputs 180-degrees out of phase (opposite to each other).

Despite the high IP3 and dynamic range, it is still prudent to provide some input band pass filtering to protect the mixer from strong out of band signals. In this CW transceiver design, the T1 transformer provides a simple solution to all of these problems with a very low parts count.

The primary couples the incoming RF into the two secondary windings which feed the double-balanced detector. One end of the primary is grounded which neatly keeps the DC potential of the input at ground, so the transmit/receive switch is easily implemented by a

QCX-mini assembly Rev 1.12

single MOSFET (see previous section). The two secondaries are connected as a centertapped single winding, which means that the outputs have 180-degree phase difference as required.
The secondary “center-tap” is connected to a DC bias formed by R1, R2 and C6 at mid-rail i.e. 2.5V. This simple bias does not source or sink any significant current due to the balanced nature of the system, therefore no buffering is required. The DC bias feeds through the pre-amps, and into later stages including the 90-degree phase shift network and the first three op- amps of the CW filter. It is a great benefit not to have to AC couple each stage with coupling capacitors, and then bias each stage individually. In this circuit, the same DC bias flows through all the way from this center-tapped input transformer. This reduces component count and – you guessed it complexity and cost. Another benefit is that since much of the receiver signal path is DC coupled, it might be easier to reduce the inevitable “thumps” on switching between receive and transmit.
Finally, the band pass filter is implemented by a fourth winding on the same transformer T1, with some fixed capacitors and a trimmer capacitor forming the resonant circuit. It is only a single resonant circuit band pass filter so has limited stop-band attenuation, but it does have the benefit of low parts count, and simplicity of adjustment due to the single adjustment control.
Note that the I and Q outputs each have a 0-ohm resistor (R246 and R245 respectively); removal of these resistors disconnects the I & Q signals from the rest of the audio processing; separate I & Q pads allow the signal to be routed to the uSDX daughtercard if required.
4.6 90-degree audio phase shift
By this stage the I and Q outputs are each double-sideband, and we need to process them to demodulate single sideband.
The circuit used here is an active two-path all-pass phase shift network based on four operational amplifiers. The circuit is based on the same phase shift block as the Norcal NC2030 http://www.norcalqrp.org/nc2030.htm
In the real world, nothing is perfect there are component tolerances to think about. The unwanted sideband suppression is maximized when the amplitude of the two paths is equal, and the 90-degree phase shift is accurate.

QCX-mini assembly Rev 1.12

To improve the accuracy of the 90-degree phase shift, R17 and R24 allow adjustment of the phase shift at higher and lower audio frequencies respectively.
R27 allows adjustment of the balance between the I and Q channels, to equalize the amplitude from each path.
This CW transceiver kit includes built in alignment and test equipment, with a signal generator that can inject a test signal into the receiver input. It makes it easy to perform these adjustments, as described previously.
Note the 2-pin header pads JP9, JP10 and JP11 at the output combination potentiometer which allow experimental modifications or connections.
4.7 CW filter
The CW filter used in this receiver has a 200Hz bandwidth. The circuit is based on the HIPER-MITE CW filter kit design by David Cripe NM0S, available from the Four State QRP group: http://www.4sqrp.com/HiPerMite.php (thanks David for permission to use it here). This is a high performance circuit specifically designed to avoid objectionable ringing.

There are three stages of low-pass filtering and one stage of high-pass filtering. The first three stages retain the 2.5V “midrail” bias all the way through from the input transformer T1. The final stage IC9A is biased using the 5V supply (avoiding a few extra components to create a real 6V mid-rail at half the supply). The CW filter also provides a measured 18dB of gain. Sidetone is injected at the input to the CW filter, so as to make is sound nice and clean by cutting off the squarewave harmonics leaving a clean 700Hz sidetone.
4.8 Audio amplifier
The final stage in the receiver signal path is the audio amplifier, to drive earphones at a comfortable listening level.

QCX-mini assembly Rev 1.12

There is a 5K potentiometer on the controls board, used as the gain control. With the wiper fully clockwise the receiver is at full volume. As the potentiometer is turned anticlockwise it forms a potential divider which attenuates the audio signal from the CW filter output. This potentiometer is located on the controls PCB and is not shown on this diagram section (below).

There is also a TX mute switch formed by Q7, another BSS123 MOSFET. This was a late addition to the design: despite all attempts, I could not remove the nasty click on receive/transmit switching. The mute switch helps to attenuate it. The switch is operated by the microcontroller Receive/Transmit switch output. When the BSS123 switch is on, it has a low resistance to ground which greatly attenuates the audio signal.
To reduce the audio “thump” when the transceiver is switched from transmit back into receive, the mute switch needs to remain switched on for a short while after the receiver is switched back on. A small wait while the thump subsides. This delay is achieved by the R-C network formed by R60 and C52. This would also introduce a delayed switch-on of the mute switch, which would allow the thump when switching to transmit to be heard. To prevent this, diode D5 was added, which bypasses the resistor R60 at the receive-totransmit switchover. It ensures that at the receive-to-transmit event, the mute switch is enabled instantly; but on the transmit-to-receive switchover there is a short delay.
C21 and C22 were originally 10uF electrolytic capacitors in the early QCX PCB revisions. Some constructors experimented and found that if these capacitors are reduced to 0.1uF the residual Transmit/Receive switchover click is even further attenuated. However, 0.1uF also reduces the gain of the receiver chain by 14dB; although overall the receiver has quite high gain, a loss of 14dB may still be too much particularly on higher frequency bands where the noise levels are lower. Therefore, the current kit is supplied with 1uF capacitors, which provide the click attenuation but have negligible effect on the gain.
IC10B is a simple amplifier configured for 41dB of gain. The ½-V mid-rail bias is created by R39, R40 and C24. Using the 5V power line as “mid-rail” was found to add too much noise. Finally, IC10A is a simple unity-gain buffer. Although it is just an op-amp it is found to be perfectly adequate for driving standard earphones.
The effect of the 5K LINEAR (not Log) volume control and the 1K load resistance of the IC10b input is to create an overall logarithmic characteristic. If you would like a more

QCX-mini assembly Rev 1.12

aggressive logarithmic characteristic or further analysis and discussion on this topic please see http://www.qrp-labs.com/qcx/qcxmods/qcxvolume.html
4.9 Transmit signal routing and PA driver

The 74ACT00 is a quad NAND logic gate. The input threshold voltage for a binary “1” is 2.4V which means that the gate is easily switched on by the ~3.3V peak-peak squarewave output from the Si5351A. The output of the 74ACT00 is 5V peak-peak, perfect for driving the BS170 MOSFETs in the Class-E PA into saturation.
The Clk2 signal from the Si5351A is used as the transmit oscillator as previously mentioned. It would have been easy to enable/disable the Clk2 output in software in the Si5351A chip configuration. However, this transceiver design also includes the built-in signal generator feature, for aligning the Band Pass Filter and adjusting the I-Q balance and phase adjustment controls. So, some of the spare gates in the 74ACT00 are used to switch the signal generator on/off and the RF Power Amplifier (PA) signal on/off, separately.
When the SIG OUT control line from the microcontroller is high, the Clk2 signal is enabled as signal generator, and routed via a 120K resistor straight to the RF input of the transceiver.
A TX signal is produced using IC3D as a plain inverter, to invert the logic level “RX” output from the microcontroller and produce a “TX” signal. This TX signal is used elsewhere in the circuit also (audio muting during TX). R36 pulls the RX signal high during the part of a second at power-up that the microcontroller is booting up and has not yet enabled or switched the RX signal high.
When the TX signal is high, the Clk2 signal is routed to the PA. The final inverting gate IC3A is added to make sure that when the TX gate IC3B is off, the driver voltage presented to the BS170 gate is low, so the transistors are off.

QCX-mini assembly Rev 1.12

4.10 Class-E Power Amplifier
A Class-E power amplifier is a wonderful thing. It has a very high efficiency, sometimes over 90%. This has several important benefits:
a) Since not much power is dissipated, we can use smaller (and cheaper) transistors
b) So little power is wasted as heat that the requirement for a heatsink is reduced or eliminated
c) During transmit the radio requires less current, so the drain on a battery is less important for people who want to operate portable.
A Class-E Power Amplifier contains a resonant circuit at the frequency of operation, so it is only suitable for single-band use. A lot has been written about Class-E, much of it is very technical and mathematical.
Some excellent background reading are two papers by Paul Harden NA5N:
http://www.aoc.nrao.edu/~pharden/hobby/_ClassDEF1.pdf and http://www.aoc.nrao.edu/~pharden/hobby/_ClassDEF2.pdf
Paul NA5N describes two defining features of Class-E:

Use of a square-wave drive to reduce switching losses: the transistors are either on, or off… no lossy region in between
Reducing the effects of the transistor capacitances. Class-E has a resonant tuned circuit. The capacitance of the transistors, normally an unpleasant lossy aspect, is now a part of the tuned circuit.
Class-E also has a reputation for being difficult to achieve. All those intense mathematics Google will help you find, don’t help. In reality, once you realize the secret it is not so difficult. Calculation of the impedance of a resonant circuit is simple, and there are many online calculators which will do the job for you. For example, http://toroids.info/T50-2.php which allows you to type in the operating frequency, and the desired resonant circuit impedance. Then the calculator computes the required inductance, capacitance, and the number of turns required for a certain toroid (in our case we use a T37-2).
The Class-E design process is simple. Choose the output impedance. We choose 50-ohms, because this is the input impedance of the Low Pass Filter we will use. The online calculator will tell you what inductance is needed, and how many turns to wind on the toroid. The online calculator also tells you the required capacitance to bring it to resonance at the operating frequency. Here we resort to experiment, because it is a little difficult to know what the output capacitance of the transistor is. The device capacitance varies depending on supply voltage and whether it is on or off. A simple experiment is required,

QCX-mini assembly Rev 1.12

adding different small capacitances to the circuit, and measuring the efficiency (measure supply voltage and supply current to calculate power input; then measure RF power output. Divide one by the other to get the efficiency). It is easy to find what additional capacitance is required to peak the efficiency. The resonance is quite broad and non-critical.
In this implementation, three BS170 transistors are used in parallel. The BS170 is inexpensive and small, but is rated for 500mA drain current and up to 830mW of dissipation. Per device. Three in parallel provides plenty of capability to achieve a 5W output on a single band.
There are always minor variations between device characteristics from one transistor to the next. If these were bipolar NPN transistors, we would not be able to parallel them in this way. If one transistor takes more of the load and starts to heat up, its resistance further decreases and this causes it to get even hotter. This process is known as “thermal runaway” and results (quickly) in destruction of the transistor. Emitter resistors are used to help balance the load. But with MOSFETs, their resistance INCREASES as the temperature goes up so there is an inherent selfbalancing when multiple devices are used in parallel, without any need for additional balancing resistors which would increase component count and waste some power.
This oscilloscope screenshot shows the classic Class-E waveform. Please ignore the ringing due to poor set-up of the `scope probes etc. The lower (blue) trace is the 5V squarewave at the gate of the BS170 transistors. The upper (red) trace is the voltage at the BS170 drain. It peaks at approximately 40V in this example. This measurement was done with 12V supply and on 40m (7MHz).
The important point to note is that when the BS170 are switched ON (the gate voltage is 5V), the drain voltage is zero. When the BS170 is OFF the drain voltage pulses nicely to a large amplitude. Class-E!
The summary: Class-E is actually quite easy to achieve in practice! Perhaps all the complicated mathematics might help to squeeze out another % or two of efficiency. But for practical purposes, it’s a wonderful building block to use in a single-band CW transceiver.

4.11 Low Pass Filter
The transmitter output is rich in harmonics and must be followed by a good Low Pass Filter, to attenuate the harmonics and satisfy regulatory compliance.
QCX-mini assembly Rev 1.12

The standard, well-proven QRP Labs Low Pass Filter kit http://qrp- labs.com/lpfkit is used here. To save space and cost, the components are installed directly on the PCB, not on a plug-in board.
It is a 7-element filter design originally by Ed W3NQN then published for many years on the G-QRP Club web site’s technical pages.
4.12 Key-shaping circuit
A hard-keyed CW transmitter generates clicks many hundreds of Hz away from the transmitted signal that can annoy users of adjacent frequencies. This is purely a consequence of the mathematics of the Fourier transform and is unavoidable. Any time you switch a signal instantly on or off, you WILL splatter energy onto unwanted nearby frequencies.
To combat this, any good CW transmitter should include an RF envelope shaping circuit to soften the key-down and key-up transitions. The ideal envelope shape is a raised cosine, but this is difficult to implement without significantly increasing the complexity of the circuit.
The simple key-shaping circuit used here uses only a few components but produces good results.
This circuit was derived from one published by Don Huff W6JL, see https://www.qrz.com/db/W6JL/ though as he says, “this integrator-type keying circuit is found in many published homebrew designs over the past 40 years or so, so it is nothing new”. It uses a PNP transistor (Q6) and R-C integrator circuit. Don W6JL uses this keyshaping circuit to drive a 600W Power Amplifier.
On key down the Q4 switch is “closed” by a high signal coming from the microcontroller. In a really simple transmitter, Q4 could just be replaced by a straight Morse key to ground! But in our case, the microcontroller implements automated stored message sending, beacon modes, and Iambic keyer so we need the microcontroller to be the boss of everything. The microcontroller reads the state of the straight key or paddle, and processes it to produce a key output. When in straight key mode the microcontroller transfers the signal straight through from the key input, to the key output control line but in other modes the processor must generate the keying signal.
The component values set the rise and fall time. With the components shown, the rise and fall time is about 5 milliseconds.
The following oscilloscope screenshots show a 40m band (7MHz) transmission, keyed with a continuous series of CW dits at approximately 24 words per minute. The amplitude is approximately 3.8W into a 50-ohm dummy load (with 12V power supply).

QCX-mini assembly Rev 1.12

4.13 Microcontroller
The ATmega328 microcontroller circuit controls many aspects of this transceiver. Below is this section of the transceiver circuit. Several points are worthy of discussion.

ATmega328P processor
The ATmega328P was chosen because it has enough processing power and I/O to handle all the tasks required here. It is also common and inexpensive, and lots of QRP Labs products already used it, bringing economies of scale in both the kit preparation and the coding. The processor is operated at its maximum rated 20MHz system clock speed.
The code is all written in C and is not open source. While the same ATmega328 processor is used in the popular Arduino Uno products, there is no relation between code written for the Arduino environment and the custom code written for this CW transceiver.

QCX-mini assembly Rev 1.12

Elimination of tuning clicks
Some constructors of radio receiver projects that use the Si5351A report loud clicks every time the frequency is changed. The cause of these clicks is one or both of two underlying issues:
a) Faults in the software configuring the Si5351A b) Power line or radiated noise from the microcontroller/LCD back into the sensitive
receiver
The first of these is not an issue here since we have already extensive experience using the Si5351A and have perfected its configuration.
The second issue is important to address. Every time the microcontroller updates the Si5351A configuration to cause it to change frequency, it typically also writes the new frequency to the LCD. There is a burst of activity in the microcontroller, and on the digital control signals to both the Si5351A and the LCD. The LCD controller chip will also be doing some work to effect the changed display. All of these digital transitions can radiate noise into the receiver front end. Changes in power consumption cause noise on the supply lines which can also be converted into noise detected in the receiver front end.
To combat the radiation issue, the ATmega328P microcontroller is sited away from the RF and audio signal paths, right at the front of the PCB so that the connections between the processor and LCD module and the microcontroller are kept as short as possible, to minimize radiated noise.
To keep noise out of the supply, the 5V supply to the microcontroller and LCD module is filtered by 47uH inductor L6 and 470uF capacitor C47.
In combination, these measures ensure that there are no “clicks” in the audio when tuning the receiver; just a small “flutter” as I call it, which is a natural consequence of the sudden change in frequency (Fourier rules).
Liquid Crystal Display module
The transceiver uses an HD44780-compatible LCD Module with 16 characters by 2 rows. The LCD is operated in the 4-bit mode in order to minimize the I/O pins used. No data is read back from the LCD which means the Read/Write pin can be grounded. In total only 6 I/ O pins are used for writing to the LCD.
A yellow/green LCD module is used in the QCX-mini kit because even with the backlight switched off, this type of LCD are still perfectly viewable in ordinary lighting conditions, and even in direct glaring bright sunlight.
The LCD is mounted on the display PCB and the circuit section (below right) is a part of the display PCB.
The usual contrast adjustment trimmer potentiometer is R47 and must be set to obtain a readable display.

QCX-mini assembly Rev 1.12

The LCD back-light consumes about 15mA of current with a 560-ohms series resistor (at 12V supply). A 270-ohm series resistor was used in the QCX/QCX+ and resulted in 3035mA current consumption. The larger resistance value was chosen here because the backlight brightness really does not need to be extreme, and for portable operations on battery power, minimizing current consumption is more important.
The back-light could be connected directly to the 5V supply but this would somewhat increase the power dissipation of the 5V voltage regulator. In order to avoid overheating the regulator, this back-light is powered instead directly from the +12V rail via R48, a 560-ohm resistor.
A different resistor value could be installed for the LCD backlight series resistor; large pads are provided on the PCB for this purpose (facilitating easy installation of a standard through-hole ¼W resistor).
Unlike its predecessors the QCX and QCX+, the QCX-mini transceiver has a means to switch on or off the backlight under firmware control, via the configuration menu. This is achieved using LCD data pin 7. Whilst this pin is used for communication from the microcontroller to the LCD module, this is only a brief data burst lasting a few microseconds. The rest of the time, the pin can be left in a high or low state, under processor control.
MOSFET Q100 is used as a switch, in series with the backlight LCD. It is switched ON by +5V on the gate pin. However, to avoid potential RF noise getting back into the sensitive QCX-mini receiver circuits, capacitor C100 and resistor R100 form an integrator, the effect is to filter out the burst of data that occurs when the microcontroller writes to the LCD. The time constant is 0.26 seconds. Its very slow, plenty slow enough for the data burst to be totally ignored by the LCD, and therefore no RF interference is generated.

Sidetone
In the early firmware versions of this transceiver, the sidetone was generated by Pulse Width Modulation using the ATmega328’s Timer1 peripheral. The frequency and volume of the sidetone were configurable in the software via the configuration menu. In order to control the volume, the microcontroller adjusted the duty cycle from 50% (maximum volume) down to under 1% (for minimum volume).
In firmware version > 1.02 and above, the sidetone generation method was changed. The former method was a simple squarewave, with variable duty cycle, in order to change the volume. However, this also changed the average level, leading to a DC bias at low volume levels; on switching from Transmit to Receive (and indeed, vice versa), the DC bias through the audio chain, being suddenly restored the nominal 2.5V, generated a large click.

QCX-mini a

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QRP QCX-mini 5W CW Transceiver kit Instruction Manual

QRP QCX-mini 5W CW Transceiver kit

Product Information: QCX-mini CW Transceiver

Features

Product Usage Instructions: QCX-mini CW Transceiver

Introduction

Parts list

Assembly general guidelines

PRECISION

Install LCD module

References

Read User Manual Online (PDF format)

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QRP QCX-mini 5W CW Transceiver kit

Product Information: QCX-mini CW Transceiver

Features

Product Usage Instructions: QCX-mini CW Transceiver

Introduction

Parts list

Assembly ­ general guidelines

PRECISION

Install LCD module

References

Read User Manual Online (PDF format)

Download This Manual (PDF format)

QRP User Manuals

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Assembly general guidelines