QRP QCX-Series AGC Module Instruction Manual
- June 6, 2024
- QRP
Table of Contents
QCX AGC module
QCX-series AGC module installation instructions
Introduction
QCX-series transceivers (QCX, QCX+, and QCX-mini) do not have built-in
Automatic Gain Control (AGC).
This small 19 x 16mm ready-assembled AGC module is based on the proposal by
Jim AJ8S in this thread on the QRP Labs groups.io forum:
https://groups.io/g/QRPLabs/topic/a_simple_qcx_agc/80821592 The shape of the
AGC module is specifically designed to fit snugly into the QCX-mini, where it
should sit on top of IC8 and IC9, a few mm above the main PCB. The perimeter
of the AGC board is designed to fit between the surrounding through-hole
components on the QCX-mini. On the QCX and QCX+, there is much more space
available so the installation position is less critical.
It’s not the most perfect performant AGC circuit in the world. But it works
very well for a very small number of components and avoids any modifications
to the QCX apart from making the four (or five) connections. The circuit
provides a very useful degree of AGC and will protect you from sudden very
loud signals.
Additionally, a few additional components have been added such that a control
signal can switch on/off the AGC. In QCX firmware version 1.08 and above,
there is a menu item in the “Other” menu, which allows you to switch AGC
On/Off. This requires a connection from LCD Pin D6 to the “AGC” control input
of the AGC module. If you prefer to keep the AGC always on, there are a pair
of pads on the PCB that you can short with a solder blob, a small wire, or an
0603 zero-ohm resistor.
The AGC module can be installed in ANY of the QCX-series transceivers. This
document includes instructions for each model.
On a QCX-mini Rev 3 (and above) PCB, there are 5 holes on the main QCX-mini
PCB that line up exactly with the 5 holes on the AGC module. Installation is
therefore exceptionally easy using 5 component lead off-cuts to make the
connection between the two boards, and no other changes.
On other QCX-series models, all of which essentially share the same schematic,
the principles of circuit operation are the same, but you will need to install
wires to make the connections.
Design details
2.1 Schematic and circuit explanation
The AGC module schematic is shown above.
The “AF OUT” connection at the top left is connected to IC10 pin 1, which is
the output of the QCX receiver. This audio signal is rectified and applied to
the gate of MOSFET Q202. The drain of this MOSFET is the module’s “MUTE”
connection which is connected to the wiper of the QCX volume control. As the
amplitude of the audio increases, the gate voltage rises, switching on the
MOSFET. The MOSFET’s resistance in parallel with the lower part of the volume
control potentiometer, therefore, reduces the volume of the receiver.
Trimmer potentiometer R202 provides an adjustment facility so that you can
choose a level of AGC action that is most comfortable to you. It applies a
bias voltage to the rectified audio. The more positive the bias voltage is,
the more sensitive the AGC circuit becomes; since it will require a smaller
amount of audio to switch on the MOSFET.
Note that the MOSFET action is not an ON/OFF switch. There’s a range of
operations where the gate voltage gradually lowers the resistance of the
MOSFET.
Transistor Q201 provides a way for the control input named “AGC” to switch the
AGC on or off. When a voltage (+5V) is applied to the transistor gate, the
transistor turns ON, and the Q202 gate voltage is pulled to the ground through
Q201, causing the muting MOSFET Q202 to be switched OFF completely.
This signal is provided from the QCX signal LCD_D6. During write operations to
the LCD, the data on this signal would cause transistor Q201 to switch on and
off rapidly and this could potentially be audible in the earphones. To avoid
this, R205 (10K) and C204 (2.2uF) form an integrator with a time constant much
longer than the micro-second data pulses when the processor talks to the LCD.
Resistor R204 is not fitted on the board. The pads of this resistor can be
shorted together if you want to keep the AGC circuit always active. In this
case, you would not need to connect the “AGC” signal to the QCX signal LCD_D6
(though if you do, that will not harm anything either).
2.2 Board layout
The diagram below shows the part’s layout. The five connections to QCX are the
header pads colored yellow. The diagram also highlights in red, the position
of the un-installed resistor R204; if you want the AGC circuit to be always
active, you simply short this resistor, either with a solder blob, a small
wire, or a zero-ohm 0603 sized resistor. In this case, you do not need to
connect the “AGC” signal at the top left corner of the board.
2.3 Performance
The graph below shows the measured performance of the circuit. Input audio
amplitude is shown on the horizontal axis, and output audio amplitude on the
vertical axis.
The various curves are shown in the legend with a number such as “AGC 1.25”.
This number is the voltage measured at the wiper of the adjustment
potentiometer.
The top line (dark blue) is a standard unmodified QCX+. The next line (red)
shows the AGC action when the adjustment is all the way clockwise (0V bias,
lowest sensitivity). As the bias voltage is increased by turning the
adjustment potentiometer more and more anticlockwise, the AGC starts acting at
lower audio levels (becoming more sensitive).
Next, the “AGC” control signal was connected to the QCX signal LCD_D6. The
pale green line “AGC 1.25 ON” (control signal is zero volts) shows the
resulting response curve and exactly overlaps the pale blue line “AGC 1.25″
before the connection was made. The black line `AGC 1.25 OFF” (control signal
is +5V) shows the resulting curve when the QCX menu switches off the AGC
circuit it overlaps the “standard unmodified QCX+” case, as you would
expect. This validates the correct operation of the control signal switching
the AGC on/off.
Note that AGC On/Off control form the QCX menu requires firmware 1.08 or
higher.
2.4 Adjustment
Adjustment of the sensitivity of the AGC action is made using the onboard
trimmer potentiometer R202. Be very careful to use the correct sized
screwdriver and do not force it: the potentiometer is a small SMD device and
is quite delicate. The least sensitive AGC action is fully clockwise, and the
most sensitive is fully anticlockwise. Take great care when adjusting this
potentiometer which is small and delicate.
One way to adjust the AGC is:
- Ensure AGC is switched ON in the menu (or you have shorted R204).
- Turn the adjustment potentiometer to its clockwise limit (least sensitive)
- Tune QCX to an empty to tune to an empty part of the band (no received signals),
- Turn the adjustment potentiometer slowly anticlockwise until you just about hear a slight reduction in volume.
Even so, you will certainly wish to experiment with the adjustment according to your preference.
Installation
Installation instructions for this AGC module vary depending on which model
and which PCB revision of QCX you have. Assuming that you know the model (QCX,
QCX+ or QCXmini), the PCB revision is written somewhere prominently on the PCB
silkscreen.
3.1 Installation in QCX-mini PCB Rev 3 and above
The five connections of the AGC PCB are connected to schematic points as shown
below.
The AGC board fits snuggly in the area of the PCB above IC8 and IC9. The AGC
PCB shape is designed to fit between the surrounding through-hole components.
Don’t worry if it seems a very tight fit or jams with something. If necessary
you can carefully file away up to an mm or so at the right edge of the AGC
board. Even then, if it doesn’t fit totally square, this is not a problem.
On the QCX-mini Rev 3 (and above) PCB, all five connections from the AGC board
to the mainboard, are simply made using component lead offcuts (or similar).
The five holes on the AGC board line up perfectly with five corresponding
holes on the main QCX-mini PCB. All you have to do is thread five pieces of
wire through and solder on the top side and bottom side. The five holes are
identified in the diagram below.
The photograph below shows the completed assembly. This is a QCX-mini 17m,
with both TCXO and AGC options installed.
3.2 Installation in
QCX-mini PCB Rev 2
The five connections of the AGC PCB are connected to schematic points as shown
below.
The AGC board fits snuggly in the area of the PCB above IC8 and IC9. The AGC
PCB shape is designed to fit between the surrounding through-hole components
as shown in the photograph below.
Don’t worry if it seems a very tight fit or jams with something. If necessary
you can carefully file away up to an mm or so at the right edge of the AGC
board. Even then, if it doesn’t fit totally square, this is not a
problem.
The following diagram is for PCB Rev 2:
The “AF OUT” and “GND” connections are the easiest because the holes on the
AGC board line up precisely with corresponding existing holes on the main QCX-
mini PCB. These holes are the ones nearest to the 470uF capacitor and are also
identified by the yellow arrows in the photograph above.
Before connecting “AF OUT” and “GND”, you should connect three hookup wires to
the remaining three connections “+5V”, “MUTE” and “AGC”. I used the same
0.33mm (AWG #28) wire used for winding the toroids. There is always some
leftover. This is convenient because the wire is thin enough to pass easily
through existing holes on the PCB. The reason for connecting these wires NOW
is that you can then ensure that you have tightly trimmed any excess wire on
the underside of the AGC board. If you install the wires later and solder them
from the top, it would be too easy to have long enough excess wire on the
underside of the board, to risk contact with the main QCX-mini PCB.
If you are sure that you want the AGC always ON, you can omit the AGC
connection (top left pad) and instead add a solder bridge across R204 as
described in section 2.2 above.
If you want the capability to switch the AGC ON/OFF from a QCX-mini menu item,
then you need to make sure you have firmware 1.08 or higher; and you connect
this hookup wire.
Once the three hookup wires are soldered to the “+5V”, “MUTE” and “AGC” pads,
you can proceed with making connections for the “AF OUT” and “GND” pads, using
component offcut wires for example.
All you need to do is solder a wire between the two boards (refer to the
yellow arrows in the photograph above). For example, you can use two pieces of
component lead offcut. Before soldering, ensure that the AGC PCB sits nice and
flat on top of the op-amps IC8 and IC9.
The remaining three connections need to be made using hookup wire. In the
photographs below, I am using enameled copper wire, leftover from the toroid
winding:
Firstly, the +5V connection must be made from the +5V pad of the AGC board to
the voltage regulator. Connect this wire to the rightmost pin of the 7805
voltage regulator (as shown in the photograph).
Next, connect the “MUTE” AGC board pad. This is the bottom right corner
connection of the AGC board, nearest the blue rectangular trimmer
potentiometers.
The best route for this wire is to take it through the hole which is just
between the metal tab of the voltage regulator, and the DC power connector, in
the bottom left corner of the photograph above. Now turn the board over and
connect the wire to the one leg of capacitor C22; the wire crosses the board
almost horizontally to reach this capacitor leg.
Please refer to the photograph below. Remember to first remove the wire enamel
if you are using enameled copper wire; either by sanding it or scraping with a
knife or my preferred method: heating it in a blob of solder on the tip of the
soldering iron.
Finally, the connection that controls AGC ON/OFF, if you are installing it.
The “AGC” control signal connection is in the top corner of the board and is
shown in the photograph on the preceding page. This should be passed through
the hole in the board that is labeled `DVM”; this hole is on the right, next
to the 2×5-pin display board header, near the top left of the main QCX-mini
PCB in the photograph on the preceding page.
Now turn the board over and solder the wire carefully to the bottom left pin
of the 2×5-pin header, which is the pin nearest the board edge and the corner
of the board. Refer to the photograph below. Remember to first remove the wire
enamel if you are using enameled copper wire; either by sanding it or scraping
with a knife or my preferred method: heating it in a blob of solder on the tip
of the soldering iron.
3.3 Installation in QCX-mini PCB Rev 1
The AGC board fits snuggly in the area of the PCB above IC8 and IC9. The AGC
PCB shape is designed to fit between the surrounding through-hole components
as shown below.
Don’t worry if it seems a very tight fit or jams with something. If necessary
you can carefully file away up to an mm or so at the right edge of the AGC
board. Even then, if it doesn’t fit totally square, this is not a problem.
The following photograph is of PCB Rev 2 but applies equally to PCB Rev
1.
The “AF OUT” and “GND” connections are the easiest because the holes on the
AGC board line up precisely with corresponding existing holes on the main QCX-
mini PCB. These holes are the ones nearest to the 470uF capacitor and are also
identified by the yellow arrows in the photograph above.
Before connecting “AF OUT” and “GND”, you should connect three hookup wires to
the remaining three connections “+5V”, “MUTE” and “AGC”. I used the same
0.33mm (AWG #28) wire used for winding the toroids. There is always some
leftover. This is convenient because the wire is thin enough to pass easily
through existing holes on the PCB. The reason for connecting these wires NOW
is that you can then ensure that you have tightly trimmed any excess wire on
the underside of the AGC board. If you install the wires later and solder them
from the top, it would be too easy to have long enough excess wire on the
underside of the board, to risk contact with the main QCX-mini PCB.
If you are sure that you want the AGC always ON, you can omit this connection
and instead add a solder bridge across R204 as described in section 2.2 above.
If you want the capability to switch the AGC ON/OFF from a QCX-mini menu item,
then you need to make sure you have firmware 1.08 or higher; and to connect
this hookup wire.
All you need to do is solder a wire between the two boards (refer to the
yellow arrows in the photograph above). For example, you can use two pieces of
component lead offcut. Before soldering, ensure that the AGC PCB is sitting
nice and flat on top of the op-amps IC8 and IC9.
Once the three hookup wires are soldered to the “+5V”, “MUTE” and “AGC” pads,
you can proceed with making connections for the “AF OUT” and “GND” pads. These
remaining three connections need to be made using hookup wire. In the
photographs below, I am using enameled copper wire, leftover from the toroid
winding. The following photograph is Rev 2, but applies equally to Rev 1,
except that on Rev 1 the +5V wire is soldered to the center pin of the voltage
regulator (see diagram above).
Firstly, the +5V connection must be made from the +5V pad of the AGC board to
the voltage regulator. Connect this wire to the center pin of the
AMS1117-5.0 voltage regulator (as shown in the diagram on the preceding page,
NOT the above photo which is a Rev 2 board).
Next, connect the “MUTE” AGC board pad. This is the bottom right corner
connection of the AGC board, nearest the blue rectangular trimmer
potentiometers.
The best route for this wire is to take it through the hole which is just
between the metal tab of the voltage regulator, and the DC power connector, in
the bottom left corner of the photograph above. Now turn the board over and
connect the wire to the one leg of capacitor C22 as shown; the wire crosses
the board almost horizontally to reach this capacitor leg. Please refer to the
photograph below. Remember to first remove the wire enamel if you are using
enameled copper wire; either by sanding it or scraping with a knife or my
preferred method: heating it in a blob of solder on the tip of the soldering
iron.
Finally, the connection that controls AGC ON/OFF, if you are installing it.
The “AGC” control signal connection is in the top left corner of the board and
is shown in the photograph on the preceding page. This should be passed
through the hole in the board that is labeled `DVM”; this hole is on the
right, next to the 2×5-pin display board header, near the top left of the main
QCX-mini PCB in the photograph on the preceding page.
Now turn the board over and solder the wire carefully to the bottom left pin
of the 2×5-pin header, which is the pin nearest the board edge and the corner
of the board. Refer to the photograph below.
Remember to first remove the wire enamel if you are using enameled copper
wire; either by sanding it or scraping with a knife or my preferred method:
heating it in a blob of solder on the tip of the soldering iron.
3.4 Installation in QCX+ Rev 4
The five connections of the AGC PCB are connected to schematic points as shown
below.
The AGC board fits snuggly in the area of the PCB shown in the diagram below.
There is an outline of the AGC board footprint on the QCX+ PCB silkscreen.
On the QCX+ Rev 5 (and above) PCB, all five connections from the AGC board to
the mainboard, are simply made using component lead offcuts (or similar). The
five holes on the AGC board line up perfectly with five corresponding holes on
the main QCX-mini PCB. All you have to do is thread five pieces of wire
through and solder on the top side and bottom side. The five holes are
identified in the diagram below, followed by a photograph showing the
installation.
3.5 Installation in QCX+ Rev 1, 2 and 3
The five connections of the AGC PCB are connected to schematic points as shown
below.
The AGC board needs a place to sit. An appropriate location where there is
enough space, is on top of the “Labs” part of the QRP Labs logo, right in the
middle of the QCX+ PCB. You can secure it to the PCB using double-sided tape
or hot glue or some other type of glue. Before gluing it down, solder on five
hookup wires for the five connections.
If you are sure that you want the AGC always ON, you can omit this connection
and instead add a solder bridge across R204 as described in section 2.2
above.
If you want the capability to switch the AGC ON/OFF from a QCX-mini menu item,
then you need to make sure you have firmware 1.08 or higher; and you need to
connect this hookup wire.
The five hookup wires should be soldered as follows. Refer to the photograph
below.
+5V (white wire in the photograph): this can be conveniently soldered to the
+5V pin header of the GPS connection strip. There are also other alternative
locations if you prefer, such as at JP21 behind the PTT jack connector.
GND (black wire in the photograph): this can be conveniently soldered to the
GND pin header of the GPS connection strip, or any other convenient location
if you prefer, such as JP23 behind the CAT jack connector.
AF OUT (purple wire in the photograph): should be soldered directly to IC10
pin 1. Note that IC10 pin 1 and pin 2 are connected together. So if you
accidentally create a solder bridge between pins 1 and 2, there is no harm
done.
MUTE (pale grey wire in the photograph): should be soldered to the left-hand
side of capacitor C22 (the side of C22 that is furthest from chip IC10 as
shown in the photograph). With care, you will be able to solder the wire to
the pad on the top side of the board. You can use a DMM to check for short
circuits if you are worried that you may have created a short with a solder
bridge.
AGC (blue wire in the photograph): IF you are connecting the AGC On/Off
control signal: This should be connected to the middle top pin of the middle
pin header connector between the front and mainboards. This is the LCD_D6
signal.
3.6 Installation in QCX (QCX Classic) Rev 1, 2, 3, 4 and 5
The five connections of the AGC PCB are connected to schematic points as shown
below.
The AGC board needs a place to sit. An appropriate location where there is
enough space, is on top of the audio output jack connector on the left side of
the QCX PCB. You can secure it to the PCB using double-sided tape or hot glue
or some other type of glue.
Before gluing it down, solder on five hookup wires for the five connections.
If you are sure that you want the AGC always ON, you can omit this connection
and instead add a solder bridge across R204 as described in section 2.2
above.
If you want the capability to switch the AGC ON/OFF from a QCX-mini menu item,
then you need to make sure you have firmware 1.08 or higher; and you need to
connect this hookup wire.
The five hookup wires should be soldered as follows. Refer to my diagram
above.
+5V (white wire in the photographs): this can be soldered to the +5V pin of
the 7805 voltage regulator chip, on the top side of the board or the bottom
side, as you prefer; it is the pin closest to the top left corner of the
board. You may prefer other locations but make sure they are really connected
to + 5V.
GND (black wire in the photographs): this can be conveniently soldered to the
GND pin header of the R36 off-board header pads if not in use, or any other
convenient location if you prefer. It is the pad close to the bottom left
corner of the board.
AF OUT (purple wire in the photographs): should be soldered directly to IC10
pin 1. Note that IC10 pin 1 and pin 2 are connected together. So if you
accidentally create a solder bridge between pins 1 and 2, there is no harm
done.
MUTE (pale grey wire in the photographs): should be soldered to the left
middle pad of the R36 off-board headers. .
AGC (blue wire in the photographs): IF you are connecting the AGC On/Off
control signal: This should be connected to D6 of the LCD module directly;
this is the 4th pin from the right, as shown in the above diagram and the
photograph below.
Resources
For updates relating to this kit please visit the QRP Labs AGC module page
http://qrp-labs.com/ agc
For any questions regarding the assembly and operation of this kit please join
the QRP Labs group, see http://qrp-labs.com/group for details
Document Revision History
1.00 03-Dec-2021 1.01 15-Feb-2022
First version 1.00
Updated for QCX+ Rev 4 PCB
Fixed error on QCX-mini Rev 1 / 2 diagrams
References
- Groups.io: Powerful Email Groups & Collaboration Platform
- QRP Labs Kits
- QCX-series AGC module
- Discussion group
- [email protected] | A simple QCX AGC