QRP Labs NK8O QDX 5 Band HF QRP Digital Transceiver Owner’s Manual
- June 9, 2024
- QRP Labs
Table of Contents
QRP Labs NK8O QDX 5 Band HF QRP Digital Transceiver
Digital Transceiver
The QRP Labs QDX Digital Transceiver is a lowpower, low-cost radio for digital operations. The reviewed unit covers fi ve HF bands — 80, 60, 40, 30, and 20 meters — and the maximum power output is 5 W, with support for digital modes only. It comes in a kit to be built, and you can buy a fully assembled unit for an extra $45. See Figure 1 for the kit parts packaging. The printed circuit board (PCB) comes with preinstalled surface-mount device (SMD) components (Figures 2 and 3 show each side of the PCB). It includes an embedded software-defi ned receiver (SDR), 24-bit 48 kilo samples per second (kS/s), a USB sound card, CAT control, and a synthesized VFO with TCXO reference. The QDX transmits a single, clean output signal, as it is not an SSB modulator with associated unwanted sideband and residual carrier, or intermodulation due to amplifi er non-linearity (more on this later).
The QDX is suitable for single-tone operations. It is reported on the QDX forum that successful RTTY operation has been accomplished. It is not suitable for CW operation using fl digi or similar programs. It is my understanding that there is no waveform shaping that would prevent key clicks. Modes that require multiple simultaneous tones, such as Winlink, are not possible with the QDX. Also, the QDX is not capable of phaseshift keying, such as PSK31. Per the product description,
QDX is suitable only for single tone FSK modes, which covers the majority of digital modes in use today. This includes everything in WSJT-X, JS8Call, some fl digi modes e.g. RTTY, Olivia and more. QDX is not suitable for on/off keyed modes such as CW because it does not have click-reducing RF envelope shaping; furthermore, it is not suitable for phase shift keyed modes such as PSK31 or modes involving multiple concurrent tones such as WinLink.
Description and Kit Assembly
While it does require a computer, the unit has only four connections on the rear panel: an antenna BNC port, a power connection, a 3.5-millimeter PTT, and a single
Reprinted with permission; copyright ARRL.
USB type B port that handles both CAT control and audio (see Figure 4). When
all the necessary physical connections are made, the unit becomes a “black
box” that requires no other user intervention.
Since Revision 3 (Rev3, now Rev4), QRP Labs added a standard 3.5-millimeter
plug for an external PTT connection that can be used without modifi cation
between the QDX and the QRP Labs 50 W PA. The PTT output can also be confi
gured for use with any other amplifier.
Bottom Line
The QRP Labs QDX is a low-cost digital transceiver with a clean RF output signal for the supported digital modes. Setting it up is fast and easy, perfect for portable and permanent installations.
I have considerable experience with QRP Labs products. I assembled at least
eight original QCX models, repaired several more, and built a QCX+. The QRP
Labs instruction manuals are clear, they contain concise technical
descriptions, and they are generally easy to follow. The QDX documentation is
no exception. It is, perhaps, among the easiest of the QRP Labs kits to
assemble. My assembly time was no more than a few hours.
Assembly is straightforward, and the parts count is low.
Most of the board is pre-populated with the SMD components, so the user
doesn’t need special tools to complete the kit. There are 16 through-hole
capacitors, six diodes, four transistors for the fi nal amplifi er, an LED, a
transformer, and a series of toroids to wind and install. Finally, a few
connectors complete the build. Depending on your level of experience, the kit
can be assembled in a few hours. There is only one construction option, and
that is whether to build the device to operate on 9 or 12 V dc. To quote the
operating manual, “No test equip
ment is required to build, align, and operate this digi modes transceiver.
There are no alignment tasks.” For more details, you can download the manual
from the manufacturer’s website (www.qrp-labs.com/qdx.html). Linux (Xubuntu 22.04) is my operating system of choice.
But as stated on the manufacturer’s website, the drivers for these types of
audio devices should be already present on your computer if you are using
macOS, Linux, or Microsoft Windows 10 or 11. If you are using older versions
of Windows, then you need to install a driver for the virtual COM port (audio
and USB fl ash drivers are already present, even on older versions of
Windows).
The details can be found in the QDX manual. I made the connection between the
QDX and my laptop via a USB A/USB C cable. Linux immediately recognized the
built-in sound card, and it was easy to select the correct sound settings in
WSJT-X. The serial device followed the typical naming scheme for Unix-like
systems. The serial port test confit rimed operation immediately, and after a
brief check I was on the air. I selected the KENWOOD TS-440S confi gyration
for CAT settings in WSJT-X. The sound card appears as ALSA_INPUT. USB-QRP-
LABS_QDX_TRANSCEIVER-02.ANALOGSTEREO and ALSA_OUTPUT.USB-QRP-LABSQDX
TRANSCEIVER-02.ANALOG-STEREO, respectively, for audio input and output. With
the QDX designation in the sound card description, there is little possibility
of making a mistake in selecting the device. The serial port appears as
/DEV/TTYACM1 on my laptop. I have not connected the radio to a Windows or Mac
computer to date. Most importantly, the radio was not designed around a single
operating system.
On the Air
With many hours of operation behind me, I can say with certainty that this transceiver is unparalleled in ease of operation within the scope of my experience. The radio’s front panel has no controls and only a single
Table 1
QRP Labs QDX HF Digital Transceiver (Rev4)
Manufacturer’s Specifi cations
Frequency coverage: 80, 60, 40, 30, and 20 meters.
Power requirement: 9 V or 12 – 13 V dc with modifi cation.
Receive
Modes of operation: Digital only.
Transmitter
Power output: Up to 5 W.
Spurious-signal and harmonic suppression: Not specifi ed.
Size (height, width, depth, including protrusions): 1 × 2.48 × 3.49
inches. Weight: 0.30 pounds/(135 grams).
*RF power output was 3.0 W on 14 MHz with 9 V dc input voltage.
Measured in the ARRL Lab
As specifi ed.
At 12 V dc: transmit, 0.9 A; receive, 166 mA
As specifi ed.
Transmitter Dynamic Testing
At 12 V dc: 3.5 MHz, 4.5 W; 5.3 MHz, 4.8 W; 7 MHz, 4.2 W; 10.1 MHz, 5.4 W; 14
MHz, 5.5 W.
Typically >52 dB; worst case, ed. 44 dB (60 meters). Complies with FCC
emission standards.
USB cable, power, and antenna connection on the back of the radio. While no adjustment or alignment is needed, it’s possible to access confi guration and test menus. To access these features, you will need to use terminal software like PuTTY or Minicom, which will give you access to several menus within the radio. The fi rmware is upgradable. The radio seems exceptionally stable, likely due to the TCXO that controls the reference oscillator.
There are no relays within the radio. All switching is done with PIN diodes. I
have used the radio only for FT4, FT8, and JS8Call, but operations with these
modes have been nearly trouble-free. Modes that will work shift frequencies
one at a time. The QDX cannot be used for phase-shift keying, multi-tone
modes, or CW. It is reported to work with RTTY (with some compromises),
DominoEX, and Olivia.
There was one instance of a brief failure, but it was really a computer
operating system problem, and it had nothing to do with the QDX. The USB port
designation changed from /DEV/TTYACM1 to /DEV/TTYACM0, but this was easily
remedied in the fi eld. This can happen with any operating system. With Linux-
based systems it is possible to make a permanent port assignment to avoid this
problem. While I have had some experience with Mac OS X, it has been quite a
while, and I am unfamiliar with recent changes within the OS. I also have very
little recent experience with Microsoft Windows.
I use the transceiver primarily for portable operations, as I am an avid activator for both World Wide Flora and Fauna and Parks on the Air® programs. Rapid deployment is a distinct advantage for fi eld operations, and the QDX greatly facilitates this. Previous fi eld setups included multiple peripherals, careful adjustment of sound and drive levels, and many extra wires that were easily misconfi gured. The setup of the QDX is nearly foolproof. Due to the nature of propagation, portable operations, and the parks programs, the majority of my operating has been on 20 meters. I’m sure other bands would work well, but I typically do not spend a lot of time away from 14074 kHz. I have, however, tested the transceiver and made a few contacts on other bands.
Operating voltage and power output are supposed to be determined by the number of turns on the secondary winding of transformer T1. I chose the 12 V operation voltage, although I fi nd that my transmitter is a bit more optimistic than one might like. The maximum recommended output is 6 W, and my transceiver puts out nearly this much and more on several bands. So far, I haven’t encountered any difficulties, but there could be a replacement for the BS170s transistor in the future. My battery of choice is LiFePO4, and the voltage is a very steady 13.2 V under normal operating conditions. The power output of the QDX is strictly a function of input voltage and not related to audio drive. Manipulation of the input voltage in the fi eld does present certain problems. The recommended input of 12 V can be obtained by placing a diode inline with the power source (see Table 1). I may try a LiPO4 pack (about 11.1 V) to see if that drops the power output to a more acceptable level.
There is a jack for operations with an external power amplifi er, but I have not found it necessary to move to QRO operations. I have made thousands of contacts with plenty of DX using low power and a good antenna.
An LED indicator shows that the radio is powered up. In transmit, the LED has a medium blink rate. It shows a fast blink rate on boot-up, or very slow and steady blink rate if it is in the fi rmware upgrade mode. Boot time is less than 1 second, but the fast LED will blink for 5 seconds. For portable operations, setup takes a little longer than setting up the antenna. There are no settings to fi ddle with. The transmit level in WSJT-X does not affect
power output, and it is recommended to simply leave the transmit power at the maximum setting. I reduced the computer’s audio input drive to keep the receive level indicator out of the red, but that was the only adjustment needed in my confi guration.
My portable operations have taken me very far afi eld, at least through a swath of the central United States. My favorite portable operating position is from a picnic table, although the small size of the QDX makes it easy to set up nearly anywhere. I have a portable aluminum table that also works well. It is rare that I operate exclusively in digital modes, so I need enough room for a CW transceiver. I may be something of a heretic, but I rarely operate QRP when I am activating a park unless there is a considerable hike involved. Then the station is as light as possible. My “tailgate” and picnic table operations are usually accompanied by a Yaesu FT-991A for CW. There is always plenty of room for the QDX. Keep in mind that the QDX requires some sort of computer, although there are many confi gurations that can be used for this purpose. A Raspberry Pi, which is about the same size as the QDX, could be linked wirelessly to a tablet or iPad, making for a compact and versatile setup.
Hans Summers, GØUPL, founder of QRP Labs and the designer of the QDX, provides a better technical description of the operation of the radio, but here’s my take. As a musician, I understand the term “transposition.” To transmit, the radio uses an analog-to-digital conversion of the audio input signal, which is sampled. The sample of each audio frequency is then “transposed,” or transformed, directly to appropriate RF frequencies. These are not sideband signals injected to produce a suppressed carrier SSB transmission. The transmitter produces pure frequency shift keying without any spurious mixing products.
The receiver is an SDR that uses an intermediate frequency for the fi nal extraction of the tones — fi rst in digital form, then fed as audio to a built-in sound card, and fi nally sent via USB to a computer. Anecdotally, the receiver passband is nearly devoid of noise. In very unscientifi c terms, the radio doesn’t seem to “hear” much of anything aside from the intended signals. This may, in part, have to do with portable operations, but even still I fi nd that, compared to other QRP FT4/FT8 setups I have used, the signal-to-noise ratio appears to be much better with the QDX. Compared to other QRP setups, my experience is that the QDX hears well enough that there is bigger spread in SNR readings, and because the QDX hears so well, a reply with the other station being unable to hear it seems to be more frequent than with other radios I have used.
The QDX does an excellent job of leveraging the modes available in WSJT-X and several other popular modes.
Conclusion
Overall, the QDX is an excellent transceiver. It is compact, easy to use, and nearly foolproof. I highly recommend purchasing the case unless you are highly skilled at fabricating your own enclosures, or if you have a 3D printer, as some templates are available online. The radio is a worthwhile addition to my go-box for portable operations, but it would function equally well as a dedicated radio in the shack.
Manufacturer: QRP Labs, www.qrp-labs.com. Available in two versions at the same price: 80, 60, 40, 30, and 20 meters (the reviewed unit), and the new 20, 17, 15, 12, and 10 meters. Price: $69 for the kit version, and $20 for the optional enclosure; an extra $45 for the assembled version.
Reviewed by Charles Powell,
NK8O/VE3ISD/5H3DX
[email protected]