QDX-M: a feature-packed, high performance, SINGLE-BAND 5W Digi-modes transceiver kit, including embedded SDR, 24-bit 48 ksps USB sound card, CAT control, synthesized VFO with TCXO reference. 

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The "QDX-M" (QRP Labs Digital Xcvr - Monoband): a feature-packed, high performance, 5W Digi-modes transceiver kit, including embedded SDR receiver, 24-bit 48 ksps USB sound card, CAT control, synthesized VFO with TCXO reference. QDX transmits a SINGLE SIGNAL, it is not an SSB modulator with associated unwanted sideband and residual carrier, or intermodulation due to amplifier non-linearity. QDX-M outputs a pure single signal.

QDX-M is based on the QDX transceiver but is designed for a single band only, so omits the band-switched filtering. This provides a slight improvement in performance particularly on the higher bands, but it also allows us to offer a version for 160m, 630m and 2200m. The multi-band QDX cannot access these lower bands because the PIN diode band-switching doesn't work at such low frequencies. 

QDX-M 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 fldigi 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,

FIrmware: The same firmware is used for QDX and QDX-M

Bands: QDX-M will be available eventually for any single band from 2200m to 10m (initially the band selection will be limited; more bands will be added in coming weeks).

The Optional enclosure is black anodized extruded aluminium, very sturdy and elegant. The enclosure size is 89 x 63 x 25mm without protrusions. The front and rear panels are drilled and cut to match the QDX PCB with laser-etched lettering. The enclosure includes four self-adhesive feet. 

List of features: 

  • Single band for any choice of 2200m to 10m (initially, limited selection)
  • 5W output at 9V supply (can be built for 4-5W at 12-13V supply) - note lower power at higher bands e,g. 10m, approx 3.5-4W
  • Single signal transmission (zero unwanted sideband, zero residual carrier, zero intermodulation distortion)
  • Solid-state transmit/receive switching under CAT control
  • High performance embedded SDR SSB receiver with 60-70dB of unwanted sideband cancellation
  • Built-in 24-bit 48ksps USB sound card
  • Built-in USB Virtual COM Serial port for CAT control
  • Si5351A Synthesized VFO with 25MHz TCXO as standard
  • Easy to build single-board design, Professional quality double-sided, through-hole plated, silk-screen printed PCBs
  • All SMD components factory assembled
  • Connectors: 2.1mm power barrel connector, USB B (for audio and CAT control), BNC RF input/output
  • Built-in test signal generator and testing tools
  • Receive current 150mA, Transmit current 1.0-1.1A for 5W output with 9V supply (around 0.7A for 5W with 13V supply).
  • I-Q output mode if you wish to use a PC SDR program for demodulation rather than the internal SDR
  • QFU firmware update for lifetime free firmware udpates, easy installation on any PC without drivers, software or hardware e.g. programmer
  • Optional aluminium extruded cut/drilled/laser-etched black anodized enclosure (88.6 x 63 x 25 mm)

PC Software

Standard digi modes software like WSJT-X and JS8Call works fine with QDX, because QDX acts as a USB Sound card (for Audio) and a Virtual COM port (for CAT control).

There are three types of USB devices in QDX: USB Audio, USB Virtual COM port, and USB mass storage (like a USB Flash memory stick) for firmware update. Drivers for all of these types of audio device are already present on your PC if you are using Mac, Linux, MS Windows 10 or MS Windows 11. If you are using older versions of Windows, then you need to install a driver for the Virtual COM port (audio and USB Flash drivers are already present even on older Windows), the details are in the QDX manual. 


The following videos apply to the QDX, and largely also the QDX-M (which is a QDX without band-switching):

Here's a YouTube video about QDX: https://youtu.be/j6Vnh8eiEpg 

Here's a build-video series by Ryan W7RLF https://www.youtube.com/playlist?list=PLDP9oGiu60jJdiQOQx6-5DRn3l5GrfQQM (for Ryan's interesting website see https://miscdotgeek.com/)

Here's a YouTube video about QDX PCB Rev 4: https://youtu.be/PSYGBGHH_00 


Latest documentation (Rev 5M)

You need THREE manuals, one contains the Operating instructions, one contains the Assembly instructions for the output transformer and one contains the remaining assembly instructions, theoretical design information, and performance measurements.

1. Make sure you download the correct manual version for your PCB revision (indicated clearly on the PCB silkscreen) and firmware version! AND the transformer construction manual!

2. Always install the latest firmware revision (see below). 

Assembly manual for PCB Rev 5M (document version 1.03 published 12-Sep-2023)
PA transformer manual - you need this with the assembly manual (rev 1.00b, 13-Sep-2023)
Operating manual for firmware 1_08 (document version 1_08 published 17-Jan-2022)
Hi-res schematic, PCB Rev 5M (.png file)


QDX-M in its beautiful black-anodized extruded aluminium enclosure, CNC machined to suit the QDX and with durable laser-etched lettering:

These are screenshots of the built-in terminal applications, that may be used for testing, investigating, learning and configuration - but are not needed for operation of the transceiver. 

Selected performance measurements - see manual for full details...

WSJT-X operating screenshots

FDIM 2022 seminar on QDX

Presented 19-May-2022 by Hans G0UPL at Fairborn Holiday Inn, Dayton. 

Conference proceedings article (1.9M PDF)
Seminar slides (3.1M PDF)
YouTube video of the same talk presented to Norfolk Amateur Radio Club (UK) on 01-Jun-2022


Please refer to the manual for the firmware update procedure, which is very easy and does not require any special hardware, software, drivers etc. It requires only a PC and a USB cable. Click the file in the first column of the table below, to download the firmware file of interest. Note that firmware files are encrypted and can only be used on the QRP Labs QDX/QDX-M. 

The downloads are a zip file; please unzip the file before copying the .QDX file into the QDX/QDX-M radio. 

Firmware version history:

Version Date Contents
1_10 19-Jul-2023 - Faster rendering of the Configuration screen
- Added "Custom USB name" parameter to Configuration screen - facilitates multiple QDX on one PC
- Improved precision of RF output frequency at given audio offset (3rd term Taylor expansion)
- Bug fix: 2200m and 630m transmit hung the QDX, this is now resolved
1_09 10-Feb-2023 - Bug fix: fix initialization of Band Configuration screen for some single-band bands (non-production yet)
- Bug fix: reading band selection jumper wires did not work properly in some cases
- Bug fix: Transmit screen no longer prevents transmission if the default freq is out of any configured bands.
1_08a 18-Jan-2023 - Fix year on menu title, now says 2023 (not 2022)
1_08 17-Jan-2023 - Change band jumpers now only resets band config, not entire configuration
- Add mono-band default configurations based on jumper wires fitted
- CAT MD; correctly returns 2 for USB, 1 for LSB (was 3 = CW)
- CAT IF; correctly returns the mode in the result string (was always 3)
- Fix bug with 80m frequency error
1_07 23-Dec-2022 - Enable default configuration set-up for high-band 20, 17, 15, 12, 11 and 10m band version
- Big fix: Transmitter Test utility was broken in 1_06
1_06 12-Dec-2022 - Resolved bugs and accuracy issues to greatly improve transmit tone setting accuracy
- Add "Night mode" to configuration screen - makes panel LED very dim
- New "TX shift threshold" parameter in Configuration screen, determines when TX freq updated (mHz)
- Now successful TX is a 1Hz pulsating indication; 2 blinks is wider spaced for easy counting
- When bias mA is 5mA, disable SMPS completely
- Change default 80m Frequency min to 3200000
- Firmware support for 2200m
- Changed length of band name in the Band Config screen to 4 digits (to allow 2200m)
- Correctly implement split mode (in split, RX on VFO A and TX on VFO B)
- Changed minimum TX freq from 300kHz to 75kHz (to allow 2200m)
- Qn CAT commands return string length is now longer to fit larger numbers
- Implement fix to SP; command; now correctly returns SP1; if Split is set.
- Added TP CAT command to return STM32 ADC temp sensor value (128-sample average)
- More aggressive timeout for lack of audio to stop TX (to work with HF pager app)
1_05 30-Sep-2022 - RF Sweep locks band settings and PLL divider, at the center frequency divider
- Default Cyan colour for grid on AF sweep, RF sweep and Input analysis screens
- Pressing . on AF or RF sweep changes the grid colour, like Input screen
- Colour of the grid in each screen is persisted for the operating session
- RF Sweep, addeed R to refresh current band (re-sweep)
- Fix gain bug in RF and AF sweeps (should be fixed at 5000. not use the BandConfig value)
- Proper log10 conversion to dB for sweeps etc
- New Image sweep screen, sweeps and measures superhet image 24kHz down the band
- Fix CAT AG command (audio gain)
- Add CAT VN command (firmware version); read-only, VN; returns VN1_05_003;
- VGA and VT220 terminal option by soldering wires to microcontroller pins
- Added three new serial ports 1/2/3
- AUX PTT - positive and negative PTT outputs independently controlled on TX and RX per band
- Config screen now has "PTT Port as Serial 3" as enable/disable
- Config and BandConfig screens don't stop radio operation now
- TX disabled during Input Analysis screen
- Press M in Input Analysis screen shows sub-window with CAT comms; and scroll only when signal present
- Change TX limits from 1-30MHz to 0.3-60MHz
- Version supports 160m and 630m (delay method of 90-degree quadrature generation)
- Added "PIN fwd bias (mA)" to Band Config screen
- Finalized SMPS control loop for Rev 4 PCB PIN diode forward bias buck converter
- Update year to 2022 on firmware version in terminal
1_04 20-Jul-2022 - Resolution to the Rev3 PCB "popping" issue (RF discontinuities during TX).
- Audio streamed to the PC during Transmit is now set to zero (silence).
- 50 millisecond transmit timeout on cessation of audio from PC (protects QDX e.g. if PC crashes).
- No CAT command TX timeout (configurable enable and timeout duration).
- Removed unnecessary 48ksps clock domain sync PC vs QDX.
- Enables PTT output connector, with grounded and +5V outputs individually controllable by band.
- 12kHz IF offset is removed when you enable IQ Mode.
- Transmit is disabled when you are in IQ mode.
- Gain is now in dB.
- Gain is now configurable per band.
- New "Band Configuration" screen where you can configure the list of supported bands.
1_03 23-Dec-2021 - Frimware for PCB Rev 2. Backwardly compatible with PCB Rev 1
- Changes OpenDrain RX, TX and LPF switching signals to Push-Pull (pull-up resistors removed in Rev 2)
- Allows transmit on 160m, and receive acts as direct conv., poor performance through the 80m filters. 
1_02 25-Nov-2021 - New "transmitter test" terminal application, lets you test PTT and Transmit
- New items in "Configuration" terminal application, let you disable transmit by band
- 'Log end' timestamp written at end of log, to indicate current elapsed time since power-up
- Bug fix: prevent log file overflow
- Bug fix: White noise intermittent on power-up is fixed
1_01e 25-Nov-2021 - Bug fix: Now AF and RF sweep terminal applications work
1_01d 24-Nov-2021 - Log entries now have a timestamp, HH:MM:SS since power-up.
- Error entries now appear in red, ordinary logging in white.
- Startup log entry "START" appears in yellow.
- Bug fix: Occasional CAT communications dropping should not happen anymore.
1_01c 12-Nov-2021 - Status LED flashes more info during TX to aid audio debugging on PC:
  3 flashes means all OK, TXing power;
  2 flashes means transmit mode, but audio level (volume) is too low;
  1 flash means transmit mode, but there is no audio at all
1_01b 08-Nov-2021 - Apply validation to config ref clock to make sure 25MHz +/- 1kHz.
1_01a 05-Nov-2021 - Bug fix: RX did not work after changing bands, until you changed freq again, or TX then RX again
1_01 27-Oct-2021 - IQ Mode, if enabled, transfers raw I & Q data to the PC for SDR programs.
- Japanese band limits mode, if enabled, prevents transmission outside Japanese band limits.
- Prevent QDX trying to transmit outside the range 3.4 MHz to 14.5 MHz, to avoid damage.
- CAT command MD can be used now to set mode LSB or USB
- CAT commands Q0, Q1 to QB allow get/set of all parameters on the Configuration screen
1_00_003 11-Oct-2021 - Initial firmware release for QDX.

QDX on RTTY using fldigi software, tips from Peter G3SPL

The following notes are provided by Peter G3SPL in this forum post https://groups.io/g/QRPLabs/message/79386.

Well, I have had my first QSO today with the QDX running RTTY. Yippee! I used the fldigi software (on Windows 10 laptop) which took quite a bit of effort to set up; certainly compared to WSJT-X.

The first thing was to change the QDX configuration using my terminal emulator to only require one sample of tone before setting the output frequency (the default being 480). This is to allow the QDX to keep up with the tone frequency potentially changing every 22mS.

With the fldigi configuration, under the soundcard, devices option, choose the QDX as both capture and playback device. I checked "port audio" - not sure what that does but it works!

Under "Audio device shared by Audio Alerts and Rx Monitor", choose Speaker/Headphones and check the "Enable" box.

Next, under the rig control option, select Hamlib and check the "Use Hamlib" box at the top.
The rig should be Kenwood TS440S, device is COM3 (i.e. whatever you use with your terminal emulator to get to the QDX menu) and check the box "PTT via Hamlib command". Other items can remain at their default values. Finally, press the "Initialize" button to make it all work.

Then you need to go into your Windows sound settings and put all the volume controls for the QDX to maximum.

Finally, choose the "Save config" option from the top of screen menu (otherwise you'll have to do all this next time you boot up fldigi!)

Expect to spend an hour or two browsing the help files to see how to operate the various buttons and options. One thing which I found difficult to find was how to turn on the Rx Monitor to hear what's going on on receive. This is done from the "View" menu: choose the top option "Rx Audio Dialog". On my setup, the received audio was very quiet for some reason. Maybe I will discover how to adjust it to a reasonable volume in due course.

By the way, I spent an hour or two wondering why the QDX wasn't going into transmit, to find that I had not checked the "Use Hamlib" box for rig control. Quite exasperating, but with perseverance I got there: it does work, so I am happy!