Intro

The 'Direct Digital Transceiver' (DDX) project. DDX is a radio transceiver (rig) which is rugged, simple and reliable.

DDX-1 is a 'silent' radio which can be heard almost all over the world!

Complete BOM (everything) estimate: ~1500 INR (15 to 25 USD depending on the configuration).

For a truly portable backpack setup you can combine DDX-1 with the FT8 Radio Android app.

Sample QSO (from Pune to Medellin on 10m, 15691km) on DDX-1:

Who doesn't want a QSO from Medellin after binge-watching Narcos? ;)

Another sample QSO with WW1WW (Pune to USA):

Features (DDX-1)

• Single USB-C cable for handling both Audio + CAT control

• True 5W output on all HF bands. It even outputs ~1-2 W @ 50 MHz! (We now just need a 6m capable receiver section to build a 6m digital transceiver).

• Rock-solid PA which handles open, short, and bad SWR conditions. We knowingly stress-test it with untuned antennas and still manage DX (not a recommended practice for sure though).

• Rock-steady VFO which does NOT drift even under exposed conditions. Yes - it receives WSPR at 28 MHz just fine.

• Reliable T/R switching based on a DPDT relay

• Switchable BPF support to get rid of BCI and other interferences (tested against OTH radar + 100 kW AM station in close visual proximity)

• No toroids to wind (assuming an external LPF bank is in use) ;-)

• The IRF510 final remains as cool as a cucumber (Class-D operation) - a small heat sink is more than enough.

• Avoids problems associated with the 'audio frequency counting' approach used by earlier designs. DDX-1 has a bit-perfect, deterministic TX output.

• It works beautifully with the 'FT8 Radio' app (https://github.com/kholia/DigitalRadioReceiverSupport/). The 'FT8 Radio' app handles FT8 decoding well in real time (Native MCU decoders offer significantly reduced performance at the moment). The app can decode 35++ stations at a time, and even large screens can fill up very rapidly - such is the receiver's performance (with a BPF)!

• Supports largely automated assembly from JLCPCB (all the required files are in the same GitHub repository)

• DDX-1 comes with a free, open-source AGPLv3 licensed firmware.

• No calibration is (ever) required. Just pick up DDX-1 and it is ready to go on air.

• Low cost (20 to 30 USD is our BOM estimate)

• Stress tested by WSJT-Z for multiple days

• DDX-1 routinely receives FT8 traffic from 80 to 100+ countries using a simple 5m long EFHW antenna.

Features (DDX-EVO-1)

DDX-EVO-1 will have the following additional features.

• SSB receiver with DSP!

• Software volume control

• AGC!

• Smaller PCB size (90x70)

• Even more simpler design - less things to go wrong

License

COMMERCIAL USAGE IS FORBIDDEN (aka NOT ALLOWED).

NO SUPPORT and NO GUARANTEE is provided to individuals! This is because we are a small team, and we want to make progress with maximum velocity to bring you more goodies. This said, we are willing to help out with the group builds or club builds of DDX-1 to maximize our dV / dT.

ATTENTION: Using a Si5351 module without a TCXO is a recipe for troubles. You have been warned ("educated").

Note: The firmware is now under AGPLv3 license.

Note 2: Some folders in this repository are encrypted as they are NOT ready for public consumption yet. With time and more testing, more stuff will be released.

RX Performance

Performance of CD2003-with-BPF is >= 80% of that of RSP1 SDR, at least!

TX Performance

VFO stability

It receives WSPR @ 28 MHz ;)

WSPR TX Performance

This is from a HF-PA-v10 board (@ 2W output setting) driving another HF-PA-v10 board with IRF530 installed on it.

QSOs

DDX-1 has already made hundreds of QSOs on air.

Known Gotchas

• DDX-1-R1.10:

  • Building the Pluggable-RX-BPF BPF might be the hardest part of this project. Well, we are trying to replace a 200 USD Professional-BPF (https://surgestop.com/ one) with a ~2 USD BPF after all!

    I recommend getting the BPF boards assembled from JLCPCB, if possible.

    In case you are NOT able to build the BPF, then also no worries. The BPF can be bypassed (by a regular jumper cap) for many use cases. Or a QRP Labs' BPF can be used along with some twisted jumper wires.

    All this said, our BPF filters are within ~0.3dB (S21 LOGMAG value) of the QRP Labs' BPF filters, and are much quicker to build and test (assuming you can solder 0805 SMD components).

Known failure modes

We believe in documenting the various failure modes so that we all can learn from them, and produce more rugged designs with time and experience.

• After making a few hundred QSOs at home and in local parks, the transmit functionality on DDX-1 became wonky. TX would NOT fire at all, or would fire for only a few seconds. It will turn on and off and then on and so on.

  This chaotic, random behaviour was indeed tough to debug.

  This seems to have been caused by the earlier missing following line of code:

  si5351_drive_strength(SI5351_CLK0, SI5351_DRIVE_8MA); // Set max. power for TX (UCC driver)

  After increasing the output drive of SI5351_CLK0 from 2mA (default) to 8mA, the problem went away for good! This is NOT the first (or perhaps the last) time we have run into this bug.

Power on sequence (Laptops along with USB-C to USB-A cable)

• Turn on the main power to the PCB.

• Now connect the USB-C cable between DDX and the PC.

• See wsjtx_helper/README.md for further steps.

Power on sequence (Android phone / USB-C to USB-C cable)

• Connect the USB-C cable between DDX and the Android phone.

• Turn on the main DC power to the radio.

• You may have to explicitly enable the File Transfer mode in USB settings on your Android device.

Files

With these files, you can get around 80% pre-assembled boards (with the SMD components soldered) from JLCPCB at a minimal cost.

• BOM

• Gerbers + PCBA files

• Schematic

Note: The Si5351+TCXO boards can be ordered in fully assembled form from JLCPCB. https://github.com/kholia/Si5351-Module-Clone-TCXO/ has the required files (use the -v3 folder).

JLCPCB link: https://jlcpcb.com/

JLCPCB ordering guide: https://github.com/WB2CBA/ADX-UnO-V1.3/ has the JLCPCB ordering guide in it.

Build Notes

In case of any doubts / confusion, refer to the latest version of the HF-PA-v10.pdf document. Note: Not everything needs to be populated on the PCB.

Render

Building the firmware (for Linux users)

Shortcut: Upload the pre-built .uf2 file to the RP2040 Board and skip to the next section.

Install dependencies:

sudo apt install cmake gcc-arm-none-eabi libnewlib-arm-none-eabi \
  libstdc++-arm-none-eabi-newlib git libusb-1.0-0-dev build-essential \
  make g++ gcc

mkdir ~/repos
cd ~/repos

git clone --recursive https://github.com/kholia/tinyusb.git  # use this in pico-sdk
git clone --recursive https://github.com/raspberrypi/pico-sdk.git
git clone --recursive https://github.com/kholia/DDX.git

Now build the firmware using instructions in the firmware_open folder.

Load the built .uf2 file into the MCU board. Process: Hold the BOOT button on RP2040-Zero, and then connect it to a computer. Release the BOOT button. A new drive will open on the computer. Just drag the .uf2 firmware file (included in the repository) to this new drive. Done!

Power sources

DDX-1 was tested with the following power sources.

• RUIDENG DPS5020 power supply module (@13v) powered by MEAN WELL / Mornsun SMPS

• 3S "12V" Li-ion 18650 battery pack (RECOMMENDED!)

Planned Action Items

• Test DDX-1 end-to-end on MS Windows.

• Upstream major portions of the firmware to the TinyUSB or other projects.

Safety Tips

• Use a Common-Mode-Choke (attached between the rig, and the antenna) for safety against return/back RF.

Resources

• https://rf-tools.com/lc-filter/

• https://github.com/WB2CBA/ADX (the design that started it all)

• http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php

• https://github.com/kholia/ConsensusBasedTimeSync (receiver comes from here)

• https://github.com/kholia/Si5351-Module-Clone-TCXO/ (VFO comes from here)

• https://github.com/kholia/HF-PA-v10 (PA comes from here)

  Ruggedness philosophy: https://www.youtube.com/watch?v=4r7wHMg5Yjg

• https://www.kk5jy.net/three-wire-gp/ (recommended antenna - superb for DX)

• https://github.com/kholia/HF-Balcony-Antenna-System (another portable antenna setup)

• https://github.com/batica81/microphone-library-for-pico (Vojislav's work makes DDX-1 possible)

Joining In

Are you interested in such projects, and spam-free technical discussions?

If yes, you might be interested in joining the 'HAMBREWERS' Telegram group.

DDX-1 Learnings (26-Jan-2024)

The power section design is the trickiest part, and we are sure that it is NOT foolproof! With USB-C power in picture, we have to resort to power-on sequencing tricks additionally.

Our next design DDX-EVO-1 will be simpler and more rugged. We might also switch to a SMPS-styled 7805 replacement! Perhaps the CAPUF DC-DC converter modules work well without too much noise after heavy CLC filtering. We can keep the audio (output) path completely analog and separated from the MCU. Generous usage of PPTC fuses will be made in order to ensure graceful failure(s). CAT control over WiFi will be possible by shifting from RP2020-Zero to WeMos D1 Mini / ESP32-S3-Zero.