A QRP Labs QDX digital transceiver kit is on order but in the mean time I was looking for information on the technique Hans uses to modulate the Si5351 from a computer generating FSK modes, such as WSPR. The manual mentions that the way it works is to measure the frequency of the audio coming in from the computer and then directly set the Si5351 frequency. This means there's no mixing and no opposite sideband. Very clever!
It turns out that this idea is not new and dates back many years, even implemented on a Z80.
While QRP Labs keeps much of their source code secret, another vendor - QRP Guys - have a transceiver that uses the same system and they supply the source code.
I have just finished building and debugging their Digital FSK Transceiver. It's not as neat as the QDX as it uses analog audio in and out. Transmission is by VOX so it's not a huge problem.
The kit went together well but I couldn't get it working even though the display showed the right things. It didn't auto-detect the low pass filter band (there's a resister divider to signal the band by voltage).
I downloaded the source code and wrote to QRP Guys to tell them they linked to the wrong source. I was wrong of course and failed to read the file called README First which explained that the same source file is used for a few different projects.
Ken LoCasale replied with LOL after I realised my error. Nice quick response from the QRP Guys.
Unable to get the kit working, I built the code and flashed an Atmel ATmega328p chip in an Arduino Uno. When I swapped that chip into the VFO board the receiver sprang to life.
As you can hear, SSB reception is good but the 1kHz tuning step is a little large for SSB reception. I haven't calibrated my oscillator so that might help.
Driven with audio from WSJT-X I saw 38V peak to peak into a 50 ohm dummy load running on 13V.
After calibration I had an FT8 contact without any problems.
Interestingly the code that made version 3 of this project possible was shared by Kazu Terasaki AG6NS. There's a discussion here. His GitHub page has several projects and there's a branch with the official QRPGuys version.
I'm very much looking forward to the QDX which has a single USB interface to the computer and an apparently very good SDR receiver. This is a good kit and fun to tinker with.