Archive | March, 2020

Corona Virus Quarantine Project! – The QSD Receiver

29 Mar

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It’s been a while since my last post. Over the past year or so, I’been quietly running simulations of circuit ideas I come up with and designing PCBs to test out those ideas. Consequently, I now have a bunch of boards for a lot of radio sub-systems and I’m finally getting around to putting some together to make functional radios. During this Corona Virus quarantine, I decided to put some of them together to form a high performance QSD phasing receiver. It’s now somewhat functional, but I want to see it through and eventually turn it into a multi-band transceiver.

Check back here regularly as I’ll be updating all the pages as I progress.

The QSD Receiver Project


The display running the system GUI

I’m especially smitten with the quadrature sampling down-conversion (QSD) method of receiver design, with hardware as well as DSP backends. In the coming days, I’m going to roll out a receiver (upgradable to a transceiver) using the ‘phasing’ method of demodulation/modulation. I have a cute mini-Altoids size demod board that implements a QSD down-converter and an  on-board Si5351 LO module directly outputting quadrature LO drive for the QSD.  I also have a board for a hardware phase-shift network as well as boards for the audio stages. I wrote my own ‘slim’ Si5351 handler functions for that (no pesky libraries). Another board has a color TFT display with 2 rotary encoders as well as touch-screen and hardware push-buttons.

It’s all controlled by an Arduino Nano board that piggy-backs on the display board. Code is written for the system and today I put all the pieces together and demodulated a 40M signal from my signal generator, outputting I-Q audio signals to my scope. These will go to a hardware audio phase shift network, an audio lowpass filter an an audio power amp. Alternatively, the I-Q signals can be inputted to an Arduino Teensy to implement a full DSP receiver. All of the PCB blocks have been built and tested, so now it’s a task of system integration, testing, tweaking and implementing changes.

I’ll start off in a few days with photos and descriptions of some of the boards, then regular progress reports as the system comes together. I’ll post schematics and PCB gerber files as soon as each block matures to the point where I think others can reproduce it. Along the way, I intend to write some ‘tech notes’ about how things work in a non-mathematical and intuitive style. I can’t promise how quickly I’ll progress, especially with Summer and my outdoor activities, but I’ll be spending more time on this as I ease into retirement from my company.

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Update: April 13, 2020

As of today, the system is built up to the audio output, minus the final 8-pole lowpass filter and AGC loop. All the printed circuit boards are mounted on a temporary breadboard. Sensitivity (by ear) is around 0.3 uV in a 2.5 kHz SSB bandwidth. There is some digital noise down around the microvolt level which I’ll be working on. Next major step is to build up the 8-pole audio lowpass filter and insert it in the signal chain. This will set overall system selectivity and provide very sharp skirts. I also want to start making accurate instrument measurements of performance, which I’ll post here.

Stay tuned!



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