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RPI Pico2 SDR - First Attempt

scottiebabe

Have been experimenting with implementing an SDR on a RPI Pico 2.

I put together a bare bones SDR for narrowband FM

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Diagram created with https://app.diagrams.net/ Source: (remove the .txt extension)/cfs-file/__key/communityserver-discussions-components-files/91/pico2sdr.drawio.txt

The input is IF is oversampled, frequency translated to baseband, filtered, and then decimated by a factor of 3. A polar discriminator demodulates the narrow FM signal and is low-pass filtered and output to an I2S DAC.

As a first test I am applying a narrowband FM signal directly to the ADC input of the pico2

image

The I2S DAC I used has a class-d BTL output. I had to add an LC filter to the output for the scope to display the audio output and not the class-d pulse train.

A few different input signals and demodulated output shown on my o-scope

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When the input is below 10 mVpp the SDR starts to struggle.

The output amplitude is a function of the frequency deviation, not the input amplitude. 

Pretty cool.

  • Cool!, couple of years ago I didi this with a PSOC (https://www.agri-vision.nl/portal/projects/5-software-defined-radio-with-psoc) and somewhat later with a Teensy, for the latter I needed a codec to get the audio into the Teensy.

  • in reply to gpolder

    That is awesome! Thank you for sharing, my google has struggled to turn up sdr projects. I will have to borrow some ideas. Neat that the psoc has hardware accelerated digital filters. On the Pico2 the floating point co-processor has a multiply accumulate instruction (VFMA) and a divide instruction, otherwise it would be a struggle 

  • Impressive.  Just so I am on the right track, the blocks depicted within the dotted lines and labeled "RPI Pico2 Software Defined" are all implemented as code being run on the Pico2?

  • in reply to dang74

    I don't want to pre-empt the actual answer, but generally, yes : ) once it's been ADC'd, all the math can happen internally in code.

  • in reply to scottiebabe

    A friend pointed me to this project recently:

    https://github.com/dawsonjon/PicoRX

    However there's no nice flow chart so it's not easy to follow (I didn't get time to peek at the source code yet).

    Hardware-wise it's very cut-down, they are trying to do everything in the Pico, whereas personally I would have used I2S DAC too, and maybe Si5351 or some other external freq synth.

    Looking at the schematic, they went with zero-IF it seems, since they generate IQ externally to the Pico. I didn't think that would work, but I guess the Pico ADC is fast enough to sample both close enough. I think they are using Pico and not Pico 2 so that must be impacting performance (just speculating, I have not tried that project). 

    For 90 deg oscillator output I don't know if say Si5351 can be programmed to do that, it would then be a useful freq synth for zero IF. For a PC-based SDR project I used D-type flip-flops to generate quadrature from a single clock, but that's limiting, since it won't operate at the high speeds for say 144MHz Rx. But personally I think low-IF is the way to go when there's only one ADC in the pico.

    image

  • in reply to shabaz

    Thanks   I was pretty sure that was the case... just needed assurance.

  • Tuning to 462 MHz

    I tuned my LO such that my FRS radio was down converted to a low IF

    image

    I have a T5725 from years ago.

    Pressing the paging/call tone button

    Unfortunately the local weather radio station officially went off the air April 1, otherwise I would tune to that.

  • in reply to shabaz

    Thank you so much for the project link!

    I believe the author has shared some more details on their personal site

    https://101-things.readthedocs.io/en/latest/radio_receiver.html 

    I will definitely have to try it out once I find some low rdson quad switches 

    I wanted to avoid the complexities of IQ sampling so its easier to tune in VHF. There may be tuner ic's with IQ outputs. I am trying to just start simple.