A Mini SDR Receiver Using An Audio DSP

Software defined radio or SDR is the most exciting frontier in the field of radio, transferring as it does all signal functions from the analogue to the digital domain. Radios using SDR techniques can be surprisingly straightforward and easy to understand, and [Ray Ring]’s little SDR receiver manages to combine this with the novel use of an audio DSP rather than a computer to perform its SDR functions.

The front end is a conventional enough direct conversion design with an Si5531 clock generator providing I and Q phase-shifted local oscillator signals to a TS3A5017 analogue switch used as a mixer. An unexpected presence is an LTC6252 op-amp as an RF amplifier, but the special part comes after the I and Q baseband signals have been filtered. The SDR part of this receiver is an audio DSP, but it’s one that might not be an immediate choice. The Spin Semiconductor FV-1 is a dedicated digital reverb chip for musical effects boxes, but it comes with the feature that its internal DSP core can access custom code from an external ROM. [Ray] has written his own code for demodulation of AM, USB, and LSB signals rather than musical effects, and used the device’s left and right audio channels to process I and Q quadrature signals. The use of a single purpose chip to do something its designers never intended gives it the essence of a good hack, and we’re mightily impressed at his spotting the potential for an SDR in a musical effect. Hear it in action in the video below the break.

Meanwhile if the operation of a receiver such as this one is a mystery to you, we published a handy primer back in 2017.

Thanks [Ziew] for the tip.

18 thoughts on “A Mini SDR Receiver Using An Audio DSP

  1. Tha analog switch part of the design goes back to the Softrock SDR’s which used the si570 oscillator, and sent audio to a PC soundcard.
    The audio DSP is a remarkable feature, replacing the entire PC the Softrock needed for decoding the audio signal.

      1. I was looking at my Mini-3 Bat Detector since I’m adding a BNC connection so I can use for electromagnetic wave reception too (goes up to 160kHz) and not just the pressure waves with the transducer… and am reminded like I found before that after a certain date there are lot’s of SDR chips for typical AM/FM radio since I see the Mini-3 design is using an AM tuner chip TEA5551A and Kieth Maries was kind enough to provide a schematic and detail a little where “It is a somewhat unusual heterodyne detector, because the tuned oscillator (VC1 and T4; 295 – 445 kHz) operates at lower frequencies than the fixed local oscillator (QA1 and T1; 455 kHz).” “The U30 used a special capacitance microphone, and operated up to 200 kHz.” I’m guessing the older S-25 used the same special capacitance microphone.

        An neat article can be making or hacking something into a good wideband ultrasonic microphone/transducer.

        I’ve been reading about the si570 also since thinking about making a Specan spectrum analyzer as a project and at the least have been wanting to test a si570 and really more-so Si569, Si564, Si549 and ADF4351. Only the ADF4351 are on cost effective modules I’ve found so far.

        Neat SDR radio build for sure and circuit salad has some great posts for sure.

        @bob: “do you think it would be possible to add fm support to the dsp?”

        Or were you thinking something more complex logic related on the chip via firmware?

        1. sdr’s were always known to be flexible in modes but i should have phrased the question differently. did the author leave out fm due to lack of interest in bands above 20mhz or due to there being limitations in the circuit.

  2. Cute component use but when the DSP costs £11 ea even from distribution then you would be better off with an entire Raspberry Pi or something silly like any other SBC with much more puff and less cost

    1. Yes you can get a embedded computer and just load software on it and get a SDR dongle and avoid building anything, learning anything, optimizing size, power consumption. or tailoring the design to your specific taste..but you will save $3.74. Even better..I can just go online and use a WebSDR and spend no money! Wait a minute… this whole project was a pointless waste!

        1. What he meant was clear…the design is not worth the effort because of better alternatives..one of which he spelled out…of course in some contexts that would be true …but in others he is incorrect. It was condescending in tenor with the “cute” crack……He didn’t mention anything about DSP programming being easy so I am not sure what you referencing? In any case, it’s all good..I hope someone gets some value out of my experiments.

          1. > DSP programming being easy

            pointless and silly. Dedicated DSP chips nowadays make as much sense as mainframes, fax machines and pagers. Yes, it works, and might even still be in use in very niche applications, but technology moved on. Today DSP is considered legacy technology.

        2. DSP programining is still relevant, weither the platform you are doing it on is is a totally different question.
          Normally these DSP chips are very cost concious but this is still quite a price.
          That said if you have a tube of them on your bench then cool!
          Now an entire linux box can be bought for fiver then you need to look at what’s most practical

    2. Hi all What I meant was that A) I like the project ! B) Unless you are doing music widgets then you are unlikely to have this chip as a spare and a new one costs £12 in UK for what is basically a DSP chip .
      You can do more with one of those PIC32 DSP efforts again with integrated ADC/DAC and RAM and a more well known tool chain and al ower cost or even a PI zero which you could just use as an AUDIO DSP or even as an entire munging device using a RTL dongle (doesn’t do HF then though). I like the case and display feature best ! ;-)

      1. No I get it…I was just being a sarcastic in fun……..I think those are all valid points but just so you understand..I started with what will be the easiest path to a small unitized SDR…and went with this approach because it was really simple to develop code,..very small, low power, low noise too…plus the ADC/DAC’s are 24 bit and it’s a very quiet part. So yes I would love to see someone give me some ideas for a better approach and I would build it!

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