5 Replies Latest reply on Oct 8, 2019 1:26 AM by neuromodulator

    El cheapo SDR

    neuromodulator

      I've been playing a lot with my FPGA lately, its such a fun toy! I'm currently trying to participate in the computer vision/computer graphics Project14 with it, but got a bit sidetracked to test something different. As FPGAs can read or generate very fast signals, they are used a lot in higher end SDRs. Usually you would use the FPGA with ADCs, DACs, transceivers, mixers, PLLs, filters, antennas, and what not! But when you are an "el cheapo" you don't need any RF gadgets, just the plain FPGA board to program it and let its tiny pins act as crappy antennas!

       

      So I fed a 440 Hz test tone into an NCO, and let the magic happen:

       

       

      And it sounds pretty good!

       

      UPDATE:

       

      I made a few modifications and now it can be used to transmit Morse (which I really don't know).

      Here is a demo, left button turns the transmitter on/off, right button sends 750 Hz, ~150 kHz bandwidth FM modulated pulses.

       

        • Re: El cheapo SDR
          14rhb

          Looks interesting but to be honest, I'm not quite sure what you've done :-)

           

          It looks like the 440Hz digitise controls the NCO on the FPGA....that oscillates at 106MHz and is radiated as you say via the device pins/PCB tracks. Is your screenshot showing you demodulating that 106MHz RF carrier and recovering the 440Hz tone ?

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            • Re: El cheapo SDR
              neuromodulator

              Yeah the screenshot is from an "SDR surfer" (not sure how these programs are called actually), it shows at the top, the spectrum & waterfall around 106.3 MHz, and at the bottom, the spectrum & waterfall of the FM demodulation of top's "marked" block (which happens to be 192 kHz wide). .

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            • Re: El cheapo SDR
              mudz

              nice work.. Keep it coming..

              • Re: El cheapo SDR
                14rhb

                Hi neuromodulator ,

                 

                Nice experiment explaining what is going on, the video update is really useful. Are you able to change the NCO to place this on another RF frequency other than 106MHz?

                  • Re: El cheapo SDR
                    neuromodulator

                    Yes I could use other frequencies too, the FPGA board doesn't come with any RF facility, I'm just switching pin voltages from 0 to 3v3 quickly with specific patterns to generate the desired RF signal. The limitation is that I'm using a 240 MHz clock for this, so by nyquist I could generate signals between 0 and 120 Mhz. But since the signal is very "squarish" its got many harmonics, so I could use the harmonics to generate signals at frequencies higher than 120 Mhz. The reason that I'm using 106.1 Mhz is because where I live there are no strong FM signals in that frequency. Now in terms of bandwidth I could use as much as I want.

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