Build Your Own Software-Defined Radio

We should be able to build a complete, simple direct conversion receiver on a chip using current IC technology. The possible receive frequency would be limited by the IC technology used for the mixers. Every day, it seems, operating frequencies move a little higher, so we can count on regular improvements.

In an article published 11 years ago in QEX Magazine, Gerald Youngblood shows us that putting a whole radio on a chip would be easy.

In a conventional superhetrodyne receiver, you mix the local oscillator signal with the incoming RF to produce an intermediate frequency (IF). Then you amplify that as needed and detect the result. This is applicable to the detection of AM, FM, or SSB signals. In a software-defined radio (SDR), one technique uses a direct conversion (no IF), so the output of the mixer is the baseband signal. Here's an example that uses a low-pass filter to limit the passband to no more than 1.5kHz. Thus, the received signal bandwidth is 14MHz to 14.0015MHz

(Source: QEX Magazine, July/Aug. 2002)

(Source: QEX Magazine, July/Aug. 2002)

Unfortunately, this simple receiver would also receive 13.9985MHz to 14MHz just as well. This is the image frequency (band). To work around this and to avoid the need for super selective tunable filter networks at the front end or an IF amplifier section, an IQ converter section provides an excellent solution. IQ stands for in-phase and quadrature (shifted by 90° or sine and cosine signals). Using this quadrature sampling mixer section and two low-pass filters, we can get baseband signals that can be fed to two ADCs.

(Source: QEX Magazine, July/Aug. 2002)

(Source: QEX Magazine, July/Aug. 2002)

Once the signals are in the digital domain, further processing is done. In the original QEX article, Youngblood makes use of the sound card on a PC and available software to do the fast Fourier transform and inverse fast Fourier transform.

(Source: QEX Magazine, July/Aug. 2002)

(Source: QEX Magazine, July/Aug. 2002)

As an example of how simple some of this circuitry can be, Youngblood shows the front-end sampling quadrature detector.

(Source: QEX Magazine, July/Aug. 2002)

(Source: QEX Magazine, July/Aug. 2002)

There aren't very many parts, and the ones that are there are dirt cheap. Youngblood was using this as the basis for a SDR that could operate from 100kHz to 54MHz. He was also making a transmitter to go with it, but that part is easy — just a simple frequency synthesizer for the ham bands from 160m through 6m.

Here's one more look at a block diagram of an SDR, this time from an article in the March/April 2012 edition of QEX Magazine.

(Source: QEX Magazine, March/April 2012)

(Source: QEX Magazine, March/April 2012)

Again, we see that it's pretty simple. The whole thing calls out for fabrication on a chip.

Have you done any work on SDRs? How well did they work?

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9 comments on “Build Your Own Software-Defined Radio

  1. eafpres
    October 15, 2013

    One of the challenges of really broad band SDR is that somewhere in there you need a couple of antennas (Tx, Rx on both ends, or Tx/Rx combined one on each end).  There are some difficulties if you need high performance–the maximum bandwith for a given gain and mismatch may not be enough in a single, static antenna.  People have worked on this for a long time, and some active antennas are going to make their way into things like handsets, allow tuning and/or switching to increase the effective bandwidth.  That of course requires sensing and feedback, adding complexity.

  2. n47
    October 16, 2013

    matching specs of conventional superhetrodyne receiver difficult

  3. paulmcv
    October 16, 2013

    G'day Brad

    Not quite one chip, but pretty amazing anyway:

    You can do SDR from 50MHz to 2GHz with a $20 DVB-T digital TV tuner dongle…

    Search for RTL SDR and be prepared to be amazed!

  4. Brad_Albing
    October 16, 2013

    @eafpres – fortunately (for me), if I ever design and build an SDR, it'll just be a receiver, thus simplifying at least part of the design.

  5. Brad_Albing
    October 16, 2013

    @n47 – we'll see….

  6. Brad_Albing
    October 16, 2013

    @paulmcv – initially I was interested in 100kHz to 50MHz – just a general purpose communications receiver for listening to… well, whatever was there. But 50MHz to 2GHz would be cool too.

  7. Brad_Albing
    October 16, 2013

    @paulmcv – I'll check those out now. Thanks.

  8. WKetel
    October 19, 2013

    Unless the prices have dropped quite a bit fairly recently the dongle to do a decent radio is quite a bit more than $20. That one is a TV only type and does not serve well as a radio.But the radio type ones are less than $100, nota bad price for a SDR. But the form factor is just plain BAD, with the thing plugging in to a USB connector. What would work quite nicely would be a package that would slide into those smart-card connectors on the sides ppf older computers.The connection would be well protected and the device could stay attached to the computer during transport without fear of damage.

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