Bringing Some Discipline To An SDR Transmitter

The proliferation of software-defined radio (SDR) technology has been a godsend for RF hobbyists. SDR-based receivers and transmitters have gotten so cheap that you’ve probably got a stick or two lying around your bench right now — we can see three from where we sit, in fact.

But cheap comes at a price, usually in the form of frequency stability, which can be prohibitive in some applications — especially amateur radio, where spectrum hygiene is of the utmost concern. So we were pleased to see [Tech Minds] tackle the SDR frequency stability problem by using a GPS-disciplined oscillator. The setup uses an ADALM-PLUTO SDR transceiver and a precision oscillator from Leo Bodnar Electronics. The oscillator can be programmed to output a rock-solid, GPS-disciplined signal over a wide range of frequencies. The Pluto has an external oscillator input that looks for 40 MHz, which is well within the range of the GPSDO.

Setup is as easy as plugging the oscillator’s output into the SDR’s external clock input using an SMA to UFL jumper, and tweaking the settings in the SDR and oscillator. Not all SDRs will have an external clock input, of course, so your mileage may vary. But if your gear is suitably equipped, this looks like a great way to get bang-on frequency — the video below shows just how much the undisciplined SDR can drift.

Like any good ham, [Tech Minds] is doing his bit to keep his signals clean and on target. His chief use case for this setup will be to work QO-100, amateur radio’s first geosynchronous satellite repeater. We’ve got to say that we hams living on the two-thirds of the globe not covered by this satellite are just dying to get a geosynchronous bird (or two) of our own to play with like this.

16 thoughts on “Bringing Some Discipline To An SDR Transmitter

    1. There was a presentation at Black Hat a few years ago, demonstrating a spoofing attack on GPS. Useful for changing time on a target device (e.g., making use of a stolen, expired certificate or something), or for messing with location-aware apps.

      The challenge is partly calculating the correct phase differences between GPS satellites, which includes relativistic compensation and a few other interesting things. It’s not that hard to spoof the GPS satellite… what’s hard is spoofing the moving constellation of various satellites to make the target device resolve the desired values.

    2. Most GPS have a nmea port / format.. includes Julian day orbit# ,time stamp of position .. and other usefull data ..iono even ….the chip rate output varies per receiver. Chip rates on the birds standardized so l1 l2 rxcvr can decode and solve pseudorange after processing..not needed for frequency ref. Can watchit stream by with windows winterm ..a old terminal program that’ll still run on new pc..
      Yea it is bit banging…filters available

    1. All it would take is a cheap GPS chip with 10kHz or higher clock output. Some GPS chips have ability to reconfigure the frequency of time pulse signal output from about 0.25 Hz to 10 MHz. The short term jitter on that is amazingly bad – really really really bad, but the long term stability is absolutely fantastic. So you would combine that with a TCXO which have very low short term jitter, with reasonably good long term stability. And the best way to mash the two together is with a DSPLL (a Jitter Attenuator, any part similar to a AD9547/AD9549 or Si5397/Si5396 ).

      For a board running at 10 MHz, to possibly 750 MHz, to keep the signals as clean as possible, you probably need at minimum a 4 layer PCB. The costs all add up and by the time you add in and program a microcontroller (UART/SPI may be needed to configure the GPS TIMEPULSE output at power on, also to configure the Jitter Attenuator parameters on power up and possibly store and update long term drift values), add a quality metal box, factor in initial production problems. Add 30 to 40% profit margin for yourself, 30% profit margin for distributors, (without them you will sell far fewer devices worldwide, they typically add value by being a local filter/buffer between you and some major nightmare customers) – it is easy to reach $150 without a lot of effort. The real question for most people is the “fun” of creating their own device worth more to them than the convenience. People who frequent here are more likely to have the knowledge and skill to build to their own exact specification and needs rather than choose convenience.

  1. Just use a cheap but solid 10MHz crystal oscillator with a PLL and a cheap GPS receiver. That way, you have the cleanest and most reliable source for any frequency you like, even if the GPS reception isn’t that good. But not for 150 €/$ .
    Sounds more like an advertorial to me…

    1. Ya I am not sure why they don’t do this. You should be able to use a function generator and get solid frequencies. I wonder what the level of precision they are try to achieve is.

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