Bringing some discipline to an SDR transmitter


The proliferation of Software Defined Radio (SDR) technology has been a boon to RF enthusiasts. 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 of them from where we’re sitting, in fact.

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

Setup is as simple as plugging the oscillator output into the SDR’s external clock input using an SMA to UFL jumper, and tweaking the SDR and oscillator settings. Not all SDRs will have an external clock input, of course, so your mileage may vary. But if your gear is properly equipped, it seems like a great way to get the optimum frequency – the video below shows just how far unruly SDR can drift.

Like any good ham, [Tech Minds] does its part to keep its signals clean and focused. Its primary use case for this setup will be operating QO-100, amateur radio‘s first geosynchronous satellite transponder. We have to say that we hams living on the two-thirds of the globe not covered by this satellite are dying to have a geosynchronous bird (or two) of our own to play with like this.


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