Over on the TechMinds YouTube channel Matt has been experimenting with using a PlutoSDR for QO-100 amateur radio satellite communications. The PlutoSDR is a low cost RX/TX capable SDR with up to 56 MHz of bandwidth and 70 MHz to 6 GHz frequency range (with mods). The PlutoSDR can suffer from frequency instability, especially when warming up, however on the latest model C/D PlutoSDRs it's possible to inject an external clock signal.
In his video experiment, Matt uses a Leo Bodnar GPSDO as an external clock source. A GPSDO is a "GPS Disciplined Oscillator", as it uses the accurate timing information found in GPS signals to create a high quality clock signal. Matt shows how to set up the GPSDO, and how to tell the PlutoSDR to use the external clock.
He goes on to show the effectiveness of the GPSDO with some transmit experiments.
To get a high accuracy clock signal from a device such as a GPSDO can be used for both the Airspy and Spyverter. Tim was able to find a very nice GPSDO from Leo Bodnar that comes with two clock separate outputs that can be configured to output any frequency between 450 Hz and 800 MHz.
The Airspy already contains an external clock input for 10 MHz, however the present version of the Spyverter contains no such external input. To get around this Tim carefully removed the oscillator on the Spyverter and then added a second SMA connector to connect to the GPSDO.
His final setup consists of the Leo Bodnar GPSDO outputting a 10 MHz and 120 MHz GPS disciplined clock signal that feeds the Airspy and Spyverter respectively. With this Tim found that he needed no initial offset and zero drift was noticed over two days of testing.
Finally Tim also writes that this Leo Bodnar GPSDO could just as easily be used to create a 28.8 MHz clock signal for an RTL-SDR, or any other SDR or upconverter that needs it.