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.