HP Z3801A GPS Disciplined OscillatorThe HP Z3801A is probably the most common GPSDO out there. Most of the performance information published about these units has been comparing the raw oscillators. I thought it would be good to show the performance in normal operation -- in other words, while being steered by GPS rather than free running.
The plots below show two well-aged (running continuously for a couple of years) Z3801As measured against one another. One of the units is known to be a bit better than the other, but both perform well within their specifications. One plot shows both units running under GPS discipline, while the other shows one unit in holdover mode with the GPS antenna disconnected.
It's very apparent where the GPS steering kicks in (or doesn't)!