These are several photos for a GPSDO, GPS satellite disciplined oscillator, that gives near atomic clock frequency accuracy. The electronics locks onto the 1 PPS, 1 pulse per second, signal and over a long period of time and adjusts the frequency of an oven stabilized quartz oscillator to the desired value. In this case it's 10MHz. Frequency test equipment have provisions for an external frequency source to use in place of the usual internal one. I believed somebody tested one of these and measu