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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 measured the frequency, against a high precision atomic clock, at 9,999,999.9998 Hz once it stabilized.

Radio shops and labs use frequency standards, not necessarily this one, with a distribution amplifier to all their scopes, frequency generators, frequency counters and spectrum analyzers. This way any drift or calibration differences between them is eliminated when making frequency and timing measurements.