Agilent / HP / Keysight 5061A

It's sold state components and the closed loop, self checking control circuit yield an accuracy of ±1 x 10E-11.

This units high accuracy is combined with excellent reliability, making the this ideal for use as a reference standard in national laboratories plus many other applications requiring accurate frequencies.

Its front panel controls provide a circuit check switch and a meter for monitoring performance.

The front panel lights are provided to indicate any interruption of continuous operation, and that the crystal oscillator is locked to the atomic resonance.