External reference frequency option for the HP/Agilent/Keysight 33120a arbitrary function generator
The calibration can fail with "value range" error. I solved it by repeating the calibration 86 multiple times changing the readout frequency in small and bigger steps.
The ocxo is a used Vectron C4550A1-0213 (10 MHz, 5V).
Kicad 3d render pic of the rev. 1 PCB.
The PCB is mounted on the side. You need to drill holes into the metal to mount the PCB. The pics show the rev. 0 with two bodge wires.
Suitable isolated BNC jacks are Amphenol 112252.