Quite a while ago I had written about how I upgraded oscillator of my Agilent/HP 53132A. I have recently bought a broken Fluke/Philips PM6680B Frequency counter. Though i was able to quickly fixed issues with the unit by replacing power supply capacitors, the unit still has really crappy original reference oscillator. So I decided to make my own PM9692 Ultra High Stability Oven Time Base with CTS 196 OCXO.
Module is relatively simple compared to Agilent 53132A OCXO, Board only has a switching regulator to step down from 12V/24V input to Oven Oscillator Operating Voltage which is 5V. Board also has a 20 Turn Trimpot to fine adjust oscillator frequency for calibration. High resolution PDF Schematic can be found on Projects github https://github.com/circuitvalley/PM6680_PM6681_OCXO_time_base_option
CTS 196 oscillator has specifications for Temp 5PPB while 1ppb/day and 100ppb/year aging
Option board is installed on BU105 Header which is 10 pin single line header. This part of PM6680B schematic shows BU105.