OCXO Timebase

New time base module for Philips frequency counters using a quartz oven

When measuring with my Philips frequency counter PM6612, I always noticed small deviations. This is no surprise when using the basic quartz time base PM9677 - it has never been calibrated since I bought it and is also subject to deviations due to temperature changes and aging of the quartz. Philips offered other versions of the time base for the counter at that time. The accuracy could be increased with a temperature compensated quartz oscillator (TCXO) or better quartz oven (OCXO). Unfortunately these exchangeable modules are in great demand and can only be obtained at high cost or are even not available at all. Therefore I decided to build one myself.

Picture: New and old time base

The heart of the new timebase (left board) is an Ovenized Crystal Oscillator (OCXO) from Micro Crystal. I got it at a well-known online auction house for little money from a Chinese Surplus dealer. It is the OCXOVT-BV5. This one only needs a supply voltage of 5 volts and then outputs the 10 MHz clock. Another pin can be used to fine tune the frequency with a control voltage (versions ending in V5).

Picture: Circuit diagram

Only a small amount of external circuitry is required. The frequency counter provides two voltages: A permanently present, unregulated DC voltage between 12 and 25V and a regulated 12V voltage that is only present when the power is on. The original OCXO module uses the unregulated voltage to permanently heat the quartz oven. I therefore create my 5V out of this voltage. Thus the counter is always ready for use with high accuracy. Furthermore I put a tristate buffer behind the oscillator. The clock is thus only activated when the counter is switched on. This is not mandatory, but I thought it was just nicer to do so.

On the backside there are some SMD capacitors and resistors.

Picture: Backside of the board

The module is only plugged into the counter and can easily be mounted.

Picture: OCXO module in the counter

Now the oscillator has to be calibrated. It was heated in the frequency counter for several hours and then adjusted to exactly 10 MHz with my father's GPS Disciplined Oscillator (GPSDO) (right side of this picture). A later comparison with my Rubidium frequency standard confirmed the correct alignment.

Picture: Adjustment with GPSDO

With my module, Philips frequency counters like PM6611, PM6612, PM6613, PM6614, PM6615 and PM6616 can be easily and inexpensively equipped with a very accurate reference. The OCXOVT-BV5 has a short term stability of less than 5E-10 @ 0.1-30s (typ. 5E-11 @ 1s) and should be as good or even better than the original quartz PM9690!

Have a lot of fun with your rebuild!

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An attentive reader notes that the control voltage of the crystal oven is taken from the voltage regulator. A separate voltage reference should be used for this purpose since the 7805 has a strong temperature drift. There are also OCXOs with reference voltage output.