Digital LCD watch accuracy tester

Watches tend to have better stability than wall clocks because for more than half the time they are clamped to roughly body temperature. Some kit today uses GPS when available to set its local clock.

I suspect it was a high Q tank circuit or a watch crystal tuned to nominal ~32768Hz to act as a passive amplifier of the stray radiation from the watch oscillator. Amplify to a logic level and count number of pulses over fixed time intervals from the stable reference source.

Unless you oven the crystal and calibrate it periodically you really want a better reference standard. MSF Rugby or equivalent is pretty good for 1 to 10000s delays so long as the dew has evaporated (after ~10am).

We used MSF as a time reference to discipline local Rb clocks for early low frequency interferometry and could easily detect diurnal shifts in their pulse timing due to loading of the transmitter when the ground had dew on it in the mornings and late at night.

You can do it the slow way over a few days against any cheap domestic atomic clock. I doubt if it is really worth the effort of making such a tester today. So many smart watches use the internet or GPS for time reference that they are already well synced.

The digital display probably emits EMI at the display refresh rate, that with any luck is tied to the frequency reference crystal (32 KHz).

The metal case will block electrical signals but not low-frequency magnetic signals.

So use a ferrite-cored solenoid to pick up magnetic signals and analyze them.

Joe

I'd expect that removing the back affects the oscillator frequency.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

More than likely... but a re test after adjustment and the back replaced would cover that. Could what I thought was a 'piezo' at the end of the probe just have been a piece of wire?

I think the coupling was quite possibly acoustic rather than electric.

Jeroen Belleman