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Measuring clock quartz frequency accurately

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ArthurDent:
Years ago I faced the same problem with a watch I had with a trimmer cap on the time base and it was consistently drifting in one direction. What I did was connect my scope ground to the metal watch case and trigger the scope from a synthesizer set to 32,768hz. I cranked the scope input channel up to maximum sensitivity and held the probe tip close enough to the back of the open watch case until I saw (along with lots of noise) the crystal frequency drifting either right or left.

I adjusted the trimmer cap to get the drift to a minimum then, just to make sure, placed a piece of aluminum foil over the back with a small hole in it to stick the probe tip through. I needed to double check the drift to make sure that when I replaced the watch back cover that it didn't cause the crystal frequency to shift from added cover capacitance. This worked well and the watch was much more accurate. If your watch is on your wrist then it kind of has a temperature stabilized environment for most of the time.

edpalmer42:
One thing to keep in mind about watches.  I've noticed that quartz watches and clocks tend to run fast rather than slow.  I thought about that and decided that it made sense.  If your watch is fast, you'll be early to your meeting or appointment rather than late.  So you might want to nudge your watch a bit fast, but still within the typical spec of 30 sec./month.  If my math is correct, that means setting it to more than 32.768 KHz, but less than 32.76838 KHz.

Ed

JBeale:
I've noticed the same thing, every (digital) wristwatch I've ever had ran slightly fast, usually about 1/2 to 1/3 second per day. I believe people tend to think a slow watch is broken, but a watch going the same amount fast is still ok.

magic:

--- Quote from: Axk on April 09, 2019, 10:29:07 pm ---Considering it is only 32KHz, I suppose I can use a FET input opamp and measure the output with a passive probe?

--- End quote ---
Very few FET opamps have less than 1pF of input capacitance and even that might be significant compared to the 10pF or whatever normally there.
Quartz clocks are also sensitive to temperature. Just because you measure it now doesn't mean it will be the same at night or next week.
I wouldn't necessarily assume that the quartz in some random cheap scope is much better than the quartz in a wall clock.

I second the recommendation to just run it for a full day or a few and compare with a good reference (GPS, radio, NTP).
I once fixed the PSU of a mains-powered clock and noticed that it has some dependence on power supply, easily visible after a few days. So I tuned it by running it for a week at a time and adjusting the PSU.

Zero999:
With digital clocks/watches, I believe the calibration is done to some degree in software. The oscillator is built so it's slightly fast and at the time of manufacture, then the exact frequency is tested and a calibration factor is programmed into the IC. If it's found to be two seconds per day too fast, then 20 seconds of every day are made to be 0.9s, so it the correct timing is achieved. It's also true there's probably some bias towards too fast, than too slow.

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