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Measuring clock quartz frequency accurately
Axk:
I've an old quartz clock and it has (presumably) an adjustment capacitor to adjust the frequency.
Is there a way to measure the frequency from the quartz accurately enough for good adjustment?
I've tried measuring the outputs with a 10M oscilloscope probe but it appears to completely throw off the low power oscillator to the point that the clock stops, let alone measuring it accurately.
helius:
You need a probe with low input capacitance in order not to disturb the crystal.
An active FET probe should do the business.
Axk:
Considering it is only 32KHz, I suppose I can use a FET input opamp and measure the output with a passive probe?
edpalmer42:
Another trick you can try is to clip a resistor onto the end of your scope probe and then probe the circuit with the open end of the resistor. The resistor isolates the circuit from the capacitance of the probe. The value will depend on the signal amplitude. Try 1M and see what happens.
Ed
golden_labels:
Run the clock for a day and compare to a some reference. Adjust. Repeat until obtaining the target accuracy. You may start with hour-long tests if the error is already very high — but with a test lasting only a hour you can’t get better than around 0.05%.
Measuring frequency/period accurately with a scope alone might be pointless. If you would manage to measure it at 0.1%, that would still mean… 1.5 minutes of error per day.
1 hour test: error ≈ 48s / day, 24 minutes / month
1 day test: error ≈ 2s / day, 1 minute / month
1 week test: error ≈ 300ms / day, 9s / month
Since this is already in the range of single PPMs, I guess that going below that makes little sense: the error from temperature and vibrations will probably take over.
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