[time-nuts] Ancient OCXO in scope calibrator.
jmiles at pop.net
Tue Sep 9 16:07:41 EDT 2008
> Following the good advice I received about increasing the value of the
> trimmer capacitor, I replaced the trimmer which was originally
> 3-12pF, with
> another one which was 2-22pF.
> This has given the additional adjustment range to allow me to pull the
> crystal to meet the specification.
> It's not mega-stable, varying between 9.9999619 MHz to 10.0000232 MHz over
> the course of a minute or so as the thermostat cycles, but
> certainly within
> the 10ppm specification and varying up and down by about 6-7ppm. This is
> before the full 2 hour warm up period is complete, but I don't things will
> change much over the next 45 minutes or so. The stability seems to be
> improving as the warm up continues which is good.
> I may be able to tweak it so the variation is more evenly balanced around
> 10.0000000 MHz, but the trimmer adjustment is pretty sensitive.
> Still don't understand why probing with a 10Meg, 1pF load kills the
> oscillation stone dead until the next power cycle.
You might have an intermittent solder joint or component lead that's being
flexed when you touch various points with the probe. Does a plastic
alignment tool cause the same behavior?
Or, more likely, there's so much loss in the circuit due to aging/component
failure that it won't restart oscillating on its own without going through a
-- john, KE5FX
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