[IRCA] Selecting XTALs for synchros

["Dan Strassberg" <dan.strassberg@xxxxxxx>]

Charles A Taylor wrote:

You simply must know that 99.9% of AM transmitters have a frequency trimmer
associated with the crystal.

------
Yes, but the existence of a trimmer is not 100% relevant to the discussion.
Even if you have to trim the frequencies of the two crystals to get the
frequencies within, say, 0.5 or 0.1 Hz of each other, if they don't drift in
the same direction at approximately equal rates as a function of time,
temperature, and any other relevant parameter (oscillator power-supply
voltage, for example), the frequencies soon enough won't match well.

I suspect that the right way to pick crystal pairs for synchros is to put a
group of crystals into identical oscillators in an oven over a period of
several weeks and log the oscillator frequencies first at various
temperatures and then at a couple of constant temperatures over long
periods. From the data, you should be able to pick the pairs that match
best. Install one matched pair and save at least one more pair in case of a
failure.

I would also say that if the transmitters aren't the same model, you might
be well advised to retrofit one of them with an oscillator circuit identical
in every respect to that of the other. Also, unless the oscillator circuit's
power-supply-rejection ratio is really good, provide local regulation for
both oscillators. If you can't ensure that the thermal environment for the
oscillators in the different-model transmitters is comparable, you probably
should also pay the extra cost of temperature-controlled oscillators.

This is a classic analog-circuit problem that AFAIK has to be solved with
rather tedious old-fashioned technques. I don't think there are any
shortcuts. Well, maybe use a clock received from a satellite to drive local
direct-digital-synthesis-based frequency dividers. I suspect that kind of
high-tech approach has problems of its own that I have never thought about.
But if you don't know about the problems, it sounds great;>)

--
Dan Strassberg, dan.strassberg@xxxxxxx
eFax 707-215-6367




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