Re: Matching Xtals for Filters

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From: Ed Pacyna (pacyna@auratek.com)
Date: Wed May 28 1997 - 17:15:45 EDT


>Attemptng to measure each crystal by ear using just a receiver is
>not going to work well. However, if you wish to match crystals,
>then you can build two identical oscillators, use one as a reference
>and listen to the beat note between the two in your receiver.

Ladder filters require that all loops be resonant at the same frequency.
This does not however always mean that all crystals should be matched to
the same frequency. In some designs (like the COHN) the crystals should be
matched. In this design, all of the coupling capacitors are the same value
and the loops stay in tune.

However, other pass-band and stop-band responses or impedance matching may
be desirable. In these cases the shunt coupling capacitors will be
different values and the loops will resonate at a different frequencies .
They must be forced back to the same frequency. This is done either by
using additional tuning capacitors in the network toplogy (in series with
the crystal) or staggering the crystal off-sets frequencies (typically the
outer loops will be resonant at higher frequencies and require a lower
frequency crystals to compensate).

Selecting crystals by ear (either relatively or comparatively) is neither
practical or productive. For example the most direct method of picking the
4 best crystals out of a lot of 10 would be by measurement with a frequency
counter. Ten measurements and your done. This applies in the case of
selecting the 4 nearest to the same frequency or to the required off-sets.

Another possibility is to use a receiver (with digital dial) to measure and
record the 10 frequencies.

73

Ed, W1AAZ


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