Re: Matching Crystals For Filters

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From: Mike Branca (w3irz@att.net)
Date: Tue Nov 21 2000 - 15:57:07 EST


Well Ed, theoretically you should use a pierce crystal oscillator because it
puts the crystal in the series mode that you will use in the crystal filter.
The series and parallel resonance of a crystal can be 1 or 2 khz apart but
because you probably are using crystals from the same batch they will likely
have the same characteristics so it probably does not matter the oscillator
type. I always use the series oscillator for this use (see QST H&K May 1978
P38). You should use the 3 matched crystals that end in 26 or 27 for the
filter as they will be matched within 10 cycles (your figures) and
(theoretically) times 10 for the filter bandwidth of 100hz minimum. I find
it easier to use varactor diodes than to calculate capacitor values and
besides you get a variable bandwidth filter (to me this is a great asset)
that way (see QQ Jan 2000 P 29 for my 4 mhz filter). As for the RX BFO and
TX CXR crystals it doesn't matter which ones you use as you will use a
"rubbering circuit" to put these frequencies exactly where they need to be.

I will be glad to help you along the way.

Mike Branca W3IRZ in Conyers Georgia


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