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Old 29th November 2011, 01:57 AM   #1
Kuro is offline Kuro  Hong Kong
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Default How to measure Q factor of a x'tal?

I have a bag of low cost x'tal and I want to hand pick the ones with high Q factor.

How do I measure the Q factor of a x'tal?
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Old 29th November 2011, 02:18 AM   #2
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I think most crystals have a Q in the neighborhood of 50,000 or more. Not easy to measure accurately because you won't have the resolution to tune a generator by a small enough amount. I think Q is also equal to the voltage gain of a resonant circuit, but then you have the problem of measuring the gain without loading the crystal or over driving it. Do you have a sweep generator that covers the frequency range?
I may be barking up the wrong tree, but at least I'm barking!
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Old 29th November 2011, 08:43 AM   #3
Kuro is offline Kuro  Hong Kong
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I read that most crystals have Q value around 10K-20K. Someone I know is using a network analyzer and his own jig to measure the Q of a crystal with the crystal soldered in place of the test circuit. Unfortunately, he won't tell me how and beside, I do not have a network analyzer...
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Old 29th November 2011, 12:12 PM   #4
DF96 is offline DF96  England
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An alternative is to measure the motional resistance. This is a bit like measuring the ESR of a capacitor, but you have to do it at the crystal series resonance frequency.
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Old 29th November 2011, 07:57 PM   #5
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I know there are specialized test instruments to measure crystal parameters, but I don't know their operating principles. A decade or so ago I played around with an HP LCR meter and a few crystals, but concluded that accurate measurements weren't practical. The L/C ratio is very large, and the losses are very small, and the instrument just didn't have the sensitivity or resolution. (As I recall, I even experimented with adding some carefully measured series and parallel inductors and capacitors, in the hopes of detecting small changes that could be used to infer the crystal's characteristics.)

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