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?
How do I measure the Q factor of a x'tal?
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 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...
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.
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.)
Dale
Dale
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