How to measure Q factor of a x'tal?

[Kuro]

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?

 

[Conrad Hoffman]

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?

 

[Kuro]

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...

 

[DF96]

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.

 

[dchisholm]

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