# Measuring the resonant frequency of a crystal

I am trying to measure the frequency of crystals in my box. I have a signal generator set to output a sine wave at 5V p/p. The output is applied to the crystal and then the other lead of the crystal is fed to the oscilloscope. The ground of the signal generator and the oscilloscope are connected together. I changing the frequency of the signal generator around the rated frequency of the crystal. I was hoping to see large amplification at the resonant frequency. Instead I am seeing nearly the same wave amplitude at all frequencies. My signal generator is capable of generating frequency accurate up to two digits after the decimal. Are all my crystals bad somehow?

• Please show a schematic. If you don't have a load on the crystal, you'll almost certainly never see a response. Aug 25, 2020 at 21:16

Are all my crystals bad somehow?

Probably not. The "issue" with crystals is that the quality factor: Q of a crystal is very high meaning it is somewhat challenging "see" the resonance.

I've built a simple crystal tester myself and it is based on a circuit, which is a standard CMOS logic based crystal oscillator which is present in many ICs including microcontrollers.

source of that picture

It states "20 MHz" for the crystal but in my experience this works with 32 kHz crystals up to around 30 MHz. If you make the circuit small and use faster inverter than the 40106 (for example the 74HC04) then 50 MHz might be possible.

Then using an oscilloscope or frequency counter you can determine the resonance frequency.

• You can -- with a proper setup, and a really good signal generator -- do it using a signal generator. But the above circuit is much more universal. Aug 25, 2020 at 21:17
• @TimWescott I was thinking maybe feeding a very steep pulse through a resistor to the crystal and looking at the response using the scope might do it but since I've never tried that and seen that working, I didn't mention it. Indeed above circuit "just works" and is handy for checking crystals. Aug 25, 2020 at 21:20

When looking at lower frequency quartz with high impedance, the resonance is clearly visible.

Higher frequency quartz is better viewed in series with a low impedance generator.

Thus, we see the series resonance frequency, which is minimally different from the parallel.

• This is what I meant by "have a load on the crystal" in my comment to your question. You also need a signal generator that's stable enough that you can find the resonance -- it'll be 1kHz wide or less, at the crystal resonant frequency; a typical bench signal generator will have a lot of problem hitting it and staying there. Aug 25, 2020 at 21:32
• @TimWescott You're right - typical function generators are far too unstable: their residual frequency modulation sweeps right through resonance. Even a medium-quality RF generator might be too "squirrelly". Aug 26, 2020 at 1:29
• @csabahu Thank you. This answer opens a good deal of experiment options. Aug 26, 2020 at 6:54