It used to be any good electronics experimenter had a bag full of crystals because you never knew what frequency you might need. These days, you are likely to have far fewer because you usually just need one reference frequency and derive all the other frequencies from it. But how can you test a crystal? As [Mousa] points out in a recent video, you can’t test it with a multimeter.
His approach is simple: Monitor a function generator with an oscilloscope, but put the crystal under test in series. Then you move the frequency along until you see the voltage on the oscilloscope peak. That frequency should match the crystal’s operating frequency.
It is interesting that because of the resonance of the crystal, the voltage on the scope can be much higher than the input voltage from the signal generator. This is a simple test, but effective. Of course, you could also have a little oscillator and see what the crystal does in a real circuit.
We’ve tested crystals before with a network analyzer and we even made a video that shows essentially the same test [Mousa] uses, although it was on an LC circuit, not a crystal. You do need to be careful you aren’t operating the crystal in an overtone mode by accident, although presumably if it works on a harmonic, it should work on the fundamental frequency, too. We’ve also seen crystal testing and classification done with a software defined radio.