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