I am looking to create a high q passive notch filter using a crystal.
I am not very familiar with crystals, hoping that someone who's had a similar experience can enlighten. I know crystals are mostly used for generating signals, but I mainly need to filter.
My working frequency is 40KHz
I know there are such parameters as series resonance and parallel resonance.
- How do I pick the right crystal for the job? The datasheets don't always state whether the crystal is designed for series or for parallel operation.
- What is the actual typical difference in frequency between the two?
- Is it possible to use the same crystal both to generate my signal and also to notch filter it? I read something about "pulling" the resonant frequency.
- How would you build a notch filter with a crystal?
Thank you so much.
Best Answer
Crystals have an extremely high quality factor, making them only suitable for very narrow bandpass / bandreject filters.
The quality factor is the ratio Q = Centre frequency / Bandwidth.
For a crystal Q can easily be 100000 so for a 40 kHz crystal that means 0.4 Hz ! Is that what you want ?
1) Crystals aren't designed for parallel or series resonance, they have both ! It depends on how you use them what will be the dominant mode of operation.
2) that depends on the crystal, I was working with a 25 MHz crystal recently and there the frequency difference was less than 1 kHz. Typically series and parallel resonance frequencies are very close together.
3) No, once the signal is generated you cannot "pull" it to a different frequency. The only way to slightly change a crystal's resonance frequency is to add capacitance in parallel with it. But that way you can change the frequency only very slightly. Like +/- 10 kHz for a 25 MHz crystal.
4) If you google "crystal notch filter" and select to see images only you will see lots of examples. Note that they're almost all for frequencies in the MHz range !