Help me with tunning the crystal filter
Have anyone here have some experience b4 of tuning the crystal filter center frequency?
As we know, the crystal filter has a very sharp passband bandwidth and very high Q. While, the crystal filter is also very hard to tune the frequency you want. I want to have a crystal filter at frequency 27.27273Mhz, while the crystal at this frequency is really hard to find. The closed crystal I could find is 27.3MHz.(By the way, I do not have the crystal on my hand, I do not know the exact resonance frequency)
So can I ask anyone who could share the experience with me of tuning the crystal filter.
Many thanks!
Best,
Not a crystal filter expert, but my experience has been that the crystal can be used in a mode where it looks like a giant inductance. In that mode, you can externally pull the frequency a little with external inductance/capacitance. Maybe can pull it 50 KHz or so.
Here is an example how to tune a ladder crystal filter.
Beware, usually 27MHz crystals are actually overtone crystals with fundamental on 9MHz.
A quartz crystal alone could be "pulled" by external capacitor or inductor, but a filter composed of several resonators comprises also precisely adjusted coupling elements for a good response, most often, a band-pass. In principle, such filter cannot be tuned or its center frequency moved, without simultaneously varying all filter elements.
To design a variable-frequency band-pass filters, use active filters with operational amplifiers; new types extend up to >1 GHz, and their gain can increase the filter pole-zero performance while it is easy to "tune" such filter.