Electronics > RF, Microwave, Ham Radio

Electrically tunable crystal band pass filters

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rhb:
It's common practice to use a capacitor to vary the frequency of a crystal over a narrow range in a VCXO or to trim the frequency in a Clapp or Colpitts design.

The same physics applies applies to  using a crystal as a filter, but I've not seen mention  of tunable crystal IF filters.

Consider a 5 MHz IF.  The spec on ebay 10-15 cent HC-49S crystals is +/- 50 ppm Fc.  That works out to 250 Hz which is irrelevant if you are moving the frequency by 1 kHz

I've attached the section of Zverer's "Handbook of Filter Synthesis".  The last page shows the filter I want to build as a tunable center frequency, constant BW filter which I can cascade with a similar filter with the opposite shift using an emitter follower to isolate the 2 filters.

It seems to me that if the caps in parallel with the xtals are implemented with varactors one can electrically tune the filter center frequency.  The BW needs to be 1500 Hz  to get good time domain response and ideally symmetric pass bands or anti-symmetric so that the intersection pass band is symmetric.

The Icom 705 does this in DSP, but this should be a lower power drain method of implementing the feature.

Does anyone have any experience with this concept or know of prior art?

Have  Fun!
reg

David Hess:
More commonly a variable passband crystal filter is implemented with one or two fixed crystal filters, and varying the frequency of the local oscillator to the mixers.

rhb:
Would you cite some examples?  I'm having a hard time grasping how it works.

Edit:  How would this work for a NorCal 40 style radio?

David Hess:
It is called variable passband tuning.

For one IF filter, if the local oscillator for the first mixer is shifted, and the local oscillator for the second mixer is also shifted, then it is as if the IF filter is moving in frequency.

If this is done with two IF filters, then the combined width can also be changed because one filter will cutoff the low frequency, and the other will cutoff the high frequencies, producing a filter response narrower than either filter alone.

T3sl4co1l:
Would it not be better to construct a standard crystal resonator filter and vary the coupling factor (thus bandwidth) instead?  That's simply varying the shunt capacitances.

Or are you looking to make two independently tunable filters, which might be used in cascade but could also be independent therefore should be independently tunable?

Tim

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