Electronics > RF, Microwave, Ham Radio

Electrically tunable crystal band pass filters

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Thank you.  iI never knew the details.    Dad referred to that circuit, though without a schematic, 55 years ago. That's an expensive crystal. 

I've seen variants, but not with variable Q, though  I knew  how it was accomplished. I've still got  copies of QST from when Dad was in high school in 1935.

The narrower BWs of the Hammarlund circuit would inevitably tend to ring.  However, I've not gotten less than ~150 Hz -3 dB BW from a single crystal.  So it might not be a problem.  I've been fooling around with a single crystal, but not as sophisticated as the link.

FWIW I've been discussing various aspects of this on several different lists which is becoming confusing trying to keep track of what I posted where.  For my own sanity I'm going to start cross posting  to qex@groups.io.

There is a surprisingly long thread on the HPAK list on measuring motional parameters with an 8753B which is NOT where I would have expected to get so detailed.

This is an attempt to duplicate the performance of  DSP TPBT  at a single IF using crystal filters to reduce the power consumption below DSP levels.  It seems to me highly unlikely to take less than a year long project given delays for instrument repairs, etc.

I have 400x xtals from one seller and 100x from another seller.  I've been diddling around with them while I worked out a simple way to measure them quickly.

I've designed a fixture to allow using a 16047A fixture from the 4284A LCR meter on either an SA w/ TG or a VNA to measure crystals and am now waiting on some SMA-N/BNC-F bulkhead jumpers to arrive from China.

Have Fun!

The screenshot is for the 4th order Butterworth example in EMRFD.

There was a construction article in QST magazine a year or two back - transceiver with a varactor tuned (as I remember) crystal filter - to vary the bandwidth

Thanks,  I'll look for that.  It's longer ago than you think if the ARRL really did remove construction articles from QST as was stated a couple of years ago when I requested QEX as my league publication.  I've done lots of experiments tuning a single crystal filter using both series and parallel capacitance.

Single crystal filters were very popular in Dad's day.  He told me about single crystals filters when I was a teenager.  But the real clincher was  W7ZOI's experiments in EMRFD.

My 4395A arrived yesterday.  I'm still waiting on parts for the adapter to mount the 16047A LCR fixture to the 4395A.  This is proving a more complex project logistically than I expected.  The 4395a is a VNA,SA and impedance meter all in one with 1 Hz resolution from 10 Hz to 500 MHz.  It is *sooooo* cool!

Have Fun!

The biggest problem today is getting the crystals you need. Vendors of custom crystals for hobbyist all went out of business a long time ago. It's far easier to buy monolithic crystal filters of different bandwidths. Finding some as a hobbyist however is not easy.

I'm designing around what's cheap.  $0.08 USD is pretty cheap for 5 MHz crystals.  The real cost is the labor to measure them (and the test kit.  though there are lots of options besides what I'm using.

At the moment I'm waiting for parts for the fixture to mount my 16047A fixture on the 4395A and test some crystals.

Have Fun!


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