Electrically tunable crystal band pass filters

[rhb]

Historically, the only way to implement TPBT was using 2 IFs with fixed filters and adjustable LOs.  I don't know of any instances of that, but would love to know what radios implemented TPBT before the Icom 7300.

I've got about 2/3 of the math coded up in Octave.  I've got signals at 5 MHz, 30 Hz below and 50 Hz above, two filters 500 Hz wide with 50 Hz of overlap and a 50 Hz wide filter.  Filters are trapezoids with 10 Hz wide skirts on each side. KISS. 

Everything is being specified in the frequency domain for convenience.  I'm not using any of the Octave packages.  Just doing the basic math by hand.  This for the simple reason I know the operations a *lot* better than I know Octave/MATLAB.  I can count 8 different DSP software packages I used over the years.  I spent far more time fixing bugs in DSP software or writing new codes than actually using it.

I am unlikely to make things minimum phase as they should be or make the huge number of figures needed to explain it all.

My goal is an analog version of TPBT using a single fixed IF.

Reg

Edit:  This is every bit as tedious as I expected.  Octave is crashing while using a 56 GB memory allocation when I'm only part way through the calculations.  So I have to go back and reorganize to create, plot, use and delete the series as I go through that nightmare task list I laid out earlier.   I'm not happy with 2 Hz for only 1 second, so I want to make the series longer.  I'd really like 10 seconds, but that will be an overnight "tape to tape" job on this machine as it's only 16 GB of DRAM and 64 GB of swap.

[RFDx]

Historically, the only way to implement TPBT was using 2 IFs with fixed filters and adjustable LOs.  I don't know of any instances of that, but would love to know what radios implemented TPBT before the Icom 7300.

An example would be the commercial R&S EK56 receiver from the end of the 60s. It uses Twin-PBT to implement 20 switchable IF-bandwidths.

[David Hess]

I found a pile of examples going back to the 60s.  I think some of the confusion, besides various manufacturers calling it different things, is that in some implementations the lower and upper filter cutoff was adjustable, and in others the filter width and filter offset were adjustable, but they were both implemented in the same way with adjustable local oscillators.

[rhb]

Major examples?  These are obviously radios outside the amateur realm at the time.

I've been working on measuring crystals and it looks challenging with 500 to measure.  In particular lab temperature control over a 2 day period. Or lack thereof :-(

So I'm considering binning the crystals by frequency using a counter and test oscillator.  Then put them in the refrigerator overnight and bin based on cold temperature frequency.

That will reduce the number I need to measure with the 8753B/85046A   in one session to find matching crystals.  So, hopefully,  the results should be more consistent with each other.

I'm beginning to appreciate that this is not going to be a quick project if done properly.  But for 8 cents each I *have* to try ;-)  Two of the 4th order filters in the EMRFD example with varactor tuning is all I need.  So 2 sets of 4 well matched crystals out of 500.

Have Fun!
Reg

[StuartA]

I hope that I have not got my wires seriously crossed here, but the Hammarlund tube receivers all had variable IF (455kHz) bandwidth using circuitry developed in 1938. I guess it could be developed for other IF frequencies. See https://www.vintage-radio.net/forum/showthread.php?t=196991. People who used those sets spoke very highly of the IF bandwidth control.

[rhb]

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!
Reg

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

[stellavox]

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

[rhb]

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!
Reg

[busaboy]

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.

[rhb]

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!
Reg