Electronics > RF, Microwave, Ham Radio
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:
--- Quote from: rhb on June 10, 2023, 05:36:01 pm ---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.
--- End quote ---
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.
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