Somewhere (link found) I read that for Homebrew Crystal Filters the HC-49US (short) crystals do not have as high of "Q" as the standard HC-49U (standard) crystals, but as with all of my projects "small is better". This experiment is being done to see what I can do with the short crystals.
To try to find matching crystals, I used an AIM-4170B Analyzer to sort the crystals into six, 5-digit groups (19.991 - 19.996MHz). The crystals sorted into a typical standard bell distribution. I think the measured frequency, which is lower than the expected 20MHz, is due to calibration, and/or because the crystals are not actually operating in an oscillator circuit.
For this build, I decided to use the 19.993MHz group, as it provided more crystals to further select from.
|Initial Sorting of 20MHz Crystals|
|AIM-4170B 20MHz Crystal Plot|
- 2 - 19.99265
- 1 - 19.99266
- 2 - 19.99267
- 1 - 19.99269
- 2 - 19.99271
|20MHz Crystal Filter|
Toner Transfer PCB
|Completed 20MHz Crystal Filter|
I initially loaded the PCB with two parallel 50pF capacitors at each of the five locations where Farhan's circuit called for a 100pF. The goal was to parallel two capacitors to lower series inductance and/or provide the option of replacing one-of-each with a variable capacitor (if needed).
Due to the observed "spikes", I replaced one-of-each set of two capacitors with a 4-47pF NP0 variable capacitors.
|Tuning Caps Installed|
|SWR with 50 Ohm Load|
More Crystal Filter fun to follow :-)
UPDATE: Feb 23, 2014 14:04
I just received an e-mail from Jack, we should be able to evaluate the Filter soon.
Nice looking XTAL filter.
If your design is around 50 ohms in and out then my equipment should do the job.
I expect to have the interconnect cable for my Spectrum Analyzer to Tracking Generator by next Tuesday.
Assuming both pieces of gear work as planned I will be testing about 4 RF filters I have on hand and your XTAL filter would be a lot of fun to test.
I can also provide a print out of the filter response curve. A camera shot of the CRT also works.
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