Sunday, February 23, 2014

A Homebrew Crystal Filter

I ordered and recently received a batch of 20MHz Crystals to build an experimental Crystal Filter similar to as shown in Farhan's Minima Receiver and my previous post.

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
The Analyzer display provided the Resonate Frequency and other information.
AIM-4170B  20MHz Crystal Plot
I re-tested each crystal of the 19.993 group, sorting and recording the frequency to 7-digits. A green tape label was attached to each crystal to make sorting easier.
Sorted Crystals
I selected a set of eight crystals for my filter, several of which had the same values.
Selected Crystals
The selected crystals have the following frequency values:
  • 2 - 19.99265
  • 1 - 19.99266
  • 2 - 19.99267
  • 1 - 19.99269
  • 2 - 19.99271
A double sided 1 x 2 inch PCB was created to mount the crystals and capacitors, the Toner Transfer Method was used to make the PCB.
20MHz Crystal Filter
Toner Transfer PCB
This is the results after Cutting, Drilling and Loading the board.
Completed 20MHz Crystal Filter
I think the AIM-4170B is NOT the best instrument to measure overall performance of a Crystal Filter, But, initial testing indicated there were some major "spikes" in the frequency response.

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
With a little tweaking, the "spikes" were removed, and this is resulting SWR plot with a 50 ohm Load on the output. But still, the resulting curve is less flat than I expected,  . . .
SWR with 50 Ohm Load
The real test and performance evaluation will need a Spectrum Analyzer and/or a measurement done within a fully constructed receiver.

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.

Hi Eldon,

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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