As reported yesterday I have the 20 meter receiver portion of my BITX 20/40 rig up and running. I decided to take a closer look at the crystal filter I built.
Here is my method:
1) Using an Arduino/DDS sig generator, I put 11 Mhz energy into the base of Q2 (the stage immediately prior to the crystal filter).
2) Using my Rigol 1052E oscilloscope, I measured RMS voltage at the output of Q3/Q3A (the stage immediately following the filter).
3) I looked at Vrms as I MANUALLY varied the input frequency in 100 Hz increments.
4) I took the results and plugged them into a spreadsheet. I then used the spreadsheet to calculate the db drop from the peak Vrms value (So I wasn't looking at insertion loss, just the filter shape).
I used 20*LOG(Vrms/276)
5) I ended up with the chart displayed above.
I have a few questions:
1) What do you folks think about my methodology for evaluating the filter?
2) Where would you guys put the BFO frequency?
3) I know the ripple looks ugly, but the receiver sounds great. Should I attempt to get rid of the ripple?
Here is the filter I used (as prescribed by the AADE software): I estimated Q at 10000 and used LM and CM values derived by the G3UUR method, and made no effort to match impedances going into the filter:
Here is what GPLA predicted. I estimated Rin and Rout values. That probably accounts for the difference between the GPLA prediction and what I measured.
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