BITX 20/40 BUILD UPDATE #4: 11MHz SSB Ladder Filter

5:40 AM , , 0 Comments


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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