Re: CW/SSB Quarz filter with tunable bandwidth #ubitxcw #cw filter #ssb filter #ubitxcw #ssb #cw

jimamos@...
 

The kitsandparts.com multiPIG+ receiver used a variable 5 pole Cohn filter.    I built and measured one of these and found that as designed the passband was initially rather poor as you tuned for larger filter BW's. The transmitter portion of the radio itself was CW,  but the radio could be tuned anywhere in the bands if desired, so a SSB filter was also useful.     CW was good, but as you varied the BW, the mismatch caused the passband to go pretty wonky.  

The Cohn design has the problem that the BW of the filter changes, the impedance changes.   I added a resistive termination to the filter by adding a JFET source follower circuit and got a great SSB response.   I tried various termination values and found one that gave a good SSB response and a useful CW response (still a compromise I think though between the various BW Settings).   I posted some of these results on the yahoo groups for the multipigplus in January of 2005.   

Allison has pointed out the Jones filter, which is definitely an improvement in trying to keep the filter matched across all BW's.  

Jim N8CAH

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