Thanks to GW0OAJ for this article
The first hack seemed to work - so on to the second effort with the bread board 4 MHz Cohn Ladder......
The impedance matching LC network at each end being left alone, the three fixed shunt capacitors were swapped with variable ceramics of a suitable type:
Trimming the caps produced a -3dB nose of 1.16 KHz, and a ripple of just over 3 dB instead of the previous 8 odd dB. The skirts were now down to about 8.9 KHz at -90 dB. Insertion loss remains at a very low figure.... -10 dB being the zero loss figure, choose your own interpretation from the nose plot!
Now too narrow for SSB, but a very nice filter for CW WIDE. All in all, tidied up and reduced in size by better layout, it would be a very nice addition to a design for a hacked out cheapo set... The proverbial single band junk box special :D
Xtal Ladder Filter part2
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