A result with the 6th Order Cohn Ladder! So on with the tweaks...
Knowing that the Xtal shunt capacitors allow tweaking of the response, the principal was carried onto the impedance matching LC elements at each end. The blob of solder by the screw slot ain't grubby work by the way, it is a manufactured thing on these dinky little ceramic jobs: they all have one...
As well as giving adjustment to the ripple to complement the shunt caps, the impedance element changes the bandwidth too. Now about 0.5 dB ripple and 2.46 KHz at -3 dB. A squarer nose shape would surely follow with a better selection of Xtals to choose from. At -60 dB we are at 6.76 KHz and so the shape factor is looking good
For a hack, the stop band was OK'ish at -82 dB, but not wonderful:
However, much better can be achieved by simple means. It is a basic principle that outputs should not see inputs, and vice versa. A bit of brass plate as a simple screen, tacked in the middle of the ladder, lowered the floor to by about 5 dB to -87 dB and better:
Without much effort, we can improve on that! The screen was removed to one end by the impedance elements, and a similar screen thrown at the vicinity of the other end:
Now we are getting somewhere. The floor is better than -100 dB: and the screening is minimal - no sides or top, etc. Still plenty of stray leakage about! We are not yet into diminishing returns here. Properly, "canned up," we could reasonably see some further improvement!
No more improvements on this board though. It's all ripped apart for the next version. More thorough xtal matching and a better layout, etc and etc...
Thanks to GW0OAJ for this article
Xtal Ladder Filter part4
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