Richard
not entirely sure 'what' custom filter opportunity you are referencing - but 'board headers' seems to be a clue.?
I remember there are pad features to interface with a second crystal filter - this topic has certainly been on the lit.? if you seek CW - a good CW filter is best at a much lower IF than 12 MHz.? its easy to make from lower frequency xtals - designs abound.? the 'bad news' - need to to add some firmware instruction to retune the BFO when doing CW.? it needs to be proven (if it hasn't been) - but it is possible that the SSB 12 MHz and a say 4.9 or 5 MHz CW filter could co-exist in parallel without switching.?
other v4 filters of interest - well something to reduce mixer spurious at 45 MHz.? v5 adds an LC filter.? I tried it on v4, in itself it does not have enough Q - so I suspect its part of a set of solutions.? adding a second 45 MHz xtal stage is popular - as reported here I tried it, it greatly reduced my CW transmit output.? I have since added SMT inductors with some shielding on the primary 45 MHz filter, so possibly it will work better - when I try it again.?
for CW and narrower bandwidth SSB - I am using a NESCAF audio filter that I am happy with.?
Curt
