Here is a Collins voice multiplex mechanical filter centered at ~ 88 kHz. The filter is ~ 3 kHz wide, an USB filter. Its been kicking around since the mid 50-60's. Excuse the ripples for its age!. Anyway, it has the shape factor and the rejection that one would like to see from such a class of filter. Again, 1010 points now from 50 kHz to 200 kHz. About 50 Hz or so per step. I do not see any strange events occurring at this point. The cal was a full so called 2 port cal includes "isolation".
