[R-390] Transplanting R-390 IF modules to R-390A

[2002tii]

Bill wrote:

> I suspect that the tuned circuit IF lent themselves better to a
> Gaussian shape unless they were staggered tuned to get a flat
> passband.

The 390 (non-A) LC IF filters are (or should be) stagger-tuned to a
Bessel response, which provides near-constant group delay and no
ringing. Collins never said, to my knowledge, what function the
mechanical filters used in the 390A are designed to implement, but the
ones I have measured appear to have Chebyshev or elliptical alignment
with lots of passband ripple and a nasty cutoff peak (to maximize skirt
attenuation).

Best regards,

Don