This routine designs a low-pass diplexer filter, that has a constant
input impedance of Zo at all frequencies, both in the pass-band and in
the stop-band. S11 is always good. S22 is poor in the stop-band.
When used at a mixer termination, make the cutoff frequency sqrt(IF*Image)

------------------L1--------------------------------------
                   |                         |
                   |                         |
1->                C                         |         <- 2
                   |                         C
                   --------                  |
                   |      |                  |
                   R      L                  |
                   |      |                  |
------------------------------------------------------------