Hello Fred,

A diplexer is certainly the Rolls Royce method. Elegant and impressive. But
there are others.

You could also provide a buffer amp that has a matching input impedance and the
mixer will be happy. One of my favorites is the AD603 if it fits the dynamic
range requirements, frequency range and cost limits. That gives you a db-linear
gain control of 40dB which often comes in real handy. They don't use Gilbert
cells in there but a patented voltage controlled attenuator so there isn't a
dynamic range penalty with gain reduction.

Regards, Joerg.