I'm trying to build a circuit to split two signal paths apart, process them
differently, and them put them back together. In particular, I have
incoming RF signals from ~25MHz-3GHz (they're going to a wideband receiver)
and I have some notch filters that I want to be able to selectively engage
between 25-500MHz.Due to the topology and parasitics of the filters, they
tend to roll off above 500MHz and have significant loss by the time you get
to 3GHz.

Hence, I'd like to run 25-500MHz though one signal path and 500MHz-3GHz
through another (I want to leave the 500MHz-3GHz part untouched). The
'cross over' region doesn't have to be particular 'clean' (i.e., it can vary
+/-5dB easily).

I tried desgining a 5th order diplexer, and it cleanly splits the two
frequency ranges into separate path going to their own terminators.
However, if instead of terminating the low pass output and high pass outputs
to their own loads I connect the two outputs together, I get something of a
mess -- not at all a 'flat line' like I was hoping for.

I next tried hooking up two of these diplexers 'end to end,' and while the
response is almost flat, it has a very sharp null right at the 500MHz corner
frequency, and another very sharp set of nulls a couple hundred MHz above
and below this (they're mirror images). Hmm...

So... any hints how to do this properly? I thought for certain the end to
end diplexers would have done the trick.

Thanks,
---Joel Kolstad