On Monday, 14 Feb 2005 23:53:26 -500, "Asimov"
wrote:

"Joel Kolstad" bravely wrote to "All" (14 Feb 05 16:51:45)
--- on the heady topic of "Something like a diplexer"

JK From: "Joel Kolstad"
JK Subject: Something like a diplexer
JK Xref: aeinews rec.radio.amateur.homebrew:2136

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

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

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

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

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

JK Thanks,
JK ---Joel Kolstad

You missed a step called a directional coupler. It's a sort of
transformer with 2 input ports and 1 output port. The 2 inputs don't
see one another but their power is combined at the output.

You don't actually *need* a directional coupler. I have seen window
preselectors with five separate sections for sub-bands between 403 and 520 MHz.
The configuration is symmetrical (in/out) with simple bandpass filter segments
and coaxial split/combine harnesses. Not a DC or hybrid in sight. I am
presuming - not having swept one - that at "off" frequencies each parallelled
leg presents a high enough impedance to the split junction that the effect is
negligible.