To get lowband out of an 8640, I think I would simply add a mixer to the
output of the 8640.

Get a cheap oscillator, one of those computer time base ones, and feed it
into the LO of a double balanced mixer. SBL-1 or some such.

Feed the 8640 into the RF in and then you have your desired lowband on
the IF output.

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I'd add an attenuator to the IF output pin, 4 dB.

This way, assuming about 6 dB loss in the mixer, you know the output is
just about 10 dB below what the 8640 output level is and you can use that to
set the lowband output to a known level.


Jim Pennell
N6BIU