On Jun 11, 5:52*pm, "Joel Koltner"
wrote:
Hi Dave,

Thanks for the information; it's quite helpful!

I was (am) committing the faux pas of feeding the mixer output directly into a
narrow bandpass filter...

For example, the popular
Mini-Circuits SBL-1 mixer seems to have LO-to-IF isolations of
anywhere from better than 65 dB (at HF) down to around 30 dB (at UHF).
If you're trying to tune a weak signal (say, 80 or 90 dB weaker than
the LO signal) then the residual LO feedthrough can cause the sort of
swamping you're seeing.

So... say I'm using a 45MHz IF, trying to tune 414MHz using the SBL-1 and an
LO of 414-45=369MHz (low-side injection) at the SBL-1's recommended +7dBm.
Presumably I'll see a 7dBm - 30dB (LO-IF isolation) = -23dBm signal at the IF,
but it should still be at 369MHz, right? *How does the LO "bleed through" to
the 45MHz IF... and what power level should I expect to see there?

Ah - here you hit something on the head. I didn't know you were asking
about UHF, but now I can contribute!

SBL-1 balance isolation gets substantially worse as you move from HF
to VHF to UHF. Looking at the spec sheet, as you move from 3MHz (where
I use the SBL-1) to 369 MHz (where you use the SBL-1) the isolation
gets 34 dB worse.

*AND* the typical and guaranteed numbers are only if all the ports are
terminated nicely.

So while my gut feeling was originally "LO-IF isolation, that's never
been a problem for me at 3.5 or 7 MHz with the SBL-1" now I look at
the spec sheet and think "wow, isolation is a much bigger problem at
UHF than anything I've ever dealt with in the SBL-1 at HF"!

If you can arrange the IF stuff at the output of the mixer such that
you have a good flat termination match and such that you hit an IF
filter that will get rid of the LO before any substantial gain, I
think you'll come out way ahead of just switching to a different
variety of mixer.

Tim N3QE

Thanks for the advice Tim; much appreciated!