In article ,
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?

I'd say that you could be seeing the 369 MHz LO signal in your IF
pathway through a number of mechanisms.

First: some of it may be getting past your bandpass filter. If
you're using a crystal filter, you may find (if you look) that the
filter's stopband attenuation is neither wonderful nor flat at some
frequencies. Crystals often have secondary resonances at (or near)
the harmonics of their fundamental frequency - that's how overtone
filters work - and your LO of 369 MHz isn't all that far from the 8th
harmonic of your IF frequency.

You'd probably need to sweep your filter's response (wideband
oscillator plus an RF detector or a spectrum analyzer) to determine if
this is occurring.

If this is the problem, you might remedy it by adding a one- or
two-stage passive lowpass filter (maybe LC, maybe just RC) somewhere
after the mixer and before the final detector. This would attenuate
out the residual LO signal that the bandpass filter doesn't. Maybe
run the mixer output into that grounded-base/source 50-ohm buffer amp
I suggested, gain a few dB, and then run the signal into a slightly
lossy low-pass filter whose output presents your bandpass filter with the
termination impedance it expects?

Second: you may be getting parasitic transmission of the LO signal
*around* the bandpass filter - either capacitive or inductive. If
your bandpass filter is a tuned LC type rather than a crystal filter,
there might be coupling between the inductors.

Third: you might be getting some LO pickup by the gain stages in the
IF path, direct from the local oscillator - wire-to-wire coupling or
something like that.

Fourth: you might be getting LO wandering around on the power supply
lines or in the ground paths/planes - insufficient power-supply
decoupling, shared power-supply wires, shared ground paths, and so
forth.

I get the impression that careful attention to layout, shielding, and
decoupling are very important for higher-performance receivers. Some
builders seem to prefer to place the LO, mixer, and filter in separate
shielded sub-compartments (soldered together from double-sided PC
board material if nothing else), to carry the signals between these
compartments on shielded coax, and to use feedthrough caps and ferrite
beads on the power wiring. This helps minimize the various forms of
parasitic coupling between stages.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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