In article ,
Joel Koltner wrote:

that a significant amount of energy ends up at my 1st IF frequency that seems
to be coming from the LO. While I expect to see energy from LO+/-IF end up at
IF, of course, I've checked the LO+/-IF spurs (the LO is coming from a
PLL-based synthesizer), and in general it seems that a lot more energy ends up
at the IF than what the spurs alone would suggest. I remember a talk I
attended where the presenter mention that one of the biggest problems with
building receivers was "the LO getting into the IF," so I'm thinking this is
what he meant? Are there other less obvious paths for the LO getting into the
IF than just the LO+/-IF spurs?

The signal right at the IF eventually gets turned into DC and hence filtered
out, so in theory it doesn't really matter that much, but in practice with
very weak signals eventually the IF feedthru is stronger than the weak
signals, so it limits how much amplification I can provide and hence limits
the ultimate sensitivity of the receiver.

Local-oscillator "blowthrough" is almost inevitable in non-balanced
mixers. Even in single- and double-balanced mixers it's fairly
common - it's an indication of less-than-perfect balance. Commercial
DBMs usually quote an IF-isolation spec. 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.

Homebrew DBMs with hand-wound toroids, and less-than-well-matched
diode sets may have significantly worse LO-to-IF isolation.

With diode-and-transformer DBMs, performance can degrade significantly
if the mixer ports don't "look into" a nice broadband 50-ohm
impedance. If, for example, you connect the IF port directly to a
bandpass filter, the filter's impedance at the LO frequency is likely
to be very much different thatn 50 ohms, and this may affect LO
feedthrough.

The higher-performance superhet designs I've seen, generally take care
to ensure that each mixer port sees the proper termination impedance.
In the case of the LO port, the easiest way is to generate a
stronger-than-necessary LO signal (maybe 13 dBm) and then feed it to
the mixer through a 50-ohm pad (6 dB in this case).

In the case of the RF and IF ports, you can either use a resistive pad
with modest attenuation (perhaps 3 dB), or use a diplexer. For the IF
port you might want to feed the mixer into a common-gate or
common-base broadband amplifier stage, run at a current level which
gives it a 50-ohm input impedance... and then feed the output of this
amp to your IF bandpass filter.

I've seen one fairly expensive device (a Tek spectrum analyzer) which
had horrendous RF-spur problems. Turns out that its front end uses a
simple non-balanced diode mixer with essentially no RF-to-IF or
LO-to-IF isolation. The LO-to-IF isolation isn't a problem due to the
nature of the analyzer design... but the analyzer uses a relatively
low swept-IF frequency range, and the incoming RF signals can blow
right through and be detected as if they were IF mixing products.
It's an OK analyzer for bench testing of many radios, but is pretty
much useless for trying to look at VHF over-the-air signals.

--
Dave Platt AE6EO
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