Before writing my comments, I read the comments of two others who
responded to your query.

On Wed, 11 Jun 2008, Joel Koltner wrote:

I've been playing around with some homebrew superhet radios, and I'm finding
that a significant amount of energy ends up at my 1st IF frequency that seems
to be coming from the LO.

At this point I'd like to know what your problem is with "significant
amount of energy..." compared to whatever your ultimate goal is.

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.

Again, what specifically are you "seeing" (measuring?) compared to what
you think you should be getting? Could your PLL synthesizer be dirtier
than you think?

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?

One way I would think about this is to ask if you looked at known circuits
that work and ask yourself what are you doing that is different from known
circuits that work. We also had some posts maybe 1-2 years ago where a guy
was working with chips and circuits and computer modelling (IIRC) and he
was unhappy that he was not getting (with real circuits) what his computer
modeling program told him he was supposed to get.

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.

I do homebrew but with tubes and can tell many stories about what should
have been a straightforward project but electrical performance was
unacceptable. QST has had, in the past, articles on why ham-built copies
of ARRL circuits don't work and of course all the blame goes on the ham
and not ARRL but there is a lot of missing information in the handbooks,
too. I've learned a few tricks by the crash-and-burn, smoke tests with
smoke and no function, the "guess and pray" techniques, and the "dumb
looks"-after-the-smoke response. Very roughly, I'd say 50% of my projects
work the way I hoped they would work, the rest go to the glue factory.



Thanks,
---Joel Koltner