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

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

I'll get something in the ballpark of an -90dBm spur at the IF when the noise
floor is down around, say, -120dBm. Hence I have a harder time recovering
signals at, say, -100dBm even though they still have a decent SNR (and my
commercial receives have no difficulty at all hearing them). This is measured
on an Agilent 8563 spectrum analyzer.

Could your PLL synthesizer be dirtier than you think?

I've done some wideband sweeps of it, and there are some spurs that are only
~ -70dBc. It's obvious when you choose a channel that suffers from these
higher-level spurs, though... it'll add 20dB or more to the IF spur. I
ditched my homebrew PLL-based synthesizer for a good HP box borrowed for
testing, though (its spurs are more like -90dBc worst case), and the problem
is still there.

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.

The usual problem is that very few circuits found on the Internet actually
come with performance data -- unless you build them yourself, you really have
no way to know if they're just as bad or worse than your own efforts!

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.

It would almost be alarming if those circuits did work, in real life, within,
say, 0.1dB of their simulated results rather than the more typical 1-5dB
that's often still considered "good agreement!"

---Joel

See at end...

On Wed, 11 Jun 2008, Joel Koltner wrote:

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

I'll get something in the ballpark of an -90dBm spur at the IF when the noise
floor is down around, say, -120dBm. Hence I have a harder time recovering
signals at, say, -100dBm even though they still have a decent SNR (and my
commercial receives have no difficulty at all hearing them). This is measured
on an Agilent 8563 spectrum analyzer.

Could your PLL synthesizer be dirtier than you think?

I've done some wideband sweeps of it, and there are some spurs that are only
~ -70dBc. It's obvious when you choose a channel that suffers from these
higher-level spurs, though... it'll add 20dB or more to the IF spur. I
ditched my homebrew PLL-based synthesizer for a good HP box borrowed for
testing, though (its spurs are more like -90dBc worst case), and the problem
is still there.

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.

The usual problem is that very few circuits found on the Internet actually
come with performance data -- unless you build them yourself, you really have
no way to know if they're just as bad or worse than your own efforts!

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.

It would almost be alarming if those circuits did work, in real life, within,
say, 0.1dB of their simulated results rather than the more typical 1-5dB
that's often still considered "good agreement!"

I don't recall what the guy was upset about. I don't think (IIRC) that we
had any comments regarding what, quantitatively, he was looking for vs.
what he was getting, quantitatively, but he was definitely upset.

I don't have anything like the fancy gear you have and my criteria are
more or less "if you can hear it and its OK with you, then its fine" (for
receivers anyway). On transmit, I have enough tuned circuits in various
places that my spurious signals should be good enough for the FCC. And,
I'm using quite clean free running oscillators at VFO, LO points. Its
really wonderful to be able to see sine waves on any one of my three
Tektronixs scopes, thought. I'll add that if I don't have at least some
minimum number of tuned circuits, that "sine wave" develops very visible
"distortions." Maybe someday I'll be interested in quantitating them.

In the meantime, good luck on your endeavors. And, thanks for the fills on
the technical stuff. Can't help you any more than that.

---Joel

"A" wrote in message
.com...
I don't have anything like the fancy gear you have and my criteria are more
or less "if you can hear it and its OK with you, then its fine" (for
receivers anyway).

I'm in a very fortuitous position, being somewhat a novice at RF design
personally, but working at a company that's been doing it for years so I have
access to a fair amount of nice test equipment. :-) Sometimes it's a little
weird, though -- until a couple of years ago now we had network analyzers
going to 6GHz but no scope better than 100MHz!

I think projects like the TAPR VNA (http://www.tapr.org/kits_vna.html -- now
sold by Ten-Tec) are great, although it's unfortunate that it tops out at
100MHz, as in many cases one can "get by" with a lot less down at HF or even
2m than those trying to homebrew 70cm or higher gear. Even getting, say, 1GHz
spectrum analyzers with tracking generators is not at all cheap (meaning,
within most hobbyist's budgets, which I'd say is something $1k or less for
such a device) -- there's been discussion on and off for ages on how such a
tool would sell like hotcakes, given how much it would benefit homebrewers and
commercial users. I kick around the idea of trying to build something like
that myself, but I do already have enough experience to know that there's a
very large investment in time between "design looks good on paper" and "design
actually works" if you're shooting for decent dynamic ranges and resolution
bandwidths.

Thanks for your input,
---Joel