(see below)

The problem you describe is a nightmare and is not that uncommon but may
be very hard to fix. I can only speculate on a few things. One is spurious
parasitics (that term covers a lot of situations, including chip behavior
that is not part of the specifications). Your oscillator circuit may be
oscillating on frequecies (self modulation of the fundamental by the
non-fundamental or harmonics and mixing, too?) other than the crystal
fundamental. Yes, the mixing could be in your test apparatus other than
the test oscillator. Sometimes if you get the receiver hundreds of feet
away from the oscillator, then the spurious signals fade out faster.

Another way to approach this might be if you use a different receiver
(different local oscilator freq, different IF) and see if you pick up the
same sets of signals.

How about what do you get when you try different crystals? Similar
spurious emissions? Try this if you can and tell us what you get.

Do you have a _good_ crystal that you know oscillates and know what its
frequency is and put that into your test circuit. Does it behave as you
expect? I also know and have had crystals that will oscillate in one
oscillator circuit but will not oscillate in others. Also, some crystals
will NOT oscillate on their fundamental but _will_ oscillate on one or
more of their harmonics. Weird but true.


===== no change to below, included for reference and context =====

On Mon, 25 Jul 2005, Ben Jackson wrote:

I got a bunch of identically (but cryptically) marked crystals that I
wanted a ballpark idea about so I built the oscillator part of the Pixie
II (up to Q1) and put a scope on it. It was around 20MHz, so I grabbed
my HT and tuned around 20MHz to see if I could find it (this, by the
way, is why you should not borrow the lcd display from your frequency
counter for another project...). What I discovered was several signals
above 20MHz (eg 21.040) that only existed when the oscillator was on.
I assumed it was picking up something else and mixing it with 20MHz so
I tried to find the original signals at 1040kHz but I couldn't. The
"mixed" signals were very clear and strong. It was some kind of news
(shortwave?) in an Asian language.

Where else should I have looked for the base signal? ~41MHz would be
in the middle of a government band.

--
Ben Jackson

http://www.ben.com/