Inadvertant mixing -- what was I hearing?
 

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/

On Mon, 25 Jul 2005 16:16:30 -0500 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.

Could your crystals just be oscillating at 21.040? A real frequency
counter can be really useful in cases like this.

I have some for sale if you, or anyone is interested.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

If they are very strong, perhaps you can use a relatively poor antenna so it
only picks up strong signals and try putting it into scan

It helps to know:
1- How did you determined the "..around 20MHz..." number.
It could actually be 1/2, 1/3, 1/4 that freq.
2- Were you able to hear the true, exact oscillator signal with the
receiver.
3- the IF of the receiver used and
4- what frequencies you had the reveiver tuned to when you heard these
signals.

The front end of these wide coverage handhelds are pretty broad with almost
no filtering. The oscillator can easily act as an L.O.


Where F1 and F2 are the oscillator and the receiver L.O.:
The result can be F1 +/- F2 +/- the IF frequency.

Then it can be N * F1 and N * F2 that can be the culprets where N = 1, 2,
3. Chances are pretty good that the signals are not much above 20MHz.

Use a spreadsheet to do the math and it is easier to cover all these cases
when you have some frequencies to enter.


73, Steve, K.9'D;C,i



"Ben Jackson" wrote in message
...
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/

(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/