"Michael Black" wrote in message
...
PaoloC ) writes:
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at
its fundamental frequency.

The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz,
which falls into 18m HAM band when multiplied by two. I assume 27MHz
XTALs are 3rd overtone....

Could the crystal be a receive crystal? CB crystals tended to show
the channel number or the frequency of the channel, and so if you simply
looked at the marked frequency, it would not tell you if it's for
transmit or receive. Though the ones I've seen did mark them with
"R" or "T". The point is that if it's a receive crystal, it wouldn't
be 1/3 of 27.125 but 27.125-IF and then divided by three.

IFs were frequently 455KHz

I have used fundamental crystals on their overtones and overtone
crystals on the fundamental. They are not exactly 1/3 or 1/5 ratio due to
parasitics in the crystal. All crystals have the fund and OT responses, but
the responses are optimized for the intended use - by changing some of the
construction. (I'm talking AT cuts)
Overtone crystals *tend* to be used in the SERIES resonant mode, though
not always. Fundamental crystals tend to be used mostly in the parallel
resonant mode. This will also cause additinal frequency error when using
the marked frequency. The parallel freq is higher than the series (if I
recall correctly)
If I recall, the two gate oscillator is a series resonant oscillator and
the one gate is parallel. I would use a transistor Colpits oscillator
myself. The digital gate oscillators can run the crystal at a higher drive
than it should causing more crystal heating than desired (more drift
w/time).
If you are going to use a OT xtal as a fund., I'd pick a parallel type
osc like the Colpits (I think it has another name when it has a crystal
rather than a coil). I never did like the digital gate oscillators. They
tend to be a *bruit force* oscillator. Because of the Rs of the crystal,
crystals of some frequencies are easier to over drive than others.
Unfortunately, I don't remember which is which.
The fact that your 10mMHz xtal works is a good start. The other one
probably will, but since it was made for the third OT, there may be enough
difference to keep it from oscillating without some circuit change.
If you have no scope, measure the current drain. There may be a change
when the xtal is inserted if it is oscillating. Also, you could try
measuring some voltage through a large resistor or choke to see changes when
the xtal is inserted.

--
Steve N, K,9;d, c. i My email has no u's.