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.

Since the circuitry is/will be digital, the oscillator is one gate of a
74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL.
10pF and 33pF (from the junkbox) capacitors to ground on each side of
the XTAL.

No oscillation (I have no oscilloscope, I use my HF receiver to
troubleshoot oscillators at known frequencies).

If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and
clear.

I have never built something with an overtone XTAL. I know that I need
an output resonating circuit if I want to extract the 3rd harmonic. Do I
need the same if I want the fundamental?

Are overtone XTALs "harder" to resonate?

Are those old CB XTALs 3rd overtone?

I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions,
Paolo IK1ZYW

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.

I'd say there shouldn't be any problem getting an overtone crystal to
oscillate on the fundamental; if there's any problem it's getting an
overtone crystal to oscillate on its overtone frequency. Of course,
they usually oscillate at a slightly different frequency, so maybe
you need to tune around a bit.

Or maybe the circuit values are wrong for that crystal or frequency.
Or the crystal is indeed dead.

Michael VE2BVW