In message , Steve Nosko
writes

"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.


Regarding 'series' and 'parallel' modes, ALL crystals actually resonate
in a series mode.

The simplistic equivalent circuit of a crystal is a series L-C tuned
circuit with a capacitor across the whole thing. The series C (call it
C1) is relatively very small (compared with the parallel C, C2) and the
L is very large (ie the L/C1 ratio is large). The parallel C2 largely
consists of the physical capacitance caused by the plating on each side
of the actual crystal. In some circuits the value of C2 is deliberately
increased by adding actual parallel capacitance (see later).

Looking at the equivalent circuit, you will see that the series C1, the
parallel C2 and the inductance are all in a loop, ie the three form
another series-tuned circuit. As C1 and C2 are in series, the
straight-through 'series' resonance of L and C1 must be at al lower
frequency than the 'parallel' resonance of L + C1 + C2 (although the two
resonances are always very close together).

In the 'normal' series-resonance mode (L with C1), an RF current is
simply fed through the crystal. This will be the case when it is
connected between the pins of an IC and there are no other 'tuning'
elements. Adding parallel C should have no effect on the
series-resonance, but it does bring the 'series' and 'parallel'
resonances closer together. This tends to force the oscillation
frequency lower. The 'parallel' mode tends to be more applicable where
impedances are higher (eg in a Colpitts circuit).

As C2 is much greater than C1, the oscillation frequency does not change
much if C2 is varied. However, it does change. Most crystals intended to
operate in this 'parallel' mode specify their frequency with a 30pF
shunt capacitor. Such crystals are usually used at the lower frequencies
(up to a few MHz). At the higher frequencies, the 'series' mode is
almost always used.

As C1 is very small, in the 'series' mode (L and C1) adding capacity in
series with the crystal has very little effect on the frequency unless
the extra capacitor is itself very small (and this tends to kill the
oscillation).

Finally, 'overtone' crystals should always be usable on the lower
overtones (essentially the odd harmonic frequencies of the fundamental),
and may (even if some TLC is required) operate on some of the higher
overtones.

I think I have this right!
Cheers,
Ian.
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