Someone may chioose to correct me but:-
Crystals are cut to operate in different modes. The most common (at
least for micros) is the fundumental mode where the crystal operates
at the frequency corresponding to the physical dimensions of the
quartz.
However, the cut can be slightly modified to operate in a overtone
mode - essentialy at a harmonic of the fundamental. So a third
overtomne will crystal is designed to operate at 3 times the
fundamental , a fifth overtone at 5 times  etc.
The crystal will however, under the right (or wrong) conditions
oscillate at the fundamental or presumably at a different overtone
from what was intended. Therefore, if the oscillator is not designed
for the overtone mode, and has sufficient bandwidth, you can get the
wrong frequency.

(You can also get a fundimental component in addition to an overtone
output but that's a different story)

Overtone mode crystals are mostly used in VHF and UHF radio systems
where it is advantage to operate the crystal at as high a frequency as
possible to minimise the frequency multiplier stages. If a crystal was
cut to operate at 100 MHz or so in fundamental mode it might not be
physiacally strong enough to withstand the vibration requirements, but
a 33MHz crystal operating in third overtone mode would be OK.

Typically,  crystals oscillating above 30MHz or so are operated in
overtone mode.

Hope this assists.
RP
On 15/11/06, John Chung <kravnus@yahoo.com> wrote:
> 3rd overtone? Not sure what you mean.
>
> John
>
> --- Richard Prosser <rhprosser@gmail.com> wrote:
>
> > It's not a crystal designed to work in 3rd overtone
> > mode is it?
> > RP
> >
> > On 15/11/06, Przemyslaw Lopaciuk
> > <pic@amusementcity.com> wrote:
> > > Hi,
> > > I'm using PIC18F8621 with 40 MHz crystal and I've
> > just tested it with an oscilloscope.
> > > The odd thing is that it shows that the frequency
> > is about 13MHz on both outputs (X1 and X2).
> > > What could be the reason of it? Load capacitance?
> > >
> > > I would be very grateful for every advice.
> > >
> > > Thank you,
> > > Sam
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