Some sort for freq. multiplier. Thanks for the
insight. Since it is running at overtone mode how does
the current processor nowdays run at Ghz without
running into the physical limitation? 


Thanks,
John

--- Richard Prosser <rhprosser@gmail.com> wrote:

> 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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