John Fields wrote:
On 28 Feb 2005 11:23:50 -0800, "Tim Shoppa"
wrote:


Are "overtone" crystals cut differently than "fundamental" crystals?
Or are they just specified differently?

In particular, say I took a garden-variety 20MHz fundamental
microprocessor crystal and instead used it at its fifth overtone,
trying to hit 100 MHz. The LC network is there to make sure that it's
on its fifth overtone. Will this "misuse" mean that the oscillator
will be harder to start up, less stable, more noisy, ???, than a
crystal oscillator made out of a real overtone crystal? I don't mind
if I "miss" 100 MHz by a several tens or hundreds of ppm, as long as
it's stable there.


---
You can use a fundamental mode crystal as an overtone oscillator, but
even if you can get it to oscillate, it won't be generating an
overtone at 100MHz, since overtone modes of oscillation aren't
harmonically related to the fundamental. It's more like the slab of
crystal is vibrating like the drumhead of a steel drum with small
areas of the slab vibrating at higher frequencies, instead of the
entire slab virbarting at just one frequency.

Check out "Chladni patterns" if you're interested.

Here's some pattrens for violin tops and circular plates:

http://www.phys.unsw.edu.au/~jw/chladni.html
---


If anyone knows of a place that ships off-the-shelf 100 MHz fifth or
seventh overtone crystals, I can avoid this whole exercise.... :-)


---
Anybody who makes crystals ought to be able to help you out; here's a
start:

http://www.icmfg.com/


In an AT cut crystal the overtone modes are close, but not exactly on,
the odd harmonics of the fundamental. Furthermore, all of the
literature that I've read on AT cut crystals reports that they vibrate
in the bulk of the crystal, in shear mode -- see figure 7 he
http://literature.agilent.com/litweb/pdf/5965-7662E.pdf.

Perhaps you're thinking of SAW devices?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com