RE: where to get parts?

Thanks Clay. Even though my eyes starting glazing over when you started
talking about crystal cuts, I learned what I wanted to know. To
summarize/generalize/bastardize:

1) An oscillator is a crystal with supporting components to make the
signal digitally TTL compatible
2) Multiply the fundamental by overtone to get MHz (if it's an overtone
crystal)

Clay Cowgill <ClayC@diamondmm.com> on 01/19/99 12:41:17 PM

Please respond to vectorlist@mcfeeley.cc.utexas.edu

To: "'vectorlist@mcfeeley.cc.utexas.edu'"
      <vectorlist@mcfeeley.cc.utexas.edu>
cc: (bcc: Franklin Bowen/MD/CheckFree)
Subject: RE: where to get parts?

> Alright, what is the difference between an oscillator and a crystal?
> And
> what is fundamental vs. 3rd overtone?
>
A crystal is typically one of those little metal "cans" you see on a
circuit board containing (usually) quartz crystal that resonates at a
particular frequency at room temperature. The output is essentially
analog in nature, and the circuit around it usually incorporates a
couple of capacitors (a "tank" circuit) that helps the cyrstal maintain
oscillation with ample output drive. They're usually inexpensive--
under a couple bucks new (about $.25-.40 for standard grades to
manufacturers).

An oscillator is usually a larger metal "box" that fits in a DIP
footprint. It essentially incorporates the tank circuit internally and
provide a logic level signal output when supplied with ground and
voltage supply. They sometimes have a "enable" pin that can turn the
output on or off.

Fundamental mode is usually used for crystals up to about 25MHz with an
"AT" cut crystal (35 degrees). (I think) It's the cut of crystal where
the first frequency temperature coefficient at room temperature is zero.
(*gack* feel free to help me out here, analog people!) The crystal
resonates at this particular frequency.

A third overtone crystal is generally used for frequencies above 25MHz.
In this mode the external tank circuit incorporates an inductor that
tunes the circuit to a harmonic of the fundamental. (Usually 3rd, but I
think 5th and 7th overtones are out there as well.) So a 5MHz
fundamental crystal operating in 3rd overtone mode would be 15MHz.

-Clay
Received on Tue Jan 19 12:07:44 1999