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From: mcovingt@athena.cs.uga.edu (Michael A. Covington)
Newsgroups: sci.electronics
Subject: Re: XTAL not oscillating at marked frequency
Keywords: wierd crystal stuff
Message-ID: <1991Jun21.235406.26542@athena.cs.uga.edu>
Date: 21 Jun 91 23:54:06 GMT
References: <15604@exodus.Eng.Sun.COM>
Organization: University of Georgia, Athens
Lines: 29

Any crystal rated between 20 and 60 MHz is going to be a "third overtone"
type which is capable of oscillating at both F and 3F, where 3F is the
frequency marked on it.  For example, a 27-MHz CB crystal will also
oscillate (in fact will more easily oscillate) at 9 MHz.

Very simply, your 24-MHz crystal chose to oscillate at its 8-MHz
fundamental rather than its 24-MHz overtone.  (You could also get it
to oscillate at 5F, which is 40 MHz, in a suitable circuit.)

Overtone crystal oscillators almost always contain a tuned circuit
to prevent oscillation at the lower frequency. Your circuit doesn't
look like an overtone oscillator; it has nothing to prevent oscillation
at the lower frequency. You might *try* replacing the .01 uF capacitor
with something a lot smaller, but don't count on it.

The easiest thing to do, if you're clocking a microprocessor, is to
use a hybrid IC oscillator -- that is, a whole oscillator in a can
the size of an IC. These have all the frequency-determining components
built in; just apply 5 volts and you get a nice 5-volt square wave out.





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Michael A. Covington | Artificial Intelligence Programs
The University of Georgia  |  Athens, GA 30602   U.S.A.
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