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Subject: Re: simple crystal oscillator IC

Date: 04/16/00 at 10:13 PM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: simple crystal oscillator IC posted by CrisE on 04/16/00 at 12:28 AM:

Hi, Chris.

There may be those who would take issue with me about my opinion of "simple" crystal oscillators, but I hold to it anyway. Such oscillators are an insufferable pain in unmentionable portions of one's anatomy and are to be avoided!!!! wherever possible.

I hold to this opinion whether for low frequencies or high for several reasons and have had two items published in Electronic Design about this topic.

"Check Crystal Impedance" was in Electronic Design's Ideas for Design section in the issue for Nov. 2, 1992. The item pointed out that the equivalent series resistance, or ESR, of some so-called microcomputer/microprocessor compatible crystals were higher than the maximum permitted values of ESR for several popular microcomputer chips in use at that time.

"Setup Tests Crystals" was in Electronic Design's Ideas for Design section in the issue for Aug. 2, 1990. This item pointed out that crystals can have spurious series resonances at other than the intended frequency of crystal operation. Wouldn't you know it, my oscillator took of at the spurious frequency!

The first one never really hurt me, but it was a cautionary tale to be sure. However, the second problem did do some harm to my work. It was finally resolved when the crystal vendor figured out a crystal cut which suppressed the unwanted spur.

More recently, I've had no end of trouble getting the recommended crystal oscillator circuit of a major gate array supplier to work properly. A 32 MHz clock had to drop down to 16 MHz in the course of solving that one!

If you need a reliable oscillator, I very, very, very strongly urge you to purchase a commercially offered crystal oscillator at the frequency you require. Trying to roll your own as it were, can lead to a lot of gray hair and to very serious problems with oscillator reliability.

Good luck.

John Dunn - President
Ambertec, Inc.

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