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Subject: Re: simple crystal oscillator IC

Date: 04/17/00 at 10:42 PM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: Re: simple crystal oscillator IC posted by CrisE on 04/17/00 at 10:06 PM:

Hi, Cris.

I can understand your concern with cost constraints, but I must worriedly caution you against the danger of false economy. The HA7210's upper limit of 10 MHz is about right, I think, for a gate-based oscillator. Remember by remark about the crystal oscillator circuit in that gate array which I was talking about.

Still, if you really want to do it at minimum cost, you need to start with a crystal which is utterly free of spurious resonances. This is NOT an easy matter to guarantee by any means and some vendors are, in my opinion, not fully appreciative of how important this property is when there are no other frequency determining elements in an oscillator circuit!

Take a look, for example, at http://www.valpeyfisher.com/crystals_throughH.htm#tha for Valpey-Fisher's versions of high frequency fundamental mode crystals. Just be sure when you contact somebody there, or at any other vendor's personnell too for that matter, to make it absolutley clear how critical it is that there not any be spurious resonances!!

With spur free crystal, you can try to make a Pierce oscillator using an actively biased, high frequency transistor like a 2N918 or 2N2857. (I know these are fairly old part number devices, but they have ft ratings of 600 MHz and 1 GHz respectively and are the kind of high speed device you'll need. A 2N2222 or 2N3904 just won't be fast enough for this purpose.)

If you don't know the Pierce circuit, I can sketch one out for you, BUT with the caveat that I will not have tried it out beforehand. Although I've used a Pierce circuit before, it was only at a few MHz, nowhere near as high as 60 MHz.

Good luck.

John Dunn - President
Ambertec, Inc.

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