Peter Alfke wrote:

> Every xtal has parallel as well as series resonance. These two frequencies
> are very close together, and the typical Colpitts oscillator actually
> oscillates at a frequency in-between.
> For most typical low-precision applications, the difference between parallel
> and series resonance is irrelevant for the user. The crystal just picks a
> point in-between.

Maybe not close enough if you are building a clock or frequency
counter, but probably close enough for a CPU clock, I agree.

I agree that the frequencies are close.  Different oscillator circuits
work differently, and I wasn't sure which your description was about.

Also, some crystals are designed to run at a higher odd harmonic of
the fundamental, which complicates things a little.

I once knew the difference between the two types of oscillators, but
I don't remember it now.

-- glen

For a basic tutorial, on xtal oscillators, see

http://www.maxim-ic.com/appnotes.cfm/appnote_number/1017/ln/en

Peter Alfke

> From: glen herrmannsfeldt <g...@ugcs.caltech.edu>
> Organization: Comcast Online
> Newsgroups: comp.arch.fpga
> Date: Wed, 05 May 2004 00:38:34 GMT
> Subject: Re: Connecting a crystal to a Cyclone or Max PLD
> 
> Peter Alfke wrote:
> 
>> Every xtal has parallel as well as series resonance. These two frequencies
>> are very close together, and the typical Colpitts oscillator actually
>> oscillates at a frequency in-between.
>> For most typical low-precision applications, the difference between parallel
>> and series resonance is irrelevant for the user. The crystal just picks a
>> point in-between.
> 
> Maybe not close enough if you are building a clock or frequency
> counter, but probably close enough for a CPU clock, I agree.
> 
> I agree that the frequencies are close.  Different oscillator circuits
> work differently, and I wasn't sure which your description was about.
> 
> Also, some crystals are designed to run at a higher odd harmonic of
> the fundamental, which complicates things a little.
> 
> I once knew the difference between the two types of oscillators, but
> I don't remember it now.
> 
> -- glen
>