Re: gEDA-user: Using 60Hz mains frequency for timing?

[Larry Doolittle <ldoolitt@xxxxxxxxxxxxxxx>]

On Fri, May 02, 2008 at 01:06:47PM -0400, Ian Chapman wrote:
> Crystal usually cut to + or - 100 ppm for a general use like a CPU and it
> will not change too much with temperature and age.  Ethernet crystals were
> at one time cut to a better spec 50 ppm.  Special communications crystal can
> be a lot better.  The mains are very good in most places for the morning
> alarm however in remote communities with diesel generators ... no way.

Sounds like all of us are in violent agreement.
But before you start thinking that crystals
are _too_ wonderful, let me quote from my
introduction to using network time standards at
  http://doolittle.icarus.com/ntpclient/HOWTO

"First, a note on typical 1990's and 2000's computer crystals.  They
 are truly pathetic.  A "real" crystal oscillator (TCXO) usually has
 an initial set error of less than 5 ppm, and variation over time, voltage,
 and temperature measured in tenths of a ppm (and an OCXO can reach ±0.3 ppm
 stability over ten years and 85°C temperature swing).  The devices used
 in conventional PC motherboards and single board computers, however,
 often have initial set errors up to 150 ppm, and will vary 5 ppm over
 the course of a day-night cycle in a pseudo-air-conditioned space."

   - Larry


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