[time-nuts] Using a frequency synthesizer replacement for motherboard oscillator

Poul-Henning Kamp phk at phk.freebsd.dk
Sat Dec 1 08:35:04 UTC 2012

In message <ECC7F999-673E-4F70-95EF-1D888761A991 at rtty.us>, Bob Camp writes:

>It's most commonly done with things like a Soekris 45xx series board. You
>don't need anything very exotic for the frequency conversion. The jitter in 
>the PC is way worse than what the external chips will be creating. 

Actually that is _not_ true anymore.  Modern CPU's are very finicky about
clock jitter because the PLL the frequency up to GHz range.  Some of the
clock-chips used now discipline a low-UHF range oscillator to the XTAL
to cope with this, but most just PLL the frequency up there.

>The real question is - what is the "magic frequency" on the particular 
>mother board you are going to modify? Once upon a time they all were a pretty
>predictable 14.xxx MHz. These days, who knows what's going in where

It's pretty much still 14.318 Mhz pretty universally.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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