Message-ID: <21133.200103201838@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Tue, 20 Mar 2001 18:43:38 +0000
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII; format=flowed
Content-Transfer-Encoding: 8bit
Subject: LF: <Tech> DOS/Windows and PC clocks
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

Dear LF Group,

Although it may be possible to access the PC's internal clocks one 
way or another, I can't see it would be much help for timing 
accuracy - after all, it's all derrived from a very ordinary, un-
trimmed crystal on the motherboard somewhere.

I did some work at U of H a while ago with Dave Lauder, G0SNO, 
investigating PC clocks. In fact, a lot of PCs (and other things) are 
now using 'dithered clocks', where the clock is frequency 
modulated to a deviation of a fraction of a percent, by a spiky 
looking triangle wave. The reason for doing this is to spread 
radiated noise emissions from the PC over a wider range of 
frequencies, which in turn reduces the noise power seen by a 
narrow band EMC receiver when noise emissions testing is being 
performed. This slightly perverse idea is basically a fiddle to allow 
PC's to generate more RF noise, while still passing the EMC 
regulations. It saves the manufacturers some money on screening 
and filter components. I'm sure it does not do much for timing 
accuracy, though!

Cheers, Jim Moritz
73 de M0BMU