[time-nuts] Spread Spectrum Spam!
dave at uk-ar.co.uk
Thu Nov 26 09:03:17 UTC 2009
> -----Original Message-----
> From: Hal Murray <hmurray at megapathdsl.net>
> Subject: Re: [time-nuts] NMEA Time
> james.p.lux at jpl.nasa.gov said:
> > I don't think the oscillator quality in the typical PC is good
> > to ever get nanoseconds, even with tons of tweaks and temperature
> > compensation, etc. The short term variability/phase noise is too
> > high.
> > Microseconds, I think you could do.
> Yes, mostly.
> PCs have two nasty disadvantages in terms of time keeping.
> One is that the CPU clock is typically using spread spectrum
> to make the EMI
> shielding (a lot) easier.
> The other is that the temperature at the crystal depends (a
> lot) on what the
> CPU is doing.
> These are my opinions, not necessarily my employer's. I hate spam.
Hmph!... Spread Spectrum clocks do *NOT* make the shielding any
easier, it's just a fudge for the accountants who won't fund a proper
job in the first place. It only "fools" the QP detector in a measuring
receiver into showing a lower value, it does not "Fix" the problem.
It could be said, it only fools naive engineers into thinking it can
cure an EMI problem.
In any case, most of the problems that could be caused by CPU clock (or
derived signals) leaking out and into something else, are easier solved
if the blessed sprog so created is not spread over a large area.
As to spam. It's OK between two slices of bread, with a good helping of
pickle! It was also one of the best Monty Python sketches ever, in
fact, it's where the name was adopted from, for junk email.
Regards to All.
Dave Baxter. Soon to go and try to fix a couple of multi kW RF amps,
a customer has let the high pressure recirculating smoke out of,
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