[ntp:questions] Something for distributing a PPS signal?
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Fri Jul 1 22:58:10 UTC 2011
Rick Jones wrote:
> Over in comp.arch, this was mentioned as how one might get
> time to the far reaches of a chip with equal delay.
> I recall that someone not too long ago wanted to
> distribute a PPS signal to a bunch of systems,
> and thought this might be helpful.
> John Savard wrote:
> If you want to have a clock distributed on a chip that's
> accurate to a small fraction of the time that light takes
> to travel across the chip, you *can* do it.
> Think of the letter H.
> Assume you have a pin on the edge of the chip where an
> external high-frequency clock signal comes in.
> Take that to the exact center of traces having the form
> of the letter H. Clearly, when the signal reaches the
> four corners of the H, it will have traveled an equal
> distance in each of the four cases.
> Now, make each of those four corners the center of a
> smaller letter H. Repeat (but not infinitely, as we
> don't need a _true_ fractal.)
> In fact, this actually *is* how clock signals are
> distributed on some chips.
Sure, if the PPS feeds a distribution Amplifier, and the
DA has (approximately) equal cable lengths to every PPS
recipient, then the PPS signal won't differ in time much
between PPS recipients.
However this means, that if the longest run from the DA
to a PPS recipient is e.g. 100 meters, then the cable
between the DA and the closest recipient must also be
100m, regardless of the fact that the closest PPS
recipient is only 1m from the DA.
Unless I'm off by a magnitude somewhere;
Copper wire is about 0.3m/ns? 100m ~= 333+ns?
The differences in e.g. PC serial port implementations
between different PPS recipients may easily cause a
difference of that much.
So, it matters a lot if you are frequency / timing lab,
as compared to a typical PC end user,
if those kind of differences are important to you.
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