[ntp:questions] Assistance with PPS on Windows
albertson.chris at gmail.com
Thu May 5 19:59:02 UTC 2011
On Thu, May 5, 2011 at 11:46 AM, unruh <unruh at wormhole.physics.ubc.ca> wrote:
>> If you need better buy a GPS. You can get setup with one that does
>> works at the nanosecond level for under $100.
> Uh, no. There is no way of getting the signal into the computer with
> nanosecond accuracy. 1usec is about it.
You are right. While A $60 GPS can produce a PPS with 1 sigma error
on the order of a hand full of nanoseconds, the computer's interrupt
handing has about 1 or 2 uSec resolution. So nanoseconds are wasted
on an NTP server.
But there are other things one can do with a PPS signal. For example
you can discipline a local frequency standard. As long as you are
installing a gps, may as well get one that works at this level as long
as the cost is still reasonable. You can pay a lot more for a GPS
that is literally 1,000 times worse. I have an older Motorola unit I
paid $25 on ebay and it does about 50 nanoseconds one sigma.
An interesting question is, If the computer's time resolution is 1
uSecond, how good must the GPS' PPS signal be so that the majority
(90%) of the error is in the computer and not GPS. Off hand I think
you want the GPS to be about one order of magnitude better or on order
of 100 ns. But I've not done the math.
Redondo Beach, California
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