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[questions] Re: GPS+PPS vs NTP server, why a huge offset ?

David Woolley wrote:
On 17/06/2022 16:34, chris wrote:
As for compatibility, while a mismatched connection may work, it's bad
practice to do that, where you are dealing with microsecond timing
and want to avoid jitter. Use the correct interfaces and do the job
right, then you can fit and forget:-)...

RS232 isn't optimal for PPS as it is slew rate limited to 30V/µs, which means it will take at least 0.5µs from a resting level until it has fully cleared the transition region, if implemented to standard.

TTL is the more natural logic family for high accuracy PPS.

GPS should be able of achieving time transfer accuracies of better than .03µs.

GPS, even at the $80 Shure evaluation board level, have been directly measured at the ~25 ns level, which is pretty much what you claim here. :-)

The key idea is of course that in order to know where a GPS is located with better than 3 m precision, the unit by implication also knows what time it is, to within 10 ns of UTC(USNO). The only problem is to be able to convey that info to a connected NTP server.


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
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