[ntp:hackers] Reduce jitter of refclocks (connected via USB)
terje at tmsw.no
Tue Mar 15 07:17:53 UTC 2011
David J Taylor wrote:
>> USB by design operates with a 1 ms polling interval, so this is
>> exactly the jitter you should get, best case.
> I once did some casual checks of running NTP with a GPS/PPS source over
> a serial to USB converter, and got the results here:
> Jitter was in the region of 50 microseconds (Windows XP SP3). Not quite
> sure how that tallies with a 1 ms polling interval.
As you write on that page, "However, it may also be that the jitter
reduction was due to the shorter polling interval used for the ref-clock
than for LAN operation" heavy oversampling will tend to reduce the jitter.
Anyway, it seems like the achievable jitter figure depends a lot on the
specific Windows/driver version combo, I'm getting 1-2 ms jitter from
the Sure GPS board connected to a real COM1 port on W7-64. :-(
Using the USB connection is _much_ worse though, since that GPS board
doesn't even try to send the PPS signal over the USB port, so I end up
with 10-15 ms jitter from the NMEA messages.
- <Terje at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
More information about the hackers