[ntp:hackers] Reduce jitter of refclocks (connected via USB)

Terje Mathisen 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.
>>
>> Terje
>
> 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:
>
> http://www.satsignal.eu/ntp/NTP-on-Windows-serial-port.html#usb
>
> 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

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


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