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

David J Taylor david-taylor at blueyonder.co.uk
Tue Mar 15 08:05:55 UTC 2011

>> 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.

But I was seeing 50 microseconds jitter, not 1 millisecond.

> 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. :-(

One millisecond is about the jitter limit for those Windows systems which 
don't use the interpolation code, i.e. Windows Vista and Windows-7, at 
least when not using a refclock.

> 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

Thanks for your comments, Terje.

Using a GPS/PPS signal connected to a real COM port on Windows-7 32-bit, I 
am seeing between 22 and 30 microseconds averaged jitter, using Dave Harts 
kernel-mode serial-PPS driver - but it wouldn't be a lot worse with the 
standard driver.  I can't use that in a 64-bit system as it's not signed, 
but I may be able to test later in the week with my second Sure board. 
Here are the offset graphs:


Upgrading to SP1 seems to have helped on that PC (upgrade on 2011 Mar 01)

I would expect that using NMEA alone would produce far worse jitter - I 
recall that Internet sync produced better results than just serial data. 
I've also seen the "failed to retain baud rate" you noted, so I will also 
see whether I can get it working as expected at the default 9600 baud.

SatSignal software - quality software written to your requirements
Web:  http://www.satsignal.eu
Email:  david-taylor at blueyonder.co.uk 

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