[time-nuts] Cheap jitter measurements

Bob kb8tq kb8tq at n1k.org
Tue Apr 10 14:11:46 EDT 2018


Hi


> On Apr 10, 2018, at 12:23 PM, David J Taylor via time-nuts <time-nuts at febo.com> wrote:
> 
> Hi,
> 
> Yes, but isn't generating pulses OUT of a PC with low latency/jitter one
> of the difficult issues?
> 
> If we (somebody smarter than me...) flip this around and modify a copy
> NTP to grab the QueryPerformanceCounter value when it gets a PPS pulse
> and log that count, don't we now have a way to compare the high
> resolution uncorrected counter to NTP and the external PPS?

We would still be relying on NTP to report on it’s self. I can feed the PC a PPS from my GPSDO
and let it report on that. Tell it to simply monitor the input, but not use it for disciplining. I get information,
but it’s what NTP “thinks of it’s self”. Ideally one would like to get an output that can be independently 
measured.

Bob


> 
> Agreed, not trivial. But maybe fun anyway!
> 
> Dan
> ================================
> 
> Dan,
> 
> Take a look at Dave Hart's patches to the Microsoft serial port driver, that does something similar.  The source may be in the NTP distribution, or Meinberg may have a copy.  At one time using the QueryPerfomanceCounter call was an option (look for "QPC").  With Windows-8 and later there is a much more precise GetSystemTime call which NTP uses.
> 
> I played a little with those calls (in a non-serious way) and wrote some code:
> 
> http://www.satsignal.eu/ntp/TSCtime.html
> 
> based on:
> 
> http://www.lochan.org/2005/keith-cl/useful/win32time.html
> 
> and just messing around!  Perhaps it's of interest or use?
> 
> Cheers,
> David
> -- 
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
> Twitter: @gm8arv 
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