[time-nuts] Cheap jitter measurements

Bob kb8tq kb8tq at n1k.org
Tue Apr 10 12:31:28 EDT 2018


Hi



> On Apr 10, 2018, at 11:56 AM, Dan Kemppainen <dan at irtelemetrics.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?

Yes, but looping back to the start of this thread, the intent is to actually measure the jitter directly. 
That’s where the whole need for a stamped pulse comes from. To paraphrase the earlier comments,
the intent is to get away from a “self reported” result and actually do a direct measurement. 

Bob


> 
> Agreed, not trivial. But maybe fun anyway!
> 
> Dan
> 
> 
> On 4/10/2018 11:03 AM, time-nuts-request at febo.com wrote:
>> NTP already looks at incoming pulses and reports what it thinks is going on with them. The desire here
>> is to get a pulse*out*  of the device. Then you can toss it into a conventional set of gear. From the data
>> you can independently evaluate what’s going on.
>> So more or less:
>> 1) Generate pulse
>> 2) Work out when the pulse went out
>> 3) Compare that to what NTP thinks is going on
>> 4) Generate a message to describe the delta in time
>> No, not trivial ….
>> Bob
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