[ntp:questions] Re: ntp performance, jitter, etc and its implications for test/verification of a timing source

Richard B. Gilbert rgilbert88 at comcast.net
Mon May 8 13:51:44 UTC 2006


Jim Cromie wrote:

> Richard B. Gilbert wrote:
> 
> thanks Richard
> 
>> Jim Cromie wrote:
>>
<snip>

>>
> Let me re-explain what Im after.
> 
> I could shutdown ntpd, and let the hardware timer free-run.
> Id expect the frequency to go its natural way, so without the correction 
> under lock,
> it would end up with a different time, and I could compute PPM error 
> from the duration and the diff.

Ntpd already does exactly that.  Do you think you could somehow do it 
better?

> Ive told the clocksource driver that the hardware is a hires timer at 
> 1.000000 Mhz, which is
> somewhat inaccurate.  I want to add/subtract some tiny amt so that when 
> the free-running
> hardware timer, driven by a crystal, rolls over, its treated as 
> X+epsilon nanoseconds,
> not just X
> 
> Id rather not wait 12 hrs to get a reasonably accurate number,
> so Id like to take some number (the 'correction') from ntpd itself and 
> compute the free-run freq
> from it.  One of the numbers in the ntpd state represents the frequency 
> correction.
> 
What leads you to believe that the correction is a constant?  The 
oscillator frequency varies with the temperature, the age of the 
crystal, and perhaps other things as well. If the correction were a 
constant, nobody would bother to run ntpd; they would just measure the 
clock drift, compute the correction and apply it.

Nothing you have said here tells me how you are going to get the number 
of nanoseconds to add to "correct" the rollover time of your crystal 
clock!   You haven't shown anything that will measure nanoseconds!! 
Each "tick" of your 1.000000 MHz oscillator is 1000 nanoseconds!!!

Ntpd, with a GPS Timing Receiver as a reference can come close to 
microsecond accuracy.  The GPS, itself, only guarantees that the leading 
edge of it's Pulse Per Second (PPS) output is within +/- 50 nanoseconds 
of the top of the second.

Either I totally misunderstand the what and how of what you are trying 
to do, or you do!




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