[ntp:questions] Raspberry Pi error in PPM offset

JP james.peroulas at gmail.com
Sun Aug 25 14:16:48 UTC 2013


On Saturday, August 24, 2013 10:41:30 AM UTC-5, unruh wrote:
> On 2013-08-24, JP <JP> wrote:
> > It's converging to an average time offset of zero, but an average frequency offset of +2.5PPM.
> 
> Fine. That means that your RPi crystal oscillator is 2.5PPM out from
> 
> spec, or the calibration that the operating system did on the crystal to
> 
> set the clock rate is 2.5PPM out. Either way ntpd corrects that error.
> 
> If it is the calibration loop, then probably the next time you boot up
> 
> it will be -4.7PPM out. etc. 

I guess there is some mis-communication here. I'm performing several measurements using an external frequency counter and for each measurement, I'm recording the externally measured PPM error at that time as an entry in a vector, ppm_actual. At the same time (within a few seconds), I'm also recording the PPM error reported by NTP in the vector ppm_ntp. I would have expected mean(ppm_actual-ppm_ntp) to be zero, but it is not. Instead, it is about +2.5 PPM.

So, the average time offset of the Pi is zero (I have not measured this, I assume it is true since the NTP loop has converged and locked). The average PPM error reported by the Pi mean(ppm_ntp) is -45.5. The average PPM error measured by my external counter mean(ppm_actual) is -43.

BR,
James



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