[ntp:questions] how does jitter and round trip time affect the accuracy of the local clock?

Miroslav Lichvar mlichvar at redhat.com
Mon Jun 27 10:58:18 UTC 2011


On Sun, Jun 26, 2011 at 09:05:59PM +0100, David Woolley wrote:
> If the jitter is of the order of 500 microseconds, and your
> delays are perfectly symmetric, and there is no clock wander (in
> particular, the temperature is tightly controlled), the error will
> exceed 500 microseconds, a small but significant amount of the time.

That wouldn't be a very good clock discipline if it wasn't able to
keep the clock error significantly below the jitter in these ideal
conditions.

In a simulation with 500us exponentionally distributed jitter and
clock wander insignificant to the PLL time constant, the RMS time
error is about 40 us for the standard PLL and 80 us for the Linux PLL.
The 99th percentiles are about 100 us and 200 us respectively.

-- 
Miroslav Lichvar



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