[ntp:questions] Kernel PLL, microkernel and the simulator.

Unruh unruh-spam at physics.ubc.ca
Wed Jul 23 19:37:07 UTC 2008

"David L. Mills" <mills at udel.edu> writes:


>You quote the sampling theorem, which is a little more powerful than 
>Nyquist. However, you are correct in that the NTP filter is not 
>brick-wall. To compensate for that, the minimum sampling rate is at 
>least twice the Nyquist rate. Having said all that, the proof is in the 
>very many simulation runs with the actual code, which confirms the 
>expected risetime and overshoot.

Have those simulations also been run with weird clockfilter selection
simulations. Eg, the best one is always the 7th oldest, or it keeps
switching or.... Ie, the input is not a nice regular sampling, but has a
large random component.  (Mind you I am not at all sure that this does not
make the sampling better, as the aliasing is spread out and becomes noise
rather than a regular low frequency feature.) 
I suspect that it is fine even with that-- ie the random elimination by the
clock filter does not matter for the stability or reliability of the
scheme. The conservative values do make it very slow to respond, and much
worse than it could be in situations where there are changes in the system
parameter (eg temp changes during the day due to regular periods of use and
non-use of the computers)  but that has been extensively discussed in the


>David Woolley wrote:
>> blu wrote:
>>> Okay, so how is the bandwidth equal to the inverse TC, and is that the
>>> TC as in 3 or as in 2^3=7? I don't understand the meaning of the term
>> The time constant as linear time, i.e. 8.
>>> bandwidth in this context. And when you say that it works even if you
>> ntpd implements a phase locked loop in software.  Phase locked loops 
>> measure a phase error, low pass filter that measurement, and feed it 
>> back as a frequency correction.  The bandwidth is that of the low pass 
>> filter and it is a fairly fundamental consequence of definition of 
>> frequency that it is inversely proportional to the length of the 
>> (significant part of the the) impulse response, i.e. the time constant.
>>> only use every seventh sample, is that even if the sample is seven
>>> poll intervals old every time?
>> Where the reason is actually slightly flawed is that Nyquist says that 
>> you can accurately reproduce a signal which is hard band limited, but 
>> that assumes you know the sampling points.  In any case, the error 
>> signal is not limited to the bandwidth and the low pass filter is far 
>> from a brick wall filter, meaning there will be aliasing effects, which 
>> will cause a sensitivity on sample rate if you only sample at only twice 
>> the filter nose bandwidth.
>> I'm not 100% sure, but I don't think that the ntpd processing 
>> compensates fully for the positions of the samples.

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