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

David L. Mills mills at udel.edu
Fri Jul 25 16:46:31 UTC 2008


Bill,

See the papes and reports on the NTP project page, including extensive 
simulations and evaluations of the NTP algorithms, and then let's talk.

Davde

Unruh wrote:
> "David L. Mills" <mills at udel.edu> writes:
> 
> 
>>David,
> 
> 
>>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
> past. 
> 
> 
> 
>>Dave
> 
> 
>>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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