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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Wed Jul 23 07:09:46 UTC 2008

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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