Achim Gratz <[email protected]>:
> Gary E. Miller writes:
> > Glossary:
> >
> > frequency offset:
> > The difference between the ntpd calculated frequency and the local
> > system clock frequency (usually in parts per million, ppm)
>
> It's actually the current correction ntpd applies to the system clock
> (for ppm, the correct unit would be µs/s). It would converge to your
> definition for a stationary system if all noise sources were unbiased.
>
> > jitter, dispersion:
> > The short term change in a value
>
> That's too short for an explanation. Jitter is the deviation from an
> idealized periodical signal and can refer to both the devation at some
> discrete time (clock event) or more commonly the resulting
> distribution. The dispersion is the width of that distribution and can
> be expressed in different ways depending on the type of distribution.
> > ppt, parts per thousand:
> > Ratio between two values. These following are all the same: 1 ppt, one
> > in one thousand, 1/1,000, 0.001, and 0.1%
>
> When talking about precise frequency measurements I would rather expect
> ppt to refer to "parts per trillion".
>
> "Parts per thousand" is usually expressed as "per mille [‰]"
>
> > σ, sigma:
> > Sigma denotes the standard deviation (SD) and is centered on the
> > arithmetic mean of the data set. The SD is simply the square root of the
> > variance of the data set. Two sigma is simply twice the standard deviation.
> > Three sigma is three times sigma. Smaller is better.
>
> Sigma is the measure of dispersion for normal distributions. It isn't
> centered on anything and the rest of that explanation is either
> misleading or redundant.
Comments on Gary's definitions, *without a proposed definition of your own*,
just make more editing work for me without solving the problem.
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