[ntp:questions] Setting the maximum rate of change (Harlan Stenn)

David Woolley david at djwhome.demon.co.uk
Wed Mar 21 23:34:31 UTC 2007


In article <4B73D9EBE2327E40B4D69822D7C4580206F75980 at sjmail1.symmetricom.com>,
GDowd at symmetricom.com (Greg Dowd) wrote:

[ Pretty sure the threading is broken after this! ]

> If I understand correctly, the question regards slew rate limits.  In an

That's not my understanding.  My understanding is that he want to 
constrain the first derivative of the frequency correction.  That is
the second derivative of the offset.  Slew rate is used to describe
the frequency correction, particular when it is being used to reduce
an offset error.

What's probably implicit is that, in the specification he is trying
to implement, this is the derivative including the first derivative of
the crystal frequency, so the 25mHz/27MHz/s (about 0.001 ppm/s) might
be totally taken up by variations in the crystal frequency.  Actually,
given that he wants this condition met after one minute, but the machine
probably won't reach thermal equilibrium for more like 15 minutes, I think
it will almost certainly be violated by the crystal frequency on its own.

In that case, the only way of achieving conformance is to NOT constrain
the first derivative of frequency correction, so that there is some
chance of the frequency being compensated fast enough to compensate for
the crystal drift.

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