William Unruh wrote:
On 2015-02-12, Charles Swiger <cswi...@mac.com> wrote:
On Feb 12, 2015, at 1:56 AM, Rob <nom...@example.com> wrote:
Charles Swiger <cswi...@mac.com> wrote:
On Feb 11, 2015, at 7:23 AM, Rob <nom...@example.com> wrote:
However, what I observe is that the plots of the offset show the derivative
of the environment temperature, which unfortunately cannot be controlled
any better. I am considering to locate the crystal that is responsible
for the timing and see if it could be ovenized or replaced by a more
temperature-stable oscillator. However, one can argue that it could be
fixed in software as well. ntpd could sense a changing drift and extrapolate
it, if necessary helped by input from a temperature sensor.
You're describing a TCXO; using a temperature sensor to compensate for thermal
drift would gain perhaps a factor of 5 accuracy.
No, that is a hardware solution. There are software solutions-- a
termistor to meaure the temperature of the crystal ( or somethign
nearby) which feeds that measurement to the OS. the revised ntp then
reads the temperature, and corrects the drift rate as a function of that
temperature. This means that the change in the ntpd drift rate does not only
depend on
the offset meaured but also on that temperature. Since it takes a while
for a temperature to be reflected in the offset, this makes ntpd track
the correct rate of the clock much more closely. Yes, factors of 5 are
easy. Actually, I suspect that oneof the reasons that chrony does so
much better than ntpd does in disciplining the clock ( 2-20 times
better) is because it reacts to such temperature changes much more
rapidly. It can do so because it keeps a memory of the drifts and
offsets and can see changes much more quickly. It also does not throw
away 85% of the measurements to correct round trip errors, so can also
react faster because of that.
This is all without controlling the temperature of the oscillator (TCXO)
but rather measuring that temperature-- much cheaper.
Solutions that measure the temperature require calibration
for the individual crystal as with the cheap crystals used
the drift per deg C can be either positive or negative and
also depending on "cut" of the crystal can follow a
parabolic or "lazy S" curve.
The alternative of fitting a simple heater with temperature
control to the crystal seemed to be more effective and with
pps ntp source the offset was < 300n.
There might still be examples on the web where this has been
done but the references I used have long since been taken
down.
David
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