[time-nuts] DC Voltage Ramp?
Tom Van Baak
tvb at leapsecond.com
Sun Sep 4 18:44:20 EDT 2005
> Hi Tom & Robert:
> I think what this is saying is that if the control voltage to the OCXO
> is fixed, then during the 10,000 (or more) seconds needed for GPS the
> OCXO has drifted about 5E-11/day * 10,000/86400 = 6E-12 seconds which
> exceeds what we are trying to accomplish.
> BUT, if instead of using a fixed control voltage, a Ramp is used then
> this will not happen. Now instead of using GPS to control a fixed
> voltage it's used to control the slope of the ramp.
> What am I missing?
I'm not up on my PLL theory but if GPS is used with
a PLL the DAC will change to keep the loop locked.
And if you plot the DAC voltage you will see wiggles
over the short-term (seconds to hours) and a ramp
in the long-term (days to weeks). The size of the
wiggles or the smoothness of the ramp varies greatly
from oscillator to oscillator.
So the question for a PLL expert is - if knowing that
there is a ramp trend over a time frame of days or
weeks; can that knowledge somehow help the PLL
as it tries to close the loop on a second by second
or minute by minute time frame? My guess is no.
When the short-term frequency wiggles around
parts in 10^12th to parts in 10^11th a gradual
long-term average trend of 5e-10 per day is so
far below the noise that it is of no use to the
PLL which must act with a short time constant.
As an example, you can find a nice 10811 that
has a drift rate of 5e-10 / day. Using your math
that comes to 50e-11 / 24 hours or about 2e-11
per hour. But take a look at a typical 10-minute
frequency strip-chart of a 10811 oscillator:
You can see that in just the space of minutes
it wanders around more than an hour's worth
of drift. Look at some of the other plots in that
The point is, a PLL must react to phase and
frequency changes in real-time. In most cases
the normal minute-to-minute fluctuations in a
warmed-up and well-aged quartz oscillator are
much larger than the extrapolated contribution
of the long-term drift rate for the same interval,
even if the drift rate were constant.
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