Guralp Systems CMG-6TD User Manual
Page 71
Configuration with Scream!
time at the end of this process. The re-synchronisation will result in a
discontinuity in the data received.
From this point, the control process will attempt to keep the internal
time-base synchronised to the GPS 1 pulse per second (PPS) output, by
adjusting a voltage-controlled crystal oscillator. First it alters the voltage
control to minimise the error. Next, it attempts to minimise both the “phase
error” (i.e. the offset between the internal 1 Hertz signal and the GPS) and the
drift (the frequency error relative to GPS, which is the first derivative of the
phase error). During the control process the system reports the measured
errors and the control signal applied, as a PWM (Pulse Width Modulation)
value.
During the initial, coarse adjustment stage, only the coarse voltage control is
used and no drift calculation is made. If the system is operating in a similar
environment to that when the system was last powered (most importantly, the
same temperature) the saved control parameters will be appropriate and the
system should rapidly switch to the ‘fine’ control mode. The system reports
its control status and parameters each minute, with error measurements given
in nominal timebase units. In an environment with a stable temperature, the
system should soon settle down, showing an offset of only a few thousand
(average error <100 µs) and a drift rate under 100 counts (< 1 in 10
6
).
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Issue F - February 2014