Operator's Guide
The internal time synchronisation and control software will wait
for the GPS unit to report a good position fix (requires 3
satellites) before starting operation. The system actually waits
6 consecutive ‘good’ messages, which normally occur every 10
to 20 seconds.
If GPS maintains a good fix from the satellites available, the
system will then switch on the control process and set the
internal clock as shown by the status messages over the page.
The system jam-sets its internal clock at this point to be
synchronised to GPS time and will also re-synchronise the
Analog to Digital Converters so that the data is accurately time-
stamped to this new reference. The data transmitted upto this
point will be stamped with the time from the internal back-up
‘Real-Time Clock’ ; this is also now reset to this accurate time.
Re-synchronisation will also result in the received data showing
a discontinuity.
The control process will now attempt to keep the internal time-
base synchronised to the GPS 1 pulse per second output by
adjusting the voltage controlled crystal oscillator. The control
algorithm has two stages - initially it compares its internal 1 Hz
timebase with the GPS 1pps and adjusts the voltage control to
minimise the error. Once this has been achieved it then
controls the crystal to minimise both the ‘phase error’ (offset
between its internal 1 Hz and GPS) and the drift (frequency
error) relative to GPS. During the control process the system
reports the measured errors and the control signal applied, as a
‘pwm’ value - Pulse Width Modulation - digital to analog
conversion.
July 2016
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Summary of Contents for CMG-DM16R8
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