VeEX VePAL TX320s, Руководство пользователя

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GPS-Disciplined Clock
When GPS and Atomic Clock options are installed and enabled, the Atomic Clock uses the GPS signal to calibrate its frequency (10
MHz) and timing references (1PPS), to improve their accuracy and stability. The Atomic Clock 1PPS phase is disciplined to the UTC
to align it with the standard second. The raising edge of the 1PPS pulse indicates the beginning of a new second all over the world.
Disciplining can be disabled or enabled. When disabled, the atomic clock runs at its natural frequency (free-running), providing a
very stable frequency source. When disciplining is Enabled, the atomic clock will use the GPS accuracy to correct its frequency and
align its phase.
The Disciplining Profile sets a time window on the atomic clock's dynamic control loop, to filter any short term frequency and phase
variations coming from the GPS receiver, this is called Time Constant. The longer the time constant, the more accurate and stable
the atomic clock output can be. The Disciplining Profile field provides suggested options for the minimum disciplining times and sets
the equivalent Time Constant. The selection varies with applications and location, so some experimentation may be required by
users to select their own default value. For quick field Wander or Phase measurements a disciplining time of ≥ 60 minutes (TC ≥
1800 seconds) is recommended. Users can also enter a customized time constant in seconds.
Holdover
In case of GPS signal loss (e.g. indoor testing) the high stability of the Atomic Clock can maintain synchronization for a few hours
allowing users to perform Wander and Absolute Phase Error measurements in conditions that were not possible before. This is
called holdover mode.
Delay Compensation can be used to perform minor adjustments to the 1PPS phase, to align it to a specific reference or to
compensate for short cable delays.
When enabled, the low power Sleep Mode keeps the atomic clock powered up when the test set is turned off. This way it maintains
its last calibration by drawing little current from the batteries. In this state, the test set will only cut the power to the atomic clock if the
battery charge reaches a level less than 50% of charge, to conserve battery. The Elapsed Time counts the time since its first
initialization, including the time the test set has been off. If disabled, the atomic clock is shut down with the test set and will require
full initialization the next time it is powered up.
Atomic Clock Relative Phase Monitoring
VeEX test sets equipped with GPS receiver and Chip Scale Atomic Clock options include a relative phase monitoring tool that can
be used for the Relative Phase Measurements that provide a bit more visibility into the disciplining process.
In the absence of another traceable frequency source or timing reference, users have to rely on relative phase measurements. It is a
direct comparison between the GPS receiver’s “raw” 1PPS signal being fed to the high-precision oscillator (CSAC) and the filtered
(stabilized) 1PPS output from the oscillator, which ultimately would be the reference signal to be used by the test set for Wander,
Absolute Time Error (Phase) and One-Way Delay (link symmetry) measurements. Since the disciplined output combines the short-
term stability of the precision oscillator and the long-term accuracy of the GPS it provides the best of both worlds, so it can be used
to measure the internal GPS receiver output to verify they are in agreement.
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