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 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 provide 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 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.
Warm Up Times
All precision reference sources require a “warm up” time to achieve high accuracy and stability. The warm up term is being used
loosely because it is not just about attaining the right temperature. It also includes disciplining of a local oscillator, negotiating and
stabilizing a clock through protocol messages, etc. Each of the tasks involved takes time to stabilize before it can be used for testing.
Operational Temperature
It is still a major player in the warm-up waiting time and it all depends on the ambient temperature and the initial temperature of the
test set. For example, a test set left in the trunk of a car in a winter night will take longer to reach operational temperature.
Atomic Clock Warm Up
If fitted with a chip-scale atomic clock, they are housed in a miniature oven to maintain its internal temperature constant in spite
variations in ambient temperatures and it can take up to five minutes to warm up and should not be used until its status shows
“Locked”. The atomic clock temperature and status can be monitored at \Utilities \Settings \More \High Precision Clock Source
\Atomic Clock. The chip-scale atomic clock is oven-controlled and temperature-compensated to minimize the effects of ambient
temperature variations.
Beside temperature, users must be aware of other factors that require time to settle, before accurate measurements can be made.
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