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Limitations: This method of determining proper 1PPS phase disciplining convergence would only work at the beginning of the
disciplining process, which is what would be needed in the field.
Long-term, especially when long time constants are used, the oscillator will become hard to steer as it would be trying to hold what it
“believes” is true time alignment, based on a long learning process. In this case, if the GPS receiver starts to wander and becomes
somewhat inaccurate, the graph would show such discrepancy, but the oscillator’s 1PPS output would still be stable and accurate.
GPS instant accuracy could change within ±150ns during the course of a day depending on atmospheric conditions and satellites
visibility. The job of the atomic clock is to filter those slow variations, so in the long term it is normal to see the GPS and CSAC
phases temporarily disagree (relative phase ≠ zero).
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.
GPS Lock
The time to get an stable clock output varies depending on the conditions, antenna type and installation, sky visibility and whether or
not the test set has changed position. Using the test set for the first time on a new site (different geographical position) would
increase the time to its first satellite lock. Users can follow the different stages (Searching, Sync 1PPS, and Lock) by checking the
GPS status GUI at >Utilities >Settings >More >High Precision Clock Source >GPS
Disciplining Time
Disciplining the atomic clock means using the accurate GPS timing signal to correct or align the atomic clock output signals. GPS is
known for its high accuracy as it is traceable to the universal time standard (UTC). Atomic Clocks are known for their great stability.
So, the combination of GPS + Atomic Clock = Highly Accurate and Stable clock source. Achieving such levels of precision takes
time. The total time to stable output depends on the precision required. The disciplining time can be programmed in the Atomic
Clock settings and users must add it to the total “warm-up” time, before starting measurements.
Precision Timing Protocol
PTP, like IEEE 1588v2, requires some time for the two ends (master and slave) to agree on the current time. This protocol “warm-
up” time is dependent on the link conditions (traffic, latency, delay variations, PTP settings, etc.). Tests shall not be started until the
protocol has stabilized and the recovered clock has achieved its maximum accuracy and stability. This is sometimes referred as
“Convergence” or “Sync PDV” convergence. Users can use the long tail of the 1588v2 Sync PDV graph as an indication the PTP
has reached synchronous state.
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