Qualitrol INFORMA PMD-A User Manual page 49

Components and Specifications
The next level of validation is the "PPS stability check". The external PPS pulse is said to be
stable, as long as the PPS to PPS duration remains constant within the acceptable jitter limit
as mentioned in below table.
Sl. No. External Time Source (PPS)
1 GPS
2 INT RTC
3 IRIG-B
4 IRIG-J
This check is performed continuously and the status of the PPS stability is updated every 16
seconds. If the external PPS is stable for 16 seconds, then the PPS is declared as stable;
else, it is rejected. When multiple INFORMA PMD-A devices are in the locked state, ADC
sampling is synchronized.
INFORMA PMD-A is said to be "locked to External time source" when the internal time is
synchronized with the external time source within an acceptable band of time accuracy.
INFORMA PMD-A is said to be in "Not-Locked State" if any of the following conditions
occurs:
•
The external PPS does not pass the stability check.
•
The time difference is not within the acceptable jitter limit.
When there is a transition of INFORMA-PMD-A internal PPS from not-locked to a locked
state, time correction occurs. The time correction can happen in any of the following ways:
•
Gradual time correction
•
Abrupt time shift
Gradual Time Correction
The INFORMA-PMD-A internal time is corrected gradually if the difference is less than 0.5
seconds with respect to the external time. The maximum error correction allowed per cycle
is approximately half of the intersample delay. Approximately, the time required to perform
the correction for a 0.5-second error is 8.5 minutes. As a result, after the device is switched
on, the clock synchronization LED might take a maximum of 8.5 minutes to turn on
indicating the system is locked.
Abrupt Time Shift
If the difference in time is greater than 0.5 seconds, then the INFORMA-PMD-A internal time
will be abruptly shifted to the external time. The abrupt shift in time is always indicated to the
user.
Jitter Limit (+/- )
10 µs
1 ms
1 ms
1 ms
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