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5.9 Model 54593 GPS Stratum 1/PRC Track and Stratum
2/TNC Hold Clock Card
5.9010 There are two 54593 track and hold cards in a redundant Stratum 1/2 or
PRC/TNC system. The 54593-0 supports the AB input architecture (see
Appendix A); the AA input architecture is not supported. In an oversimplified
way, the Model 54593 card is a Model 54523 card with a GPS receiver added.
5.91 Functions
5.9110 The Model 54593 clock card performs the following functions:
a. Accepts one GPS reference signal and provides timing and a smoothed
and jitter free signal to the output cards from a rubidium oscillator tracking
a GPS signal. The card also accepts one of two optional 1.544 MHz
signals for a tracking reference if the GPS receiver should fail. Refer to
Figure 5-5 for a flow chart tracking the 'health' of the GPS module.
b. If the GPS receiver fails, monitors the DS1/E1 reference signal for framing,
CRC, and out-of-specification errors. An out-of-specification input signal
forces the card to hold to the last known reference, with a drift of less than
7.5 x 10
-11
in 24 hours. By means of a digital phase detector, the 54593
measures the input DS1/E1 reference bit time phase compared to the
rubidium atomic oscillator output as conditioned by a DDFS. Samples of
phase information are collected and averaged over about a seven-second
period to provide input to the control algorithm. If, at any point in the
accumulation and calculation period, a condition is detected (excessive
errors, LOS, OOF, etc.) which causes the collected information to be
suspect, the unit enters the holdover state and does not update the
correction to the synthesizer. If the data are qualified, then the update will
proceed. After 128 acquire cycles (about 15 minutes) following initial
qualification, the clock will be in tracking mode. After 45 to 60 minutes, the
tracking mode will be at minimum error if the input is within specifications.
The 54593 will track timing from another Stratum 2/TNC or Stratum 1/PRC
source having an average frequency offset of no more than ±0.04 Hz. The
signal to the output cards is continuous (no phase hits) during any switch
from track to hold. Frequency and phase corrections occur over periods of
about 10,000 seconds. A fast ACQUIRE state reduces this to 300
seconds for power-up and 1000 seconds for return from the HOLD state.
c. Accepts an alternate external 5 or 10 MHz signal from a high stability
timing source such as a GPS receiver, cesium clock, or similar
mechanism. This signal may be used as a tracking input. The external 5
or 10 MHz signal is selected as a tracking reference through user
commands to the 54550 Information Management Card.
d. Supplies monitoring and alarm information to the 54550 Information
Management Card and 54560 Alarm Interface Card.
e. Facilitates synchronization status messaging per Bellcore GR-378 (DS1
only).
5.92 Circuit
Description
