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CXR Larus 80-100-400 Issue 1, July 2006 1-5
1.216 Each track and hold card has provision for two DS1, E1, or CC tracking
reference inputs, one primary and one secondary, and contains circuitry to
remove jitter and wander from the input signal being tracked. If a StarClock
200E system loses its active tracking reference, the cards switch to holdover
mode where the digital frequency synthesizer continues to run at the last
setting before the reference was lost. Frequency accuracy in holdover
determines the timing level of the various track and hold card models. After a
track and hold card switches into holdover mode, it attempts to acquire and
track the reference signal at its secondary tracking input. If it is successful, the
card will continue normal operation.
1.217 Two input architectures are available for StarClock 200E systems with two
54522 or 54523 track and hold cards. The AA input architecture has both cards
using the same source as the primary DS1/E1 tracking reference and another
source for the secondary tracking reference. The AB input architecture uses
one source as primary on one card and the other source as secondary. The
other card reverses the references so that each DS1/E1 source is primary on
one track and hold and secondary on the other. The 54591 and 54593 GPS
track and hold cards provide only the AB input architecture. AB architecture is
the dominant choice in North America. AA architecture is used in some
international locations.
1.218
The StarClock 200E track and hold clock cards have phase alignment circuitry
that causes the secondary input track and hold card to attempt continually to
align its output phase with the primary card, regardless of the reference being
tracked. Thus, an input switch by the output driver cards from one input track
and hold card to the other will cause a minimal phase change at the outputs.
1.219
The StarClock 200E is nonrevertive which means that, if the primary DS1, E1,
or CC reference fails and the track and hold card(s) switches to use the
secondary input, the card(s) will not switch back to use the primary input unless
the secondary input fails or a Switch Inputs command is sent from the terminal
or PC.
1.220 As another alternative, the StarClock 200E track and hold cards can be
commanded to use an external 5 or 10 MHz source as a tracking reference. A
suitable source could come from a cesium beam clock, an external GPS
receiver, a LORAN-C receiver, or similar precision source. The accuracy of the
outputs from a StarClock 200E system in this configuration is related to the
accuracy of the external 5 or 10 MHz source.
1.221
If a track and hold card has never been in tracking mode since it was powered
up, there is no information available to set the frequency synthesizer for
holdover mode. In this case, the frequency synthesizer is set to run at the
nominal default frequency. This mode is called free run. Since there is no
reference available to compare to the output frequency in free run, the output
frequency could be farther off-frequency than in holdover.
