Symmetricom Z3801A, User Manual

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Chapter 2 Features and Functions
Operating Concepts
User Guide 2-11
Operating Concepts
General
The time required to acquire GPS lock as described in the following
paragraph can vary significantly depending on your local conditions.
In general, we strongly recommends that your antenna and cables be
set up in accordance with the information provided in this guide prior
to using the output signals of the Receiver to ensure they are valid.
Acquiring lock does not mean that the Receiver is fully operational and
meeting all specifications. It just means that the Receiver has detected
enough satellites to start its survey mode to determine its precise
location. An internal measurement FFOM (Frequency Figure of Merit)
becomes 0 when the internal loops reach their proper time constants,
indicating that the output frequency and 1 PPS signals are now fully
operational and meeting their specifications. Under the worst
conditions, the Receiver may take up to 24 hours to achieve FFOM = 0.
FFOM can be monitored in the Reference Outputs quadrant of the
Receiver Status screen (see Figure 3-1 in Chapter 3, “Using the
Receiver Status Screen,” of this guide). Also, using the appropriate
SCPI query command will provide FFOM value (refer to Chapter 4,
“Command Listing and Status Information,” in this guide for specifics).
The Receiver is designed to automatically detect and acquire satellites
in order to begin providing precise frequency and time information.
Until such acquisition is complete and the instrument is locked with
FFOM = 0, the signals produced on the rear panel are not precise.
Holdover Description
If the GPS signal is interrupted, the Receiver enters an intelligent
holdover mode that uses SmartClock® technology. SmartClock takes
over control of the quartz oscillator that has been steered to the GPS
reference during locked operation. SmartClock predicts the
performance of the quartz oscillator based on the information gathered
during the “learning period” (locked to GPS). Corrections are
automatically issued over time, keeping the performance of the quartz
oscillator as close as possible to the performance achieved while locked
to the GPS reference signal.
Holdover frequency is maintained to better than <1
×
10
-9
per day.
The time specifications 1 µs locked, and 7µs unlocked for 24 hours.
When the GPS reference signal is restored, the Receiver automatically
switches back to normal mode of operation.