Symmetricom SDU-2000, User Manual

Page: 228 / 470

Hardware Configuration Guide
Configuring a Subtending SSU
228 SSU-2000 User’s Guide 12713020-002-2 Revision D – April 2004
The CC Input modules make phase measurements between the input signal and
the clock oscillator(s). From these phase measurements on the selected input, the
Clock modules compute frequency offset correction and holdover value. The
selected Input module also generates a 4 kHz “C” clock that is used to align the
phase of the CC output signals.
Operating Mode
The operating mode of the SSU is set with the ICS command OPERMODE
[NORMAL| SUBTENDING]. In Normal mode, the unit operates as specified in
Chapter 1, Product Overview . In the Subtending mode, the unit uses only the CC
inputs as reference sources. The bipolar violation (BPV) occurrences in the selected
input are used to generate a signal output for phase alignment of the CC output
signals. In addition, phase measurements on the selected input are used to control
the average frequency of the outputs. If there is a significant difference in the BPV
alignment between the selected CC input and any secondary CC inputs, the unit
issues a warning that the reference signals are not phase aligned. It is the
operator’s (or installer’s) responsibility to ensure that the CC inputs are from the
same (master) BITS and that they are wired correctly to maintain the same polarity.
A valid CC input is required when starting the Subtending mode in order to align the
phase of the CC outputs.
Selecting Input References and Switching References
In the Subtending mode, the unit selects the highest priority valid CC input
available. If the selected input is lost, the unit switches to any alternate CC input as
specified by priority assignments; the phase and frequency of the CC outputs follow
the new reference signal. If all CC inputs are invalid, the output phase continues at
the last valid alignment before the input was lost; the frequency is established by the
learned holdover values in the Clock modules. When a valid CC input is
re-established, the CC output is immediately aligned to the selected input signal. A
CC input is invalid when it is in LOS or it has invalid BPV occurrences. LOS is
declared when the Input module is unable to track the expected 64 kHz input; an
invalid BPV event occurs when the expected 8000 BPVs per second varies by more
than ±10%.
Output Section
The output section operates exactly the same in Subtending mode as in Normal
mode, except for the CC Output modules. In the Subtending mode, the CC outputs
are phase aligned with the selected CC input reference signal. The output section
consists of up to 120 channels from six Output modules (60 channels from 3
redundant pairs) that reside in the main shelf and up to 1120 channels from 56
modules (560 channels from 28 redundant pairs) in a fully expanded system with
four SDU-2000 expansion shelves. Each Output module type provides 20
independent output synchronization signals.