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5−
45
5.1024 Alarm
Circuit
This circuit, under microprocessor control, provides a single line alarm, major
or minor, to the 54560 Alarm Interface Card.
5.1025 Phase
Detectors
NOTE:
For E1 and 2.048 MHz square wave operation, the E1 clock frequency is converted to
T1 frequency before going to the phase detectors. The phase detectors always work at
1.544 MHz.
The SMC contains two identical, independent phase detectors. The selected
recovered clocks from the DS1/E1s being monitored are sent to the phase
detectors for time interval measurements.
The other input to the phase detectors, reference, is the same clock that is
currently selected by the output driver cards. The SEL1, SEL0 signals from
the track and hold card select the reference, although this selection may be
overridden by the microprocessor if required.
Using counter techniques, the phase detector determines the phase difference
between its reference and input signals to 0.5 nanosecond resolution over ±8
UI. An up/down overflow (or unit interval) counter extends this range to ±256 x
8 UI or ±2048 UI (a total range of 2.6 milliseconds or 20 frames).
The hardware phase sampling rate is 600 per second. Each input signal to
the phase detector is divided by 16 (to 69.5 kHz) to extend the linear range of
the phase detector to 8 UI. A lead/lag detector determines whether the phase
counter counts up or down, giving it a total linear range of ±8 UI. If the phase
detector count nears either end of its range, the unit interval counter is
incremented or decremented and the 69.5 kHz signal being measured is
shortened or lengthened by one UI, thus keeping it within the linear range of
the phase detector.
The microprocessor is interrupted each time a phase sample is ready (600
times per second). The microprocessor combines the output of the phase
detector counter (16 bits) and the unit interval counter (8 bits) to calculate the
total phase shift. The starting phase shift is arbitrary. Each count of the phase
detector counter is equivalent to 0.5029782263 nanoseconds. This is the
resolution of the phase detector.
NOTE:
Resolution is NOT the same as accuracy. Accuracy is defined as the maximum
allowable fractional frequency offset.
(continued)
