GE Digital Energy L60, Инструкция по эксплуатации

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9-22 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
OVERVIEW CHAPTER 9: THEORY OF OPERATION
9
Because of this, all phase comparison schemes that can operate with asymmetrical channels are equipped with a
symmetry adjustment.
The symmetry adjustment is in the receiver input circuit as shown in the following figure. It is set with either a time delay
pickup or a time delay drop-out depending on whether the receiver elongates or shortens the received signal. The time
setting is made in the field after the transmitters, receivers, and coupling equipment have all been tuned and adjusted for
proper sensitivity. The proper setting is obtained by keying the transmitter on and off by means of a symmetrical sinusoidal
output from the mixing network. Then, while this is taking place, the time delay pickup or dropout of the symmetry logic is
adjusted so that the receiver yields a symmetrical output.
Figure 9-15: Blocking scheme with symmetry and phase delay adjustments
The receiver output is now symmetrical, but can be phase-shifted in the lagging direction from the actual keying signal at
the remote terminal. This latter result is not good, but it can be mitigated. In addition, there is the propagation delay in
getting the communication signal from the remote transmitter to the local receiver (1 millisecond per 186 miles) plus the
delay in the receiver itself. All of these compound to produce a receiver output that can be significantly phase-delayed
from the current at the remote end of the line.
This is undesirable because it introduces an error in the phase comparison. There is no way to eliminate this phase delay
but there is a way to compensate for it. This compensation is accomplished by the phase delay timer in the comparer input
circuit.