GE L60 Instruction Manual page 605

CHAPTER 9: THEORY OF OPERATION OVERVIEW
the filter response in the receiver and is common with ON-OFF type of equipment. It is not a constant value but rather
depends on operating frequencies as well as received signal strength. Thus, this asymmetry can vary from equipment to
equipment and from time to time (as atmospheric conditions change) in service.
Frequency shift channels are generally symmetrical in their response when the discriminator in the receiver is balanced. If
the discriminator is biased to one side or the other, the receiver output tends to favor the side to which it is biased.
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
9
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.
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-23