GE L60 Instructions Manual page 671

CHAPTER 9: THEORY OF OPERATION OVERVIEW
9.1.7.1 Symmetry adjustment
As noted in a previous section, receivers are not always symmetrical in their response. That is, if a transmitter is keyed on
and off symmetrically every half-cycle, the remote receiver output does not necessarily correspond exactly to the keying
signal. For example, if an ON-OFF transmitter were keyed on for a half-cycle and then off for a half-cycle, and so on, the
remote receiver output can be on for more than a half-cycle and off for less than a half-cycle. This affect is primarily due to
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
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L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-23