9 THEORY OF OPERATION
9.1.7 REFINEMENTS TO BASIC SCHEMES
There are a number of standard refinements that are required and normally included in all phase comparison schemes.
These will be discussed in terms of the basic blocking scheme of Figure 9-4, but will apply generally to all schemes, some-
times in a somewhat different form.
a) SYMMETRY ADJUSTMENT
As was 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 would not necessarily correspond exactly to the key-
ing 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 might 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 may 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 may operate with asymmetrical channels are equipped with a symme-
The symmetry adjustment is in the receiver input circuit as shown in Figure 9-15. 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 sen-
sitivity. 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
L60 Line Phase Comparison System