8-20 L60 Line Phase Comparison System GE Multilin8.1 OVERVIEW 8 THEORY OF OPERATION88.1.7 REFINEMENTS TO BASIC SCHEMESThere 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 8-4, but will apply generally to all schemes, some-times in a somewhat different form.a) SYMMETRY ADJUSTMENTAs was noted in a previous section, receivers are not always symmetrical in their response. That is, if a transmitter is keyedon 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, theremote receiver output might be on for more than a half cycle and off for less than a half cycle. This affect is primarily due tothe filter response in the receiver and is common with ON-OFF type of equipment. It is not a constant value but ratherdepends on operating frequencies as well as received signal strength. Thus, this asymmetry may vary from equipment toequipment 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. Ifthe 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-try adjustment.The symmetry adjustment is in the receiver input circuit as shown in Figure 8-15. It is set with either a time delay pickup ora time delay drop out depending on whether the receiver elongates or shortens the received signal. The time setting ismade 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 fromthe mixing network. Then, while this is taking place, the time delay pickup or dropout of the symmetry logic is adjusted sothat the receiver yields a symmetrical output.Figure 8–15: BLOCKING SCHEME WITH SYMMETRY AND PHASE DELAY ADJUSTMENTS
