9-22 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUALOVERVIEW CHAPTER 9: THEORY OF OPERATION9The following illustrates a dual phase-comparison blocking scheme using a two-frequency, frequency-shift channel. Sinceone or the other of the two frequencies must be on at all times, and since both are blocking frequencies, there appears tobe little need for an FDL function. Thus, it is not included. When the transmitter is not keyed, it sends low-shift continuouslyand when it is keyed by the negative squaring amplifier, it shifts to high for the negative half-cycle. This scheme is simplerthan that of the previous scheme but probably is not as secure.Figure 9-14: Dual phase-comparison blocking schemeThere does not appear to be a good purpose for a three-frequency channel in dual phase-comparison blocking schemesbecause the center frequency does not add to security or otherwise improve performance.Note that a dual phase-comparison scheme using an ON-OFF channel has to be a combined blocking and trippingscheme. During one polarity of half-cycle, it has to trip on absence of any received signal (blocking), and on the otherpolarity of half-cycle it has to trip in the presence of the received signal.In general, it can be concluded that dual phase-comparison can be accomplished in the blocking and in the trippingmodes. The overall performance of the scheme depends on the characteristics of the channel selected. While dual phase-comparison reduces the maximum tripping time, it does so at the expense of simplicity and possibly some securitydepending on how it is accomplished.9.1.7 Refinements to basic schemesThere are a number of standard refinements that are required and normally included in all phase comparison schemes.These are discussed in terms of the basic blocking scheme of Figure 9-4, but apply generally to all schemes, sometimes ina different form.9.1.7.1 Symmetry adjustmentAs noted in a previous section, receivers are not always symmetrical in their response. That is, if a transmitter is keyed onand off symmetrically every half-cycle, the remote receiver output does not necessarily correspond exactly to the keyingsignal. 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 can be on for more than a half-cycle and off for less than a half-cycle. This affect is primarily due to
