9 THEORY OF OPERATION
The main point to be gained from the foregoing discussion is that it is not always possible to determine from a logic diagram
whether a scheme is of the blocking or tripping type unless an indication is given as to the receiver output voltages. This
applies to frequency shift as well as ON-OFF communication equipment.
It will become apparent from subsequent discussion that it is extremely difficult, if not impossible, to provide a concise rigor-
ous definition of the terms Blocking Scheme and Tripping Scheme. Possibly it would be well to proceed with a discussion of
the different kinds of channels, their characteristics, and their application before attempting a definition.
b) CHANNEL TYPES
The total channel is composed of the communication equipment itself plus the path or link over which the signal is sent. For
relaying purposes there are two basic types of communication equipment.
The ON-OFF type, as the name implies, operates with the transmitter either being keyed on or off by the relay logic. That is,
the transmitter at any given instant is either sending an unmodulated signal or it is sending nothing.
There are two types of frequency-shift equipment. The most prevalent is the two-frequency kind. With this type, the trans-
mitter can send either of two closely spaced frequencies. When no keying signal is applied to the transmitter, it sends one
of these two frequencies. When the transmitter is keyed, it shifts to the other frequency. It is always sending one or the
other. The frequency-shift receiver has two separate outputs, one for each of the two transmitted signal frequencies. Thus,
if the transmitter is sending the MARK frequency, the MARK output is present in the receiver. If the transmitter is sending
the SPACE frequency, the receiver SPACE output is present. These types of receivers are basically FM receivers and uti-
lize discriminators. Because of this, the SPACE and MARK outputs from the receiver cannot both be present simultane-
ously. Also, broad band noise at the input to the receiver tends to provide a balanced signal to the discriminator which
forces its output towards zero. If the noise is severe enough to swamp out the real signal, it can cause random receiver out-
put or all output to disappear.
The other kind of frequency-shift equipment is a three-frequency type. When this type of transmitter is in its quiescent state,
it sends the center frequency. It has two separate keying inputs so that it can be keyed to shift high or low (MARK or
SPACE) from the center frequency. The three-frequency receiver receives all three frequencies but provides only two out-
puts to the relay logic, the high shift and low shift outputs. When the receiver receives the center frequency neither the high
nor low outputs are present. Here again the MARK and SPACE outputs (high and low) cannot both be present simultane-
ously, and severe broad band noise at the receiver inputs can result in receiver output.
There are several characteristics of communication equipment directly related to phase comparison relaying performance
that might well be discussed. Phase comparison types of schemes compare the phase angle of a current derived at one
end of a line with a communication signal received from the remote end. The communication signal arrives in a MARK-
SPACE arrangement that should represent the positive and negative half cycles of current at the transmitted end of the line.
Actually this is not possible for several reasons:
There is a time lag from the instant a transmitter is keyed until the output reflects a change. This build up is generally a
very short time and is usually insignificant.
There is the propagation time from the instant the transmitter sends until this signal arrives at the remote location,
approximately 1 millisecond for every 290 km (180 miles) of distance. The same applies from the instant the transmit-
ter stops until the remote signal is gone.
There is the build up time in the receiver from the instant the signal appears at its input until the output reflects the
change of state. This time plus the build up time in the transmitter is called the channel operating time.
There is the tail off time in the transmitter from the instant the keying is removed until the output signal changes or dis-
appears. This is generally very short and is usually insignificant.
There is the tail off time in the receiver from the instant the input changes until the output changes accordingly. This
time plus the tail off time of the transmitter is called the channel release time.
In ON-OFF channels the operating and release times are not generally the same. They can vary with frequency and
In frequency-shift channels the discriminator employed in the receiver can be balanced so that build up and tail off
times are equal, or it can be unbalanced (biased) to the MARK or SPACE side. For example, if it is biased toward
MARK and the input signal is symmetrical (half cycle MARK and half cycle SPACE), the output will be more than a half
cycle MARK and less than a half cycle SPACE.
L60 Line Phase Comparison System