In general wide band channels tend to operate and release faster than narrow band channels. That is, faster channels
use more spectrum than slower channels.
It is obvious from the foregoing that the received signal at any given terminal is not an exact analog of the remote current.
There are techniques used in phase comparison schemes to compensate for this and they will be discussed subsequently.
Until then it should be assumed that the received signal provides a true representation of the phase position of the remote
c) TYPES OF COMMUNICATIONS MEDIA
The communication medium over which the transmitted signal is propagated to the remote receiver can take several forms:
Directly over the power line (Power Line Carrier)
Multiplexed over the power line (Single Side Band Carrier)
Multiplexed over microwave (Microwave)
Pair of Wires (Pilot Wire)
A distinction is made between leased facilities and the other (power company owned) facilities because in many cases the
telephone company defines the characteristics of the channel without defining the type of link.
The ON-OFF type of communication equipment is used exclusively over power line carrier links. The transmitted signal is
propagated along the power line between the transmitter and the remote receiver. This equipment usually operate in the
frequency range of 30 to 200 kHz.
Frequency-shift equipment is available in several frequency ranges. First there are those in the audio range. These are
generally employed over single side-band, microwave, pilot wires, and leased facilities. There are also frequency shift
channels in the power line carrier frequency range. These are employed directly over the power line as are the ON-OFF
types of equipment. Finally there is the frequency shift equipment that operate in and occasionally outside the power line
carrier spectrum. These are employed over microwave and leased facilities.
d) POWER LINE CARRIER MEDIA
It is obvious that the performance of any channel that utilizes the protected power line itself as a communications medium
will be affected in some way by faults on the power line. A fault on a transmission line can attenuate or completely block a
signal, transmitted at one end of the line, from being received at the remote end. Faults external to the protected line have
no affect on the signal attenuation since transmission lines that incorporate power line carrier channels are trapped at each
end (See Figure 9-9).
(a) Metallic pilot wire
Figure 9–9: TYPICAL POWER LINE CARRIER ARRANGEMENT
L60 Line Phase Comparison System
9 THEORY OF OPERATION