GE L60 Instruction Manual page 596

OVERVIEW
• There is the tail off time in the transmitter from the instant the keying is removed until the output signal changes or
disappears. 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
attenuation.
• 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 is more than a half-cycle MARK
and less than a half-cycle SPACE.
• 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.
In short, 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, as outlined subsequently. Until then, assume that the received
signal provides a true representation of the phase position of the remote current.
9.1.5.3 Types of communications media
The communication medium over which the transmitted signal is propagated to the remote receiver can take the
following 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)
• Leased Facilities:
– Metallic pilot wire
– Microwave
– Cable
A distinction is made between leased facilities and the other facilities owned by power companies 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 operates 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 operates in and occasionally outside the power line
carrier spectrum. These are employed over microwave and leased facilities.
9.1.5.4 Power line carrier media
The performance of any channel that utilizes the protected power line itself as a communications medium is 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
9
end (see the figure).
9-14
CHAPTER 9: THEORY OF OPERATION
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL