Lesson 1/Learning Event 8
respect to a normal standard of 1 milliwatt (mw), designated 0 dBm on most
decibel meters. Higher levels than 0 dBm carry a plus (+) sign, and lower
levels carry a minus (-) sign.
a. Equalization. All telephone wires and cables have series inductance
caused by magnetic fields around the wires and parallel capacitance between
the wire pairs.
The combination of series inductance and parallel
capacitance causes the line to resemble a low-pass filter. Such a filter
tends to pass the low frequencies readily but to attenuate the high
frequencies. Correction of this condition is obtained by bridging circuit
elements across the line so that they produce the exact opposite effect to
the line constants.
Inserting these circuit elements constitutes the
process of equalization, which is always accomplished at the receiving end
of the line. The result of equalizing is to attenuate the low frequencies
and raise the high-frequency level.
Technically, this is accomplished by
restoring the phase shift between current and voltage to zero across the
entire range of line frequencies; in other words, current and voltage are
now in phase.
The equalizing procedure always introduces some loss of
signal power, so an amplifier normally follows each equalizer in the signal
path. The amplifier is designed to raise the equalizer output signal level
to that required by the terminating device.
b. Delay compensation.
Theoretically, transfer of the current and
voltage elements of a signal takes place instantaneously at the transmitting
end of a circuit.
Consequently there is no delay in power transfer.
However, as the signal travels down the line, the constants of inductance
and capacitance take their toll of signal quality by shifting current and
voltage components of the signal.
When the signal arrives at its
destination, the phase relationship is different from when the signal was
originally transmitted, and therefore the power within the signal is delayed
in time.
The result of this change is a variation of the signal's
characteristics at different frequencies within the signal. The waveshape
of the signal is therefore different when received than when transmitted.
Obviously, the signal is now distorted. The correction technique involves
the bridging of circuits containing inductance and capacitance across the
line to offset the effect of line characteristics which caused the delay.
This is the principle of delay compensation. Delay compensation techniques
are analogous to equalization techniques, except that different testing
devices are used. Delay has the greatest effect on digital signals because
of the change in pulse waveshape caused by delay.
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