part of both technicians.
Normally the first adjustment is for amplitude-
frequency response, followed by envelope delay. Separate test sets are used to
measure the two different parameters (amplitude-frequency response and envelope
delay).
Subsequent adjustments are made to minimize variation in the
parameters.
This is made necessary by the changes that take place in one
parameter while an adjustment of the equalizer is made to satisfy another
parameter. Knowledge of the system is important to the procedure. The circuit
conditioner muse be given a circuit layout card so that every item of equipment
is known to him. Equipment characteristics, especially frequency cutoff, are
important because no amount of conditioning can compensate beyond that response
limitation.
A normal measurement of system parameters cannot disclose
characteristics of equipment used in the system. Such a measure indicates the
total of all subsystem responses.
4-5. APPLICATION OF EQUALIZERS
Equalizers are used only when necessary to maintain or obtain the
parameters established by the DCA for the various services performed by
telephone networks.
a. Location.
Equalizers are always placed at the receiving end or
terminating point of a circuit.
However, some lines cannot be equalized
because of the facilities included in the wire circuit. For example, a long-
distance trunk circuit containing a channel of telephone carrier cannot be
equalized beyond the limits of the channel filter in the carrier terminal. On
the other hand, a pair of wires should be capable of wide equalization, because
the variation in reactance occurs relatively slowly with change in frequency,
provided there is nothing else in the line.
However, when loading coils or
impedance-matching transformers are used, the equalizing limit is determined
primarily by the frequency cutoff that results from combining these line
devices.
b. Application to Telephone Carrier Terminal. The problems encountered in
applying equalizers to telephone carrier terminals are illustrated in figure
4-6.
(1) The individual channels of a multichannel telephone carrier terminal
are not normally equalized because equalizing is accomplished across
the baseband of all combined channels at the receiving line
terminations. As shown in A of figure 4-6, one equalizer serves all
channels within the carrier terminal.
(2) An input loop circuit connecting to the channel of a telephone
carrier terminal uses a wire pair which carries signals having an
amplitude-frequency relationship. The line equalizer on the receive
line cannot compensate for this condition.
If equalization is
important to the signals arriving at the loop input to the carrier
channel, an equalizer will have to be installed at the termination
of that loop.
(3) The equalizer will have to be placed at the terminating end of each
pair of a four-wire circuit; it is not effective on a two-wire
circuit because such a circuit passes signals in either direction.
Therefore a hybrid coil must be used wherever equalization is
required for a two-wire loop. This principle in illustrated in A of
figure 4-6.
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