(4) If necessary, amplitude equalizers will be used to bring the
circuit within the required equalization parameters involved (S1, S3, etc.).
Example:
Assume that a circuit has been assigned parameter S2.
Parameter S2 allows a limit of -0.5 dB (less loss) to +2 dB
(more loss) compared with the loss at 1,000 Hz.
Assume
that the normal received test level is -10 dBm at 1,000 Hz,
and that the received test level at 1.8 kHz measures -12
dBm.
Does the circuit meet the parameter requirement at
1.8 kHz?
The circuit has 2 dBm more loss at 1.8 kHz than at 1,000 Hz
(difference between -10 and -12 dBm).
The difference in
level (loss) at 1.8 kHz as compared with 1,000 Hz meets the
parameter limit.
Keep in mind that less loss indicates a higher level, and
that more loss indicates a lower level, as compared with a
reference. Also, keep in mind that at levels below 0 dBm,
the higher the absolute value the lower the level.
For
example, -12 dBm represents a lower level than -10 dBm.
2-3. Equalizing. The two preceding tests determine the envelope delay and
amplitude-frequency response of the line or facility under investigation.
After the test results are known, the technician applies the equalizer to
correct the line's characteristics according to the parameters of the
specified service.
Upon completion of equalizer adjustments, the entire
equalizer is left in place and becomes part of the circuit as long as that
circuit remains active under those specified parameters. A large number of
equalized circuits, therefore, requires an equally large number of expensive
line equalizers. The communications planner must carefully weigh the cost
of equalizing lines against the type of service required.
Ordinary voice
communications circuits require little, if any, equalization. On the other
hand, telephone circuits which are required to carry data signals will
usually require equalization. If the flow of data traffic is in only one
direction, only the line in that direction of transmission need be
equalized.
a. Test arrangement.
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