1-8. Longitudinal Balance.
When a circuit is installed or activated, the
technical controller or circuit conditioner may have to determine
longitudinal balance before continuing with other tests.
A test for
longitudinal balance determines the dynamic balance; that is, the relative
impedance of each wire in the pair with respect to ground.
A serious
unbalance may make further circuit test ineffective. If unbalance exists,
the controller may take one of two actions: either notify wire technicians
to correct the cause of unbalance, or switch to another circuit to meet the
requirements. Two tests are normally performed; one to check balance at the
input of the circuit under test, and another to check balance at the output
of the circuit under test. Both tests are relatively easy to perform, the
only instruments being one oscillator and two voltmeters from the AN/USM-
181's, together with precision resistors and an isolating transformer.
Fundamentally, both tests involve finding the voltage between the electrical
center of the wire pair and ground, then comparing that voltage with the
test signal voltage across the line. The ratio of the two voltages will be
given in decibels.
a. Test arrangement.
Block diagrams of the two test arrangements
(input and output) are shown in figures 1-14 and 1-15.
The illustrations
show both wires in the pair so that the method of obtaining electrical
center can be indicated.
Tests will be made in both directions of
transmission.
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