Lesson 2/Learning Event 3
Input and output impedances of the test sets
must match the circuits to which they are applied. Likewise, the input and
output impedances of the corrective devices must match the circuits and test
Failure to observe these precautions will cause inaccurate level
Where impedances do not match, the technician will have to
install an impedance-matching transformer or similar device.
g. Transmission level.
All levels must be adjusted to their proper
values before circuit conditioning tests are started.
This procedure has
two benefits; it assures accuracy of results, and it protects sensitive
instruments against possible damage.
h. Location for tests of line condition. The most positive and direct
method for determining the condition of a telephone line is to pass a signal
through it and observe the line's effect on the quality of the signal. In
determining the line's condition by this dynamic method, the technician must
assure himself that no equipment is included in the initial test. Equipment
which is capable of either improving the signal quality or degrading it
gives the technician a false indication of the line's true condition. One
way the technician can assure himself that he is observing a true line
condition is to disconnect the line from the circuit at the cable patch bay.
The signal therefore cannot enter any equipment in the technical control
A careful analysis of the signal quality thereby gives the
technician a correct interpretation of line condition.
(1) A list of equipment in a technical control center that may affect
signal quality includes pads, amplifiers, signaling units, and echo
suppressors. Compandors, which also affect test results, are normally part
of equipment items; unless these items are included in the tests, compandors
need not be considered.
(2) The technician has a choice of LINE, EQUIP, and MON jacks in the
receive cable patch bay. When he uses the LINE jack, he lifts the line away
from all circuits thus eliminating the effect of equipment items on signal
quality. He cannot lift the line at the EQUIP jack. He can, however, pick
up the signal in MON jacks.
The MON jacks are intended merely for
monitoring and not to determine line condition because these jacks do not
disconnect anything; they merely provide a point where test sets can monitor
the signal with high-impedance instruments.
The high impedance inputs of
such test equipment assures the technician that the instruments will not
load the circuit under test.