(2) At the receiving station, bridge the AN/USM-181 voltmeter and
frequency counter of the 340B across a terminating resistor.
(3) At the receiving station,
adjust
the
output
level
to
the
frequency counter for optimum.
(4) Observe the level meter indication for a period of 1 minute and
record the reading. If there is any deviation in excess of 2 dB from the
initial reading, the cause of the variation must be corrected prior to
continuing with this test.
(5) Observe the reading of the frequency counter for a period of 1
minute. Record the frequency reading to the nearest tenth of a hertz.
(6) Ask the technician at the sending station for the frequency of
the transmitted signal as measured on his frequency counter, and record the
figure.
(7) The difference between the readings of the two frequency counters
will be recorded as the frequency change.
(8) Change the transmitted frequency to 2,000 Hz (0.1) and repeat
the measuring operations.
1-7. Terminal Impedance. A telephone communications system connecting two
technical control centers will usually include several subsystems.
These
subsystems may include telephone carrier terminals, radio sets, and cable
sections. Military standards used in the design of equipment items assures
Modern
telephone cables are also designed to have 600-ohm impedance pairs.
When
all of the above equipment items are integrated into a telephone system, the
impedance should match; reflection loss and level variation are, therefore,
minimized. Should any mismatch develop along the system, the resulting rise
in reflection loss, together with a reduction in received signal level,
would signify the need for checking system characteristic impedance. Such a
test is called a terminal impedance test, not because it is associated with
any particular type of terminal equipment, but rather because the test
determines whether an impedance mismatch occurs in the overall system.
Evidence of mismatch or instability of impedance indicates possible trouble
somewhere in the system. In this event the circuit conditioner notifies the
appropriate equipment or line maintenance crews
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