c. Performing the test.
(1) Terminate the send end of the line in a 600-ohm resistor, as in
the idle circuit noise test. Patching the line jack to the 600-ohm resistor
removes the signal from the line.
(2) Patch the receive end of the line to the 310 LINE jack and select
the proper weighting (WTG), depending on the signal characteristics.
A
switch-controlled filter circuit selects either FLAT, 10.2KC-51KC, or VOICE
weighting.
The VOICE position selects the circuit components within the
test set for C-message weighting, while the previous two positions provide
for data transmission weighting.
(3) After setting the INPUT switch to match line impedance, adjust
the
REF
LEVEL
DBRN
attenuators
according
to
the
DCA
parameter
specifications.
(4) Since only those peaks above the value established by a threshold
circuit in the TTS-58A will be counted, white noise has no affect. On small
peaks, the LO LEVEL counter will indicate; on peaks higher then LO LEVEL,
the MED and LO counters will indicate; on peaks higher than MED, the LO,
MED, and HI counters will indicate.
The total count of "hits" will,
therefore, show on the LO counter. This arrangement allows the technician
to determine the strength and number of noise peaks at each of three
different threshold levels over a selected time interval.
(5) The spring-driven TIMER limits the period of time over which the
counting will occur. The MON jack permits the technician to listen to the
noise on the line with a headset or loudspeaker.
1-3. Harmonic Distortion. When a signal passes through a telephone circuit,
frequencies sometimes appear that were not in the original signal. Analysis
of the received signal shows that these additional frequencies are harmonics
(even and odd multiples) of the original signal frequency.
Harmonics are
produced by nonlinear circuit elements through which the signal is passing.
The number and intensity of the harmonic frequencies is a measure of the
circuit nonlinearity.
The distortion may be measured by determining the
harmonics of a single tone. Total harmonic distortion is the ratio of the
power in the test tone frequency to the power in the total harmonic energy
observed at the output of the system.
Single harmonic distortion is the
same ratio except that the denominator is the power of a specified
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