(2) With the MODE SELECTOR switch of the 302A on NORMAL, the RANGE
switch on the full-scale meter movement desired, and the SCALE VALUE switch
to ABSOLUTE, tune in the 700 Hz signal with the FREQUENCY control. Adjust
the MAX INPUT VOLTAGE control to obtain suitable meter scale reading. Now
place the MODE SELECTOR switch to AFC, and test set will lock on the 700 Hz
incoming frequency. Read the level.
(3) Using the 302A, place the MODE SELECTOR switch on NORMAL, and the
SCALE VALUE switch on RELATIVE. In the RELATIVE switch position the meter
will read an absolute level of signal as determined by the setting of the
REF ADJUST attenuator. The technician must calculate the relative level in
dBm0 because the meter does not read in dBm0. Making the change in the MAX
INPUT VOLTAGE control setting, rotate the RANGE switch counterclockwise (to
increase sensitivity) and tune in the harmonic frequency with the FREQUENCY
control.
Tunable circuits select the harmonic frequencies for level
measurement.
Finally, place the MODE SELECTOR switch to AFC and read the
level.
(4) Continue the operation through the fourth harmonic. The 700 Hz,
rather than the usual 1,000 Hz, is used for test tone because the fourth
harmonic of 1,000 Hz would exceed the upper limit of a normal voice circuit.
In the case of a telephone carrier channel, the fourth harmonic of 1,000 Hz
would fall outside the passband of the channel filter and, therefore, would
be filtered out. The fourth harmonic of 700 Hz is 2,800 Hz, well within the
voiceband.
1-4. Intermodulation Distortion.
Intermodulation distortion is the
impairment
of
a
complex
(multichannel)
signal
due
to
nonlinear
characteristics of the transmission system.
It is characterized by the
formation of frequencies at the output of a system equal to the sums and
differences of the component frequencies present at the input.
Harmonic
components are also present in the output, but are usually excluded from the
measurement by the selective nature of the receiving test equipment. Since
harmonic components and intermodulation products both result from nonlinear
characteristics of the transmission system, reduction of harmonics normally
minimizes the development of intermodulation products.
The test setup to
isolate intermodulation interference is generally similar to that harmonic
distortion, except that two oscillators are used, sending two different
frequencies at identical levels. The wave analyzer is then tuned to select
a third frequency resulting from the interaction of the second harmonic of
one
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