frequency and the fundamental of the other. This test is seldom necessary
if the test for harmonic distortion is performed properly.
A convenient
method to determine whether intermodulation products are developed is to
view the spectrum over which the products should appear, using the spectrum
analyzer.
Two frequencies commonly used for this test are 1,575 Hz and
1,000 Hz.
The wave analyzer is tuned to select 425 Hz, which is the
difference between 2,000 Hz (second harmonic of 1,000 Hz) and 1,575 Hz. The
amplitude of the 425 Hz signal is an indication of the degree of circuit
nonlinearity.
1-5. Single-Tone Interference.
The subjective affect of single-frequency
tone interference on a listener depends on the frequency of the tone and the
power associated with it.
The interfering tone is a voice communications
channel is caused by signaling tone in its own channel or in an adjacent
channel, or both; or from such sources as carrier leak, crosstalk, and
induction from power lines.
Although the disturbances caused by single-
frequency tone interference is usually of a low level, subjective tests have
proven that a persistent single tone is more annoying to a listener than a
corresponding level of random noise. The frequency spectrum of a telephone
transmission system must, therefore, be examined for the presence of such
spurious interfering tones.
Their origin must be located and the
interference eliminated or reduced to an acceptable level. Further, single-
tone interference can be detrimental to data transmission, since noise
bursts associated with it are more likely to produce errors than the same
noise bursts in the absence of the steady interfering tones.
a. Test arrangement. A block diagram of a typical test arrangement for
single-tone interference is shown in figure 1-7.
Note that the test
arrangement is similar to the harmonic distortion test, except that the send
line is terminated in resistance and no signal source is involved.
Wave
Analyzer Model 302A is the only test equipment used because it has selective
tuning capability; this is, it can select the desired frequency and
disregard all others.
Tests are made in each direction of transmission.
The 600-ohm resistor is needed at the input terminals of the 302A to bridge
the high input impedance of the test set and thus match it to the line
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