(1) Idle noise. This noise is always present in a system, with or without modulation. Idle
noise includes thermal noise generated within the equipments by mixer diodes, low level amplifiers. It
includes shot noise from klystrons. It also includes noise generated by semi-conductors, multiplier
chains, and crossover electro-magnetic effects.
(2) Intermodulation noise. This noise is introduced into the system as a result of heavy
signal load or increased operating level. The greater the traffic load or the higher the operating level, the
more intermodulation noise introduced. Usually intermodulation noise increases slowly until a "break
point" is reached, after which, it increases rapidly. Generally, it is desirable to operate the systems at as
high a signal level as possible (short of the breakpoint) in order to improve the signal/noise (S/N) ratio.
For some equipment, however, specific signal levels must be maintained to ensure proper operation.
b. Interference in line-of-sight and tropospheric scatter systems. Most interference in line-of-
sight or tropospheric scatter systems is generally of three types; interference from natural sources, man-
made interference, and mutual interference. The prime requirement for good communications is a high
S/N ratio. Radio interference is discussed in more detail in the next three paragraphs.
c. Interference from natural sources.
(1) Radio interference from natural sources may be divided into four classifications:
(a) Atmospheric interference; electrical storms.
(b) Solar and cosmic interference; eruptions on the sun and other stars.
(c) Precipitation static from charged particles in the atmosphere. Particles may be rain, sleet,
snow, sand, and smoke or dust. Dry particles produce greater charges than wet ones.
(d) Fading from disturbances in the medium through which radio waves are propagated.
(2) The interferences listed above appear in electronic equipment as audible noise or errors in
the output of some terminal equipments. There is some interference at most frequencies, but the higher
the frequency, the less noise.
d. Mutual interference.
40
Previous Page
