If the background noise level remains constant and the signal diminishes, the reception of a weak signal becomes
difficult. The application of automatic gain control is useless, because increasing the gain to amplify the fading
signal to the normal level will cause the noise level to be amplified an equal amount, and the result is the same
poor signal-plus-noise-to-noise ratio. The peculiar atmospheric conditions that cause fading may last for hours, or
only a few minutes. Generally fading is more prevalent during the summer months and during daylight hours.
4-7.
INTERNAL AND MANMADE
a. Internal interference is present to some extent in every electrical or electronic receiver. This noise
arises from the natural action of electrons in transit within electron tubes and in other circuit components. Even if
the receiving equipment is perfectly aligned and all of the internal components are in the best condition, the
internal interference will still exist.
(1) Thermal noise is caused by the thermal agitation of electrons in conductors. Thermally agitated
electrons generate minute voltages which add to or subtract from the circuit voltage, and thereby
cause electric noise.
(2) Shot-effect noise is caused by the inconsistency of electrical currents. Electrical current is
composed of minute electrical impulses which are the result of electrons changing energy states.
This lack of continuity creates noise. Shot-effect and thermal noise are closely related in their
causes and effects.
(3) Spontaneous emission is created by electrons giving up energy when they revert to a lower
energy state. This energy induces noise voltages into the conductors.
b. Many kinds and types of equipment produce undesirable radio frequency impulses which are
transmitted and travel out through the air exactly as if they were deliberately prepared for broadcast. In addition,
some equipments radiate back through the power line to other equipments unless the radiation is stopped by an
impedance or absorbed by a reactance.
Section. II. NOISE MEASUREMENTS
4-8.
NOISE TEMPERATURE
a. The noise voltage appearing across the terminals of a resistor is proportional to the temperature of the
resistor. The noise voltage is due to thermal agitation, that is, electron motion caused by the heating of the
electrons in the structure of the resistor. If the resistor is heated to a higher temperature, the noise voltage
increases; if the temperature is lowered, the noise voltage decreases. A useful measure of these noise voltages is a
quantity that is proportional to voltage squared, or power. The rule that this noise power or voltage increases with
temperature can be expressed in a more precise way if the noise is measured as a noise power and the temperature
is measured on an absolute scale.
344 L4
141
Previous Page
