Lesson 1/Learning Event 2
c. Distortion. Distortion is the result of variation or change in the
amplitude, phase, or frequency of an audio signal, or the change in length
of DC signal pulses.
d. Total effect. The total disturbing effect on a signal is the result
of the cumulative effect of noise, interference, and distortion. Although
these disturbances can be defined and discussed, they are difficult to
extract from the signal once they have entered it.
Prevention of these
disturbances is therefore more important than compensating for their effect.
Learning Event 2: EFFECT OF NOISE.
The effect of noise on a signal is never known until the signal is
demodulated.
The effect of noise must therefore be observed at the
receiving device.
a. General.
Noise can occur on specific frequencies, or it can be a
type that is spread equally over a wide part of the frequency spectrum.
Impulse noise and AC hum usually are frequency selective--that is, they
occur only on specific frequencies. Types of noise often present on radio
circuits, such as thermal noise, cosmic noise, atmospheric noise, etc., are
generally broad and evenly distributed across a wide band.
These noise
types are classified as "white" noise, to differentiate them from impulse
noise.
b. Signal-to-noise ratio. The actual signal strength is not the most
important factor in a communication circuit, since a weak signal can always
be amplified. However, noise in the circuit is amplified at the same time.
Also, additional amounts of noise are added at each amplifier. Therefore,
the more times the signal is amplified in an overall system, the higher the
noise level. If the noise level at the receiving device is far enough below
the signal level so that the receiving mechanism can be adjusted to operate
only on the desired signal and to ignore the lower amplitude noise, then the
noise has no effect on the signal. The relationship between the amplitude
of the signal and amplitude of the noise is called the signal-to-noise-
ratio. This ratio is the limiting factor for reliable communication.
Learning Event 3: SUPPRESSION OF MANMADE RADIO NOISE.
a. An electric spark developed across relay contacts or electric motor
brush contacts generates RF noise which, unless suppressed, can cause
disruption of radio reception.
Suppression of radio noise from sparking
contacts can frequently be achieved by devices called suppressors, or spark
killers. The common type
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