rate. Normally, the source of the random noise is a
variable contact between brush and commutator bar
or slip ring, or an imperfect contact or poor isolation
between two surfaces.
Narrow-Band Interference
Narrow-band interference is almost always
caused by oscillators or power amplifiers in receivers
and transmitters. In a receiver, the cause is usually a
poorly shielded local oscillator stage. In a trans-
mitter, several of the stages could be at fault. The
interference could be at the transmitter operating
frequency, a harmonic of its operating frequency, or at
some spurious frequency. A multichannel transmitter
that uses crystal-bank frequency synthesizing circuits
can produce interference at any of the frequencies
present in the synthesize. Narrow-band interference
in a receiver can range in severity from an annoying
heterodyne whistle in the audio output to the complete
blocking of received signals. Narrow-band
interference affects single frequencies or spots of
frequencies in the tuning range of the affected
receiver.
SOURCES OF ELECTRICAL NOISE
Learning Objective: Recognize the various
sources of electrical noise and the operating
characteristics of each.
Any circuit or device that carries a varying
electrical current is a potential source of receiver
interference. The value of the interference voltage
depends upon the amount of voltage change. The
frequency coverage depends upon the abruptness of
the change. The principal sources of man-made
interference in aircraft include rotating electrical
machines, switching devices, pulsed electronic
equipment, propeller systems, receiver oscillators,
nonlinear elements, and ac power lines. Each of these
sources of noise is discussed in the following
sections.
ROTATING ELECTRICAL MACHINES
Rotating electrical machines are a major source of
receiver interference because of the large number of
electric motors used in the aircraft.
Rotating
electrical machines used in aircraft may be divided
into three general classes: dc motors, ac motors and
generators, and inverters.
DC Motors
Modern aircraft use dc motors in great numbers,
such as in flight control actuators, armament
actuators, and flight accessories. Most electronic
equipment on the aircraft include one or more dc
motors for driving cycling mechanisms, compressor
pumps, air circulators, and antenna mechanisms.
Each of these motors can generate voltages capable of
causing radio interference over a wide band of
frequencies. Types of interfering voltages generated
by dc motors areas follows:
l Switching transients generated as the brush
moves from one commutator bar to another
(commutation interference)
. Random transients produced by varying
contact between the brush and the commutator
(sliding contact interference)
. Audio-frequency hum (commutator ripple)
. Radio frequency and static charges built up on
the shaft and the rotor assembly
The dc motors used in aircraft systems are of
three general types:
the series- wound motor, the
shunt-wound motor, and the permanent-magnet (PM)
motor.
The field windings of both series- and
shunt-wound motors afford some filter action against
transient voltages generated by the brushes. The PM
motor’s lack of such inherent filtering makes it a very
common source of interference. The size of a dc
motor has little bearing upon its interference
generating characteristics. The smallest motor aboard
the aircraft can be the worst offender.
AC Generators and Motors
The output of an ideal alternating-current
generator is a pure sine wave. A pure sine-wave
voltage is incapable of producing interference except
at its basic frequency. However, a pure waveform is
difficult to produce, particularly in a small ac
generator. Nearly all types of ac generators used in
naval aircraft are potential sources of interference at
frequencies other than the output power frequency.
Interference voltages are produced by the following
sources:
l Harmonics of the power frequency. Generally,
the harmonics are caused by poor waveform.
. Commutation interference. This condition
originates in a series-wound motor.
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