Aviation Electronics Technician 1 - Historic Naval Ships Association

ate. Normally, the source of the random noise is avariable contact between brush and commutator baror slip ring, or an imperfect contact or poor isolationbetween two surfaces.Narrow-Band InterferenceNarrow-band interference is almost alwayscaused by oscillators or power amplifiers in receiversand transmitters. In a receiver, the cause is usually apoorly shielded local oscillator stage. In a transmitter,several of the stages could be at fault. Theinterference could be at the transmitter operatingfrequency, a harmonic of its operating frequency, or atsome spurious frequency. A multichannel transmitterthat uses crystal-bank frequency synthesizing circuitscan produce interference at any of the frequenciespresent in the synthesize. Narrow-band interferencein a receiver can range in severity from an annoyingheterodyne whistle in the audio output to the completeblocking of received signals. Narrow-bandinterference affects single frequencies or spots offrequencies in the tuning range of the affectedreceiver.SOURCES OF ELECTRICAL NOISELearning Objective: Recognize the varioussources of electrical noise and the operatingcharacteristics of each.Any circuit or device that carries a varyingelectrical current is a potential source of receiverinterference. The value of the interference voltagedepends upon the amount of voltage change. Thefrequency coverage depends upon the abruptness ofthe change. The principal sources of man-madeinterference in aircraft include rotating electricalmachines, switching devices, pulsed electronicequipment, propeller systems, receiver oscillators,nonlinear elements, and ac power lines. Each of thesesources of noise is discussed in the followingsections.ROTATING ELECTRICAL MACHINESRotating electrical machines are a major source ofreceiver interference because of the large number ofelectric motors used in the aircraft. Rotatingelectrical machines used in aircraft may be dividedinto three general classes: dc motors, ac motors andgenerators, and inverters.DC MotorsModern aircraft use dc motors in great numbers,such as in flight control actuators, armamentactuators, and flight accessories. Most electronicequipment on the aircraft include one or more dcmotors for driving cycling mechanisms, compressorpumps, air circulators, and antenna mechanisms.Each of these motors can generate voltages capable ofcausing radio interference over a wide band offrequencies. Types of interfering voltages generatedby dc motors areas follows:l Switching transients generated as the brushmoves from one commutator bar to another(commutation interference). Random transients produced by varyingcontact between the brush and the commutator(sliding contact interference). Audio-frequency hum (commutator ripple). Radio frequency and static charges built up onthe shaft and the rotor assemblyThe dc motors used in aircraft systems are ofthree general types: the series- wound motor, theshunt-wound motor, and the permanent-magnet (PM)motor. The field windings of both series- andshunt-wound motors afford some filter action againsttransient voltages generated by the brushes. The PMmotor’s lack of such inherent filtering makes it a verycommon source of interference. The size of a dcmotor has little bearing upon its interferencegenerating characteristics. The smallest motor aboardthe aircraft can be the worst offender.AC Generators and MotorsThe output of an ideal alternating-currentgenerator is a pure sine wave. A pure sine-wavevoltage is incapable of producing interference exceptat its basic frequency. However, a pure waveform isdifficult to produce, particularly in a small acgenerator. Nearly all types of ac generators used innaval aircraft are potential sources of interference atfrequencies other than the output power frequency.Interference voltages are produced by the followingsources:l Harmonics of the power frequency. Generally,the harmonics are caused by poor waveform.. Commutation interference. This conditionoriginates in a series-wound motor.10-3