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. 10-3