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Subelement ZLI
Interference and How to Fix it
Section ZLI29
Interference and Filtering
Electromagnetic compatibility is
  • two antennas facing each other
  • Correct Answer
    the ability of equipment to function satisfactorily in its own environment without introducing intolerable electromagnetic disturbances
  • more than one relay solenoid operating simultaneously
  • the inability of equipment to function satisfactorily together and produce tolerable electromagnetic disturbances
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On an amateur receiver, unwanted signals are found at every 15.625 kHz. This is probably due to
  • a low-frequency government station
  • a remote radar station
  • Correct Answer
    radiation from a nearby TV line oscillator
  • none of these
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Narrow-band interference can be caused by
  • Correct Answer
    transmitter harmonics
  • a neon sign
  • a shaver motor
  • lightning flashes
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Which of the following is most likely to cause broad-band continuous interference
  • an electric blanket switch
  • a refrigerator thermostat
  • a microwave transmitter
  • Correct Answer
    poor commutation in an electric motor
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If broadband noise interference varies when it rains, the most likely cause could be from
  • underground power cables
  • Correct Answer
    outside overhead power lines
  • car ignitions
  • your antenna connection
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Before explaining to a neighbour that the reported interference is due to a lack of immunity in the neighbour's electronic equipment
  • disconnect all your equipment from their power sources
  • write a letter to the MBIE
  • Correct Answer
    make sure that there is no interference on your own domestic equipment
  • ignore all complaints and take no action
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A neighbour's stereo system is suffering RF break-through. One possible cure is to
  • put a ferrite bead on the transmitter output lead
  • put a capacitor across the transmitter output
  • use open-wire feeders to the antenna
  • Correct Answer
    use screened wire for the loudspeaker leads
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When living in a densely-populated area, it is wise to
  • always use maximum transmitter output power
  • Correct Answer
    use the minimum transmitter output power necessary
  • only transmit during popular television programme times
  • point the beam at the maximum number of television antennas
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When someone in the neighbourhood complains of TVI it is wise to
  • deny all responsibility
  • immediately blame the other equipment
  • inform all the other neighbours
  • Correct Answer
    check your log to see if it coincides with your transmissions
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Cross-modulation is usually caused by
  • Correct Answer
    rectification of strong signals in overloaded stages
  • key-clicks generated at the transmitter
  • improper filtering in the transmitter
  • lack of receiver sensitivity and selectivity
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When the signal from a transmitter overloads the audio stages of a broadcast receiver, the transmitted signal
  • Correct Answer
    can be heard irrespective of where the receiver is tuned
  • appears only when a broadcast station is received
  • is distorted on voice peaks
  • appears on only one frequency
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Cross-modulation of a broadcast receiver by a nearby transmitter would be noticed in the receiver as
  • a lack of signals being received
  • Correct Answer
    the undesired signal in the background of the desired signal
  • interference only when a broadcast signal is received
  • distortion on transmitted voice peaks
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Unwanted signals from a radio transmitter which cause harmful interference to other users are known as
  • rectified signals
  • re-radiation signals
  • reflected signals
  • Correct Answer
    harmonic and other spurious signals
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To reduce harmonic output from a transmitter, the following could be put in the transmission line as close to the transmitter as possible
  • wave trap
  • Correct Answer
    low-pass filter
  • high-pass filter
  • band reject filter
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To reduce energy from an HF transmitter getting into a television receiver, the following could be placed in the TV antenna lead as close to the TV as possible
  • active filter
  • low-pass filter
  • Correct Answer
    high-pass filter
  • band reject filter
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A low-pass filter used to eliminate the radiation of unwanted signals is connected to the
  • output of the balanced modulator
  • Correct Answer
    output of the amateur transmitter
  • input of the stereo system
  • input of the mixer stage of your SSB transmitter
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A band-pass filter will
  • pass frequencies each side of a band
  • attenuate low frequencies but not high frequencies
  • Correct Answer
    attenuate frequencies each side of a band
  • attenuate high frequencies but not low frequencies
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A band-stop filter will
  • Correct Answer
    pass frequencies each side of a band
  • stop frequencies each side of a band
  • only allow one spot frequency through
  • pass frequencies below 100 MHz
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A low-pass filter for a high frequency transmitter output would
  • Correct Answer
    attenuate frequencies above 30 MHz
  • pass audio frequencies below 3 kHz
  • attenuate frequencies below 30 MHz
  • pass audio frequencies above 3 kHz
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Installing a low-pass filter between the transmitter and transmission line will
  • permit higher frequency signals to pass to the antenna
  • ensure an SWR not exceeding 2:1
  • reduce the power output back to the legal maximum
  • Correct Answer
    permit lower frequency signals to pass to the antenna
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A low-pass filter may be used in an amateur radio installation
  • to attenuate signals lower in frequency than the transmission
  • Correct Answer
    to attenuate signals higher in frequency than the transmission
  • to boost the output power of the lower frequency transmissions
  • to boost the power of higher frequency transmissions
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Television interference caused by harmonics radiated from an amateur transmitter could be eliminated by fitting
  • a low-pass filter in the TV receiver antenna input
  • a high-pass filter in the transmitter output
  • Correct Answer
    a low-pass filter in the transmitter output
  • a band-pass filter to the speech amplifier
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A high-pass filter can be used to
  • prevent interference to a telephone
  • prevent overmodulation in a transmitter
  • Correct Answer
    prevent interference to a TV receiver
  • pass a band of speech frequencies in a modulator
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A high-pass RF filter would normally be fitted
  • between transmitter output and feedline
  • Correct Answer
    at the antenna terminals of a TV receiver
  • at the Morse key or keying relay in a transmitter
  • between microphone and speech amplifier
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A high-pass filter attenuates
  • a band of frequencies in the VHF region
  • all except a band of VHF frequencies
  • high frequencies but not low frequencies
  • Correct Answer
    low frequencies but not high frequencies
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An operational amplifier connected as a filter always utilises
  • positive feedback to reduce oscillation
  • Correct Answer
    negative feedback
  • random feedback
  • inductors and resistor circuits only
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The voltage gain of an operational amplifier at low frequencies is
  • Correct Answer
    very high but purposely reduced using circuit components
  • very low but purposely increased using circuit components
  • less than one
  • undefined
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The input impedance of an operational amplifier is generally
  • Correct Answer
    very high
  • very low
  • capacitive
  • inductive
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An active audio low-pass filter could be constructed using
  • zener diodes and resistors
  • electrolytic capacitors and resistors
  • Correct Answer
    an operational amplifier, resistors and capacitors
  • a transformer and capacitors
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A filter used to attenuate a very narrow band of frequencies centred on 3.6 MHz would be called
  • a band-pass filter
  • a high-pass filter
  • a low-pass filter
  • Correct Answer
    a notch filter
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