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Subel. ZLI
30 questions
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

Correct answer: rectification of strong signals in overloaded stages

Cross-modulation occurs when a receiver stage is driven into non-linear operation by a strong signal. The overloaded device partially rectifies or mixes the strong signal with a weaker signal, causing the modulation from the strong signal to appear on the weaker one.

This is a receiver overload effect and indicates that an RF or IF stage is being driven beyond its linear range.

  • key-clicks generated at the transmitter produce broadband interference, not cross-modulation inside a receiver.
  • improper filtering in the transmitter may cause spurious emissions or splatter, but it does not create cross-modulation in another receiver.
  • lack of receiver sensitivity and selectivity affects weak-signal reception and adjacent-channel rejection, not non-linear mixing effects.

Therefore, cross-modulation is usually caused by rectification of strong signals in overloaded stages.

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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

Correct answer: negative feedback

An operational amplifier used as a filter always relies on negative feedback to control gain, stability, and frequency response. The feedback network, typically made from resistors and capacitors, determines the filter characteristics such as cutoff frequency, bandwidth, and response shape.

Negative feedback keeps the amplifier operating in a stable, linear region and prevents unwanted oscillation.

  • positive feedback to reduce oscillation is incorrect, positive feedback promotes oscillation and instability.
  • random feedback has no defined electrical meaning and is not used in circuit design.
  • inductors and resistor circuits only is incorrect, op amp filters normally use resistors and capacitors, and inductors are rarely required.

Therefore, an operational amplifier connected as a filter always utilises negative feedback.

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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

Correct answer: very high

An operational amplifier is designed to draw almost no input current so that it does not load the signal source. This results in a very high input impedance, often in the megaohm to gigaohm range for modern op amps.

A high input impedance allows accurate voltage amplification and prevents signal attenuation caused by loading effects.

  • very low is incorrect because a low input impedance would heavily load the source and distort measurements or signals.
  • capacitive and inductive may exist as small parasitic effects, but the overall input impedance is dominated by a very high resistance.

Therefore, the input impedance of an operational amplifier is generally very high.

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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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