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Subel. ZLE
10 questions
Subelement ZLE

The Radio Transmitter

Section ZLE19

Transmitter Theory

Morse code is usually transmitted by radio as

  • Correct Answer
    an interrupted carrier
  • a voice modulated carrier
  • a continuous carrier
  • a series of clicks

Correct answer: an interrupted carrier

Morse code is transmitted using continuous wave (CW) transmission by switching the transmitter carrier on and off in accordance with the dots and dashes of the code.

This produces a radio-frequency carrier that is present during key-down and absent during key-up.

  • A voice modulated carrier is used for speech transmission (e.g. AM or SSB).
  • A continuous carrier with no interruption would convey no information.
  • A series of clicks does not describe the RF transmission method.

Therefore, Morse code is usually transmitted as an interrupted carrier.

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To obtain high frequency stability in a transmitter, the VFO should be

  • run from a non-regulated AC supply
  • in a plastic box
  • Correct Answer
    powered from a regulated DC supply
  • able to change frequency with temperature
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SSB transmissions

  • occupy about twice the bandwidth of AM transmissions
  • contain more information than AM transmissions
  • Correct Answer
    occupy about half the bandwidth of AM transmissions
  • are compatible with FM transmissions
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The purpose of a balanced modulator in a SSB transmitter is to

  • make sure that the carrier and both sidebands are in phase
  • make sure that the carrier and both sidebands are 180 degrees out of phase
  • ensure that the percentage of modulation is kept constant
  • Correct Answer
    suppress the carrier while producing two sidebands

Correct answer: suppress the carrier while producing two sidebands

A balanced modulator combines the audio signal with the RF carrier in a way that cancels the carrier component at its output while still producing both the upper and lower sidebands. The result is a double sideband suppressed carrier (DSB-SC) signal.

This is the required first step in generating an SSB signal. A filter or phasing network later removes one of the sidebands.

  • make sure that the carrier and both sidebands are in phase is incorrect, the carrier is intentionally suppressed, not phase aligned.
  • make sure that the carrier and both sidebands are 180 degrees out of phase is incorrect and does not describe the function of a balanced modulator.
  • ensure that the percentage of modulation is kept constant relates to modulation control or ALC, not carrier suppression.

Therefore, the purpose of a balanced modulator is to suppress the carrier while producing two sidebands.

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Several stations advise that your FM simplex transmission in the "two metre" band is distorted. The cause might be that

  • Correct Answer
    the transmitter modulation deviation is too high
  • your antenna is too low
  • the transmitter has become unsynchronised
  • your transmitter frequency split is incorrect
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The driver stage of a transmitter is located

  • Correct Answer
    before the power amplifier
  • between oscillator and buffer
  • with the frequency multiplier
  • after the output low-pass filter circuit

Correct answer: before the power amplifier

In a transmitter, the driver stage provides sufficient signal power to properly drive the final power amplifier stage.

The typical signal chain is:

Oscillator → Buffer → Driver → Power Amplifier → Output Filter → Antenna

  • The driver is not placed between the oscillator and buffer.
  • It is not necessarily combined with a frequency multiplier.
  • The output low-pass filter is located after the power amplifier.

Therefore, the driver stage is located before the power amplifier.

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The purpose of the final amplifier in a transmitter is to

  • increase the frequency of a signal
  • isolate the multiplier and later stages
  • produce a stable radio frequency
  • Correct Answer
    increase the power fed to the antenna
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The difference between DC input power and RF power output of a transmitter RF amplifier

  • radiates from the antenna
  • Correct Answer
    is dissipated as heat
  • is lost in the feedline
  • is due to oscillating current
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The process of modulation allows

  • Correct Answer
    information to be impressed on to a carrier
  • information to be removed from a carrier
  • voice and Morse code to be combined
  • none of these
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The output power rating of a linear amplifier in a SSB transmitter is specified by the

  • peak DC input power
  • mean AC input power
  • Correct Answer
    peak envelope power
  • unmodulated carrier power
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