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Subel. ZLE
20 questions
Subelement ZLE

The Radio Transmitter

Section ZLE20

Harmonics and Parasitics

A harmonic of a signal transmitted at 3525 kHz would be expected to occur at

  • 3573 kHz
  • Correct Answer
    7050 kHz
  • 14025 kHz
  • 21050 kHz
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The third harmonic of 7 MHz is

  • 10 MHz
  • 14 MHz
  • Correct Answer
    21 MHz
  • 28 MHz
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The fifth harmonic of 7 MHz is

  • 12 MHz
  • 19 MHz
  • 28 MHz
  • Correct Answer
    35 MHz
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Excessive harmonic output may be produced in a transmitter by

  • a linear amplifier
  • a low SWR
  • resonant circuits
  • Correct Answer
    overdriven amplifier stages
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Harmonics may be produced in the RF power amplifier of a transmitter if

  • the modulation level is too low
  • Correct Answer
    the modulation level is too high
  • the oscillator frequency is unstable
  • modulation is applied to more than one stage
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Harmonics produced in an early stage of a transmitter may be reduced in a later stage by

  • increasing the signal input to the final stage
  • using FET power amplifiers
  • Correct Answer
    using tuned circuit coupling between stages
  • using larger value coupling capacitors
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Harmonics are produced when

  • a resonant circuit is detuned
  • negative feedback is applied to an amplifier
  • a transistor is biased for class A operation
  • Correct Answer
    a sine wave is distorted
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Harmonic frequencies are

  • always lower in frequency than the fundamental frequency
  • Correct Answer
    at multiples of the fundamental frequency
  • any unwanted frequency above the fundamental frequency
  • any frequency causing TVI
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An interfering signal from a transmitter has a frequency of 57 MHz. This signal could be the

  • seventh harmonic of an 80 meter transmission
  • third harmonic of a 15 metre transmission
  • Correct Answer
    second harmonic of a 10 metre transmission
  • crystal oscillator operating on its fundamental

Correct answer: second harmonic of a 10 metre transmission

The harmonic frequency of a transmitter is an integer multiple of its fundamental frequency:

\[ f_n = n \times f_0 \]

The 10 metre band is around \(28\text{–}29\ \mathrm{MHz}\).
The second harmonic of a 10 m signal is therefore:

\[ 2 \times 28.5\ \mathrm{MHz} \approx 57\ \mathrm{MHz} \]

This matches the interfering signal frequency of 57 MHz, making a second harmonic from a 10 m transmitter the most likely source.

  • seventh harmonic of an 80 metre transmission
    The 80 m band is around \(3.5\text{–}4.0\ \mathrm{MHz}\).
    \[7 \times 3.7\ \mathrm{MHz} \approx 26\ \mathrm{MHz}\]
    This does not match 57 MHz.

  • third harmonic of a 15 metre transmission
    The 15 m band is around \(21\ \mathrm{MHz}\).
    \[3 \times 21\ \mathrm{MHz} = 63\ \mathrm{MHz}\]
    This is significantly higher than 57 MHz.

  • crystal oscillator operating on its fundamental
    A fundamental crystal frequency of 57 MHz is uncommon and does not explain why the signal is interfering with a transmitter harmonic scenario.

Therefore, a signal at 57 MHz could reasonably be the second harmonic of a 10 metre transmission.

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To minimise the radiation of one particular harmonic, one can use a

  • Correct Answer
    wave trap in the transmitter output
  • resistor
  • high pass filter in the transmitter output
  • filter in the receiver lead
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A low-pass filter is used in the antenna lead from a transmitter

  • to reduce key clicks developed in a CW transmitter
  • to increase harmonic radiation
  • to eliminate chirp in CW transmissions
  • Correct Answer
    to reduce radiation of harmonics
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The following is installed in the transmission line as close as possible to a HF transmitter to reduce harmonic output

  • a middle-pass filter
  • Correct Answer
    a low-pass filter
  • a high-pass filter
  • a band-reject filter
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A low pass filter will

  • suppress sub-harmonics
  • Correct Answer
    reduce harmonics
  • always eliminate interference
  • improve harmonic radiation
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A spurious transmission from a transmitter is

  • Correct Answer
    an unwanted emission unrelated to the output signal frequency
  • an unwanted emission that is harmonically related to the modulating audio frequency
  • generated at 50 Hz
  • the main part of the modulated carrier

A spurious transmission from a transmitter is an unwanted emission unrelated to the output signal frequency

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A parasitic oscillation

  • Correct Answer
    is an unwanted signal developed in a transmitter
  • is generated by parasitic elements of a Yagi beam
  • does not cause any radio interference
  • is produced in a transmitter oscillator stage
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Parasitic oscillations in a RF power amplifier can be suppressed by

  • pulsing the supply voltage
  • Correct Answer
    placing suitable chokes, ferrite beads or resistors within the amplifier
  • screening all input leads
  • using split-stator tuning capacitors
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Parasitic oscillations in the RF power amplifier stage of a transmitter may occur

  • at low frequencies only
  • on harmonic frequencies
  • at high frequencies only
  • Correct Answer
    at high or low frequencies
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Transmitter power amplifiers can generate parasitic oscillations on

  • the transmitter's output frequency
  • harmonics of the transmitter's output frequency
  • Correct Answer
    frequencies unrelated to the transmitter's output frequency
  • VHF frequencies only

Correct answer: frequencies unrelated to the transmitter's output frequency

Parasitic oscillations in transmitter power amplifiers are unintended self-oscillations caused by stray capacitance, lead inductance, device internal feedback, and layout effects. These oscillations are not driven by the intended RF signal and can occur at whatever frequency the unintended feedback path supports.

They often appear at much higher frequencies, sometimes well into the VHF or UHF range, and are not harmonically related to the transmitter’s operating frequency.

  • the transmitter's output frequency is the desired operating signal, not a parasitic oscillation.
  • harmonics of the transmitter's output frequency are normal byproducts of non-linear amplification and are handled by output filtering, but they are not parasitic oscillations.
  • VHF frequencies only is too restrictive, parasitic oscillations can occur at many unrelated frequencies depending on circuit conditions.

Therefore, parasitic oscillations are generated on frequencies unrelated to the transmitter's output frequency.

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Parasitic oscillations tend to occur in

  • high voltage rectifiers
  • Correct Answer
    high gain amplifier stages
  • antenna matching circuits
  • SWR bridges
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Parasitic oscillations can cause interference. They are

  • always the same frequency as the mains supply
  • always twice the operating frequency
  • Correct Answer
    not related to the operating frequency
  • three times the operating frequency

With parasitic signals there is no simple mathematical relationship between the operating frequency and the interfering frequency. The effects may be the same as with harmonics - a VHF receiver being interfered with by a HF transmission. The cause is an additional and undesired oscillation from an oscillator or amplifier for which it was not designed. The circuit functions normally but the parasitic oscillation occurs simultaneously. Parasitics are suppressed by adding additional components to the circuit to suppress the undesired oscillation without affecting the primary function of the circuit. A typical solution is to add a VHF choke (an inductor) or a small-value resistor (a "stopper") somewhere close to the active component in the offending circuit.

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