Ham Radio General Class Practice Test

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What happens to the RF carrier signal when a modulating audio signal is applied to an FM transmitter?

  1. The carrier frequency changes proportionally to the instantaneous amplitude of the modulating signal

  2. The carrier frequency changes proportionally to the amplitude and frequency of the modulating signal

  3. The carrier amplitude changes proportionally to the instantaneous frequency of the modulating signal

  4. The carrier phase changes proportionally to the instantaneous amplitude of the modulating signal

The correct answer is: The carrier frequency changes proportionally to the instantaneous amplitude of the modulating signal

When a modulating audio signal is applied to an FM (Frequency Modulation) transmitter, the carrier frequency undergoes changes influenced by the characteristics of the modulating signal. In frequency modulation, the instantaneous frequency of the carrier signal is varied based on the amplitude of the modulating audio signal. Thus, as the amplitude of the modulating signal increases or decreases, the frequency of the carrier shifts proportionally. This means that when the audio signal has higher amplitudes, the carrier frequency will deviate more from its unmodulated value. Conversely, when the audio signal amplitude decreases, the carrier frequency will return closer to its original frequency. This direct relationship between the amplitude of the audio signal and the frequency deviation of the carrier is fundamental to how FM transmission works, allowing the modulated signal to encode information in the frequency changes. In contrast, the other choices suggest varying relationships that do not accurately represent the mechanics of FM modulation. While B incorrectly implies that the frequency change is dependent on both amplitude and frequency, FM specifically uses amplitude for frequency deviation. C misrepresents the role of amplitude as it incorrectly states that the carrier amplitude changes with the frequency of the modulating signal, which is not how FM operates because it does not primarily vary the amplitude

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