Question 8A5

Excessive modulation, such as "in excess of 150%" in an FM system like marine VHF, is a severe form of overmodulation. When the audio signal causes the frequency deviation to exceed the modulator's linear operating range or the maximum allowed deviation (e.g., ±5 kHz for standard FM), the modulator stage "clips" the peaks of the modulated waveform. This clipping introduces significant non-linear distortion, generating harmonics and spurious emissions that spread beyond the allocated channel bandwidth, resulting in severely distorted audio and interference to adjacent channels. * **A) The reactance modulator may be out of alignment.** While misalignment can cause distortion, "excessive modulation" is a direct condition of the signal level causing the distortion, rather than a component fault. * **B) The excessive use of pre-emphasis on audio signals above 25 kHz.** Marine VHF voice communication typically uses frequencies between 300-3000 Hz. Signals above 25 kHz are outside the relevant audio band and would be filtered out, making this irrelevant. * **C) A defective varactor diode in the transmitter’s voltage controlled oscillator.** A defective varactor can cause issues like frequency instability or non-linear modulation, but "excessive modulation" describes the *level* of the signal being applied to a *working* modulator, causing it to distort.