ARRL Technician Practice Exam

What is one effect of over-deviation in an FM signal?

1. Reduced clarity

2. Distortion of audio

3. Increased bandwidth

4. All of the above

The correct answer is: All of the above

Over-deviation in an FM (Frequency Modulation) signal refers to the modulation index being too high, which can lead to several negative effects. When the deviation exceeds the optimal level, it can cause a range of issues that impact the quality of the signal. One significant effect is increased bandwidth. FM signals occupy a bandwidth that is determined by the amount of deviation used, calculated using the Carson's Rule. If the deviation is excessive, the bandwidth of the transmission increases, potentially interfering with adjacent channels. Another outcome is distortion of audio. When over-deviation occurs, the transmitter may create unintended frequency components that were not present in the original audio signal, thereby distorting the information being transmitted. This distortion can cause the sound to be less faithful to the original source, leading to a drastically different listening experience. Furthermore, reduced clarity can result from over-deviation as well. The combination of excessive bandwidth and distortion can makes it difficult for receivers to decode the signal accurately, leading to a drop in clarity and overall signal quality. Thus, all the stated effects—reduced clarity, distortion of audio, and increased bandwidth—are indeed consequences of over-deviation in an FM signal, confirming that the best choice is that all effects are valid.