Noise in FM systems: It is known that noise phase modulates an FM wave. As the noise sideband frequency approaches the carrier frequency, how does the noise amplitude behave?

Introduction / Context:
Noise in FM receivers behaves differently compared to AM. In frequency modulation, noise components produce phase modulation of the carrier, with their effect depending on the frequency separation from the carrier.

Given Data / Assumptions:

• FM system with modulated carrier frequency.
• Noise sidebands are added due to channel disturbances.

Concept / Approach:

Noise at frequencies far from the carrier produces larger deviations, while noise close to the carrier produces very small phase deviations and thus has smaller amplitude effect at the demodulator output.

Step-by-Step Solution:

Verification / Alternative check:

FM noise triangle theory confirms noise amplitude falls as frequency approaches carrier, forming the classical triangular noise spectrum.

Why Other Options Are Wrong:

• Increase: opposite of FM noise behavior.
• Constant: not observed in practice.
• Negligible: only at exact carrier, not in general.
• First increase then decrease: incorrect description of FM noise spectrum.

Common Pitfalls:

• Confusing AM noise with FM noise spectrum characteristics.

Final Answer:

Will decrease