Consider the following statements about frequency modulation (FM): (1) The amplitude of an FM wave remains constant. (2) FM is generally more immune to noise than AM for the same received power. (3) FM broadcasts operate in the upper VHF and UHF ranges. (4) FM transmitting/receiving equipment is simpler than AM. Which statements are correct?

Introduction / Context:
FM is ubiquitous in broadcast audio and TV sound because of noise performance and capture effect. Understanding what is truly characteristic of FM helps avoid design and exam pitfalls.

Given Data / Assumptions:

• (1) Amplitude of FM: does it stay constant?
• (2) Noise immunity: FM vs AM.
• (3) Broadcast bands: common FM services.
• (4) Relative equipment simplicity: FM vs AM.

Concept / Approach:

(1) In ideal FM, information is carried by instantaneous frequency deviation; amplitude is constant (limited by limiters). (2) FM exhibits better noise immunity due to amplitude limiting and pre-emphasis/de-emphasis, for the same received C/N. (3) Broadcast FM (radio) uses VHF; TV sound historically used FM in VHF and UHF channels—thus “upper VHF and UHF” is accepted in broadcast contexts. (4) FM RF/IF and demodulation chains (discriminators, PLL) are generally more complex than AM, so (4) is not correct.

Step-by-Step Solution:

Verification / Alternative check:

Block diagrams of FM receivers include limiters and discriminators; AM uses envelope detectors, typically simpler. Spectrum allocations place FM radio in VHF; TV sound (FM) occurred in both VHF and UHF bands.

Why Other Options Are Wrong:

• Including (4) is incorrect; FM is not simpler than AM.
• Excluding (2) ignores FM's noise advantage; excluding (3) ignores TV FM sound allocations historically in UHF/VHF.

Common Pitfalls:

Equating only “FM radio” with VHF and forgetting TV sound; assuming FM equipment simplicity comparable to AM.

Final Answer:

1, 2, 3