Introduction / Context:
In frequency modulation (FM), unlike amplitude modulation (AM), the carrier amplitude remains constant regardless of the strength of the modulating signal. This unique feature of FM provides inherent noise immunity and efficiency advantages.
Given Data / Assumptions:
- FM system uses constant amplitude carrier.
- Information is carried in frequency deviation, not amplitude.
- Transmitter power depends on carrier amplitude, not modulation depth.
Concept / Approach:
The total transmitted power in FM remains constant because the carrier amplitude does not vary with modulation. Frequency deviation changes, redistributing power between carrier and sidebands, but the total RF output power remains the same regardless of modulating signal strength.
Step-by-Step Solution:
FM signal: v(t) = Ac cos(2πf_ct + β sin(2πf_mt)).Ac (carrier amplitude) remains fixed.Average transmitted power P = (Ac^2) / (2R).This does not depend on β (modulation index) or modulating signal power.
Verification / Alternative check:
Practical FM transmitters (broadcast, two-way radios) deliver constant carrier power independent of modulation depth.
Why Other Options Are Wrong:
Increase or decrease with modulating signal: describes AM, not FM.Dependence on modulation index: affects spectral distribution, not total transmitted power.'None of the above' is incorrect because there is a specific correct answer.
Common Pitfalls:
Confusing FM with AM, where power is proportional to modulation depth.Assuming carrier amplitude changes with modulating signal in FM—it does not.
Final Answer:
Is independent of the modulating signal power
Discussion & Comments