What is a disadvantage of frequency modulation?

• A. It has poor spectrum efficiency
• B. It is easy to implement
• C. It changes amplitude
• D. It is highly resistant to interference

Answer: (A)

Frequency modulation trades bandwidth for noise resilience. In FM, the carrier frequency shifts with the input signal, producing a broad spectrum made up of many sidebands. The required bandwidth grows with the maximum frequency deviation and the modulating signal’s bandwidth (Carson’s rule: roughly 2(Δf + f_m)). To improve fidelity and reduce noise, you increase the deviation, but that widens the spectrum further. So, FM is spectrally less efficient than some other modulation methods, which is its main drawback in bandwidth-constrained environments. The other statements don’t fit: the carrier’s amplitude stays essentially constant in FM, so it isn’t about amplitude changes; implementing FM and its demodulators is more complex than simple amplitude modulation; and FM offers strong resistance to interference, which is a benefit rather than a disadvantage.