Which modulation has poor spectrum efficiency?

• A. Amplitude Modulation
• B. Phase Shift Keying
• C. Amplitude Shift Keying
• D. Frequency Modulation

Answer: (D)

Spectrum efficiency measures how much data can be carried per hertz of bandwidth. Frequency Modulation tends to have poor spectrum efficiency because the information is carried in frequency variations, which creates many sidebands around the carrier. The required bandwidth grows with the modulation index, and Carson’s rule approximates this as B ≈ 2Δf + f_m, meaning even moderate frequency deviation can demand a wide bandwidth. In contrast, amplitude modulation schemes (AM and ASK) use a comparatively narrower spectrum, and digital modulation like PSK can be highly spectrally efficient by packing information into the main lobe and using filters to limit out-of-band energy. Therefore, frequency modulation is the least spectrum-efficient among these options.