Which statement best describes bandwidth-limited signals?

• A. All signals are bandwidth-limited.
• B. No signals are bandwidth-limited.
• C. Some signals are bandwidth-limited, but not all.
• D. Not all signals are bandwidth-limited; some have energy across all frequencies.

Answer: (D)

Bandwidth-limited means the signal’s spectrum is confined to a finite range of frequencies, with zero energy outside that range. In practice, not all signals have this strict confinement. Many signals carry energy at arbitrarily high frequencies, even if those components are small, so they are not bandwidth-limited. A classic example is a sharp-edged time-domain waveform (like a rectangular pulse); its Fourier transform spreads out as a sinc function and has nonzero energy at all frequencies. So there exist signals with energy across all frequencies, making them non-bandlimited. That’s why the statement not all signals are bandwidth-limited, with some having energy across all frequencies, best describes the situation.