Which statement best describes bandwidth-limited signals?

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 zero outside some finite frequency range. In practice, not all signals meet this: many have energy extending to arbitrarily high frequencies, even if those components are very small. A common example is a Gaussian pulse in time, whose spectrum stretches across all frequencies, so it is not bandwidth-limited. There are signals that are bandwidth-limited, such as a pure sine wave (energy at only one frequency, or two discrete frequencies in the spectrum) or an ideal time-domain sinc signal, but many real-world signals are not strictly confined to a finite band. Therefore, not all signals are bandwidth-limited, and some have energy across all frequencies.