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The Fourier transform of a unit impulse is

  • A.A sinc function with its first null at one hertz
  • B.Zero at every frequency
  • C.Constant at all frequencies
  • D.Another impulse at zero frequency

Answer: C

An impulse contains every frequency in equal measure, so its transform is flat - the frequency-domain statement of the fact that a perfectly short event has infinite bandwidth. An impulse in frequency corresponds to a constant in time, which is the dual pair. A sinc arises from a rectangular pulse of finite width. And a transform that vanished everywhere would describe no signal at all. This pair underpins impulse-response testing, since exciting a system with an impulse probes all frequencies at once.

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