## Impulse Trains

The*impulse*signal (defined in §B.10) has a constant Fourier transform:

(B.43) |

An impulse

*train*can be defined as a sum of shifted impulses:

(B.44) |

Here, is the

*period*of the impulse train, in seconds--

*i.e.*, the

*spacing*between successive impulses. The -periodic impulse train can also be defined as

where is the so-called

*shah symbol*[23]:

(B.46) |

Note that the scaling by in (B.46) is necessary to maintain unit area under each impulse. We will now show that

(B.47) |

That is, the Fourier transform of the normalized impulse train is exactly the same impulse train in the frequency domain, where denotes time in seconds and denotes frequency in Hz. By the scaling theorem (§B.4),

(B.48) |

so that the -periodic impulse-train defined in (B.46) transforms to

(B.49) |

*Proof:*Let's set up a limiting construction by defining

(B.50) |

so that . We may interpret as a

*sampled rectangular pulse*of width seconds (yielding samples).By

*linearity*of the Fourier transform and the

*shift theorem*(§B.5), we readily obtain the transform of to be

(B.51) |

with , we can write this as

(B.52) |

as . Finally, we expect that the limit for non-integer can be neglected since

(B.53) |

whenever and is some integer, as implied by §B.13. See,

*e.g.*, [23,79] for more about impulses and their application in Fourier analysis and linear systems theory.

Exercise:Using a similar limiting construction as before,

(B.54) |

show that a direct inverse-Fourier transform calculation gives

(B.55) |

and verify that the peaks occur every seconds and reach height . Also show that the peak widths, measured between zero crossings, are , so that the area under each peak is of order 1 in the limit as . [Hint: The shift theorem forinverseFourier transforms is , and .]

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Poisson Summation Formula

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Sinc Impulse