As the fundamental period of the time waveform increases, the
fundamental frequency of the Fourier series components making up the waveform decreases
and the harmonics become more closely spaced.
In the limit, as the time between pulses approaches infinity, the harmonic spacing becomes
infinitely small and the spectrum is in fact continuous and bounded by the sinc function as shown.
A single pulse is not of course a periodic time function and the spectrum cannot strictly
be evaluated using the Fourier series expansion. Instead the more general Fourier
transform should be used.
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