[Amps] IMD

[Fuqua, Bill L]

  The spikes at the beginning and ends of a square wave are produced when higher frequency harmonics are removed such as by using 
a low pass filter. And their amplitudes are  limited to only about 9%. That is because the harmonics amplitude drop of as 1/N where N is the harmonic
number. This is why I said "Google Fourier  Gibbs phenomenon." To have a perfect square wave all the odd harmonics must
exist at the right amplitudes. 
   It is true that a perfect ideal square wave will produce infinite odd harmonics but they drop in amplitude very quickly.

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"The leading and trailing edge portions of the square
wave stack up and create an infinite amplitude at each."