From Design and Development of Medical Electronic Instrumentation
As they operate, digital circuits constantly switch the state of lines between a high-voltage
level and a low-voltage level to represent binary states. As shown in Figure 4.1a, the resulting
time-domain waveform on any single line of a digital circuit can thus be idealized as
a train of trapezoidal pulses of amplitude (either current I or voltage V) A, rise time tr, fall
time tf(between 10 and 90% of the amplitude), pulse width τ (at 50% of the amplitude),
and period T.
The Fourier envelope of all frequency-domain components generated by such a periodic
pulse train can be approximated by the nomogram of Figure 4.1b. The frequency spectrum
is composed mainly of a series of discrete sine-wave harmonics starting at the fundamental
frequency f0 = 1/T and continuing for all integer multiples of f0. The nomogram
identifies two frequencies of interest. The first is f1, above which the locus of the maximum
amplitudes rolls off with a 1/f slope. The second, f2, is the limit above which the locus rolls
off at a more abrupt rate of 1/f2...
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