EMC Design Guide for Printed Circuit Boards
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3.5. Frequency Domain of Digital Signals
A typical square wave is shown in Figure 37.
Figure 37. Elements of Digital Signal
A square wave has an AC component during the transition times and a DC component
during the steady state. The AC current contains all of the frequency components of the
square wave. In addition to the fundamental frequency, a digital signal also contains
harmonic frequencies which are integer multiples of the fundamental frequency. For
example, a digital signal with a fundamental frequency of 10 MHz has harmonic
frequency components at 20, 30, 40, ... MHz. Therefore, digital signal current flows at
DC and at 10, 20, 30, 40, ... MHz.
The signal spectrum is a plot of the fundamental and harmonic frequencies. Figure 38
shows the signal spectrum for a square wave in amplitude versus frequency graph.
Figure 38. Digital Signal Spectrum
AC DC AC
Digital Signal Spectrum AC DC AC A A/2 t T t r t f Bandwidth Frequency Amplitude Fundamental frequency -20 dB/decade Spectral envelop -40 dB/decade t 1 f 1 × = r 2 t 1 f × = DC A 2 ×