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70 DESIGN007 MAGAZINE I MAY 2020 The term bandwidth was first used years ago in the RF world to represent the range of fre- quencies in a signal. In digital electronics, we also use the term to describe the signal spec- trum since square waves are made up of nu- merous sine waves (harmonics) of the funda- mental frequency (Figure 1). For digital signals, the lowest frequency is always the DC compo- nent (zeroth frequency) and the highest fre- quency component is the maximum frequency that is significant (typically the fifth harmon- ic). The shorter the rise time in the time do- main, the higher the bandwidth in the frequen- The Impact of Signal Rise Time on Bandwidth cy domain, and the more closely the wave- form resembles an ideal square wave. In this month's column, I will look at the relationship between signal rise time and the bandwidth of a digital signal. Rise time describes how quickly a digital sig- nal can change from a low state to a high state. A signal must have a fast enough rise time to accommodate the data being processed. Oth- erwise, information in the waveform (circuit timing) may be lost. However, a signal does not have to have a faster rise time than is re- quired by the system. Faster is not necessar- ily better as it may create ground bounce, reflec- tions, crosstalk, and elec- tromagnetic radiation. In an ideal world, one should limit the band- width so that the system performs to expectations, but at the same time, avoid high-frequency ef- fects, which cause elec- tromagnetic compatibili- ty issues. This is exactly what a series impedance termi- nation does. The resis- tor, close to the source, combines with the in- put capacitance of the receiver IC(s) to cre- ate a low pass filter (Figure 2). This fil- ter rolls off the high- frequency components of Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Harmonic content of a square wave. Higher harmonics have faster rise times.