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MAY 2020 I DESIGN007 MAGAZINE 73 In practice, the signal rise time has an im- pact on the maximum signal bandwidth. Un- derstanding the frequency band, that really matters, for digital design is very important. Traditionally, we used Equation 1 for the up- per bandwidth, assuming one can ignore all the frequency components above the band- width and not lose any valuable information about the time domain response. Equation 2 However, as rise times become faster, a more accurate approach is to use the upper knee fre- quency (which is down -6.8 dB), as in Equa- tion 2. This forms a useful translation between es the interpretation of the rise time. The rise time of a signal is inversely proportional to its bandwidth. Equation 1 BW is the bandwidth in GHz, and Rt is the rise time in ns. The bandwidth of the signal in the frequen- cy domain is where we arbitrarily define the bandwidth as the -3 dB point. This is the fre- quency at which the signal amplitude, through the circuit, is reduced down to 70% of the am- plitude of the input signal. Higher frequencies are attenuated even more. Figure 3: Odd harmonics of a 1 GHz fundament clock in the frequency domain.