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58 DESIGN007 MAGAZINE I NOVEMBER 2019 As system performance requirements in- crease, the PCB designer's challenges become more complex. The impact of lower core volt- ages, higher frequencies, and faster edge rates has forced us into the frequency domain. At first, signal integrity (SI) can look quite daunt- ing, but if we take the time to absorb the key concepts, then it is like visualizing a multi- layer PCB from a different perspective. In this month's column, I look at the frequency do- main. Perhaps one of the most fundamental steps in the process of gaining proficiency in high- speed digital, RF, and microwave design is learning to think in the frequency domain. For most of us, the vast majority of our early ex- perience with electrical circuits and signals re- mains within the context of voltages and cur- rents that are either static or dynamic with re- spect to time. Digital design, on the other hand, is a world of frequencies, so we need a different para- digm. The frequency domain can provide valu- able insight to understand and master many SI effects, such as impedance, lossy transmis- sion lines, and the power distribution network (PDN). In the time domain, the system is evaluated according to the progression of its state with time. In the frequency domain, the system is analyzed according to its response for different frequencies. In a linear system, a transforma- tion (usually Fourier transform) can convert the model into the frequency domain from the time domain. The system is changed from time to frequency to make it easy to understand the response of the system because the time do- main is more complex for higher orders. Put simply, a time-domain graph shows how a signal changes over time (Figure 1), where- as a frequency domain graph (Figure 2) shows how much of the signal lies within each giv- en frequency band over a range of frequencies (bandwidth). The Frequency Domain Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: A square wave created by odd harmonics. Figure 2: Frequency transform of a square wave.

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