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Beyond Design: The Frequency Domain

As system performance requirements increase, the PCB designer’s challenges become more complex. The impact of lower core voltages, higher frequencies, and faster edge rates has forced us into the frequency domain. At first, signal integrity (SI) can look quite daunting, but if we take the time to absorb the key concepts, then it is like visualizing a multilayer PCB from a different perspective. In this month’s column, I look at the frequency domain.

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 experience with electrical circuits and signals remains within the context of voltages and currents that are either static or dynamic with respect to time.

Digital design, on the other hand, is a world of frequencies, so we need a different paradigm. The frequency domain can provide valuable insight to understand and master many SI effects, such as impedance, lossy transmission 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 transformation (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 domain 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 given frequency band over a range of frequencies (bandwidth).

To read this entire column, which appeared in the November 2019 issue of Design007 Magazine, click here.