Part 1 of this series described how digital signals propagate through PC boards, and Part 2 described specific board stack-up designs to achieve low EMI. Part 3 will address partitioning of circuit sections, routing of high-speed traces, and a few other layout practices to help reduce EMI.

Besides proper layer stack-up, the next most important consideration when laying out the circuitry on your board is partitioning of circuit functions, such as digital, analog, power conversion, RF, and things like motor control or other high-power circuits.

Before we get to circuit layout, we must first understand and visualize how return currents flow and how the electromagnetic fields are distributed under high-speed circuit traces. At low frequencies—below about 50 kHz—return currents tend to follow the path of lowest resistance. They tend to travel along the shortest distance between source and load, as simulated by the green area in Figure 1.

Figure 1. Return current field distribution below 50 kHz follows the path of least resistance. Image courtesy Keysight Technologies.

At roughly 50 kHz to 100 kHz, return currents tend to follow the path of lowest impedance due to mutual impedance coupling effects between the signal path. These currents tend to travel directly underneath the signal path between source and load, as simulated by the green area in Figure 2.

Figure 2. Return current field distribution at frequencies above 50 kHz, will return to the source through the path of least impedance. Image courtesy Keysight Technologies.

You can now understand why analog circuitry should be located well away from digital or other noisy circuits. Keep these "spread out" return currents from intermixing with the return currents from noisy circuitry. That's the main reason why partitioning is so important.

Partitioning
Part 1 of this series described how the digital (and other high-frequency) signals propagate through a board's dielectric space. To avoid signal coupling and crosstalk, you must not allow the various return signals from intermixing within the same dielectric space. Thus, you need to partition major circuit functions. Figure 3 demonstrates one example of partitioning. Of course, this gets more challenging as board size shrinks. Henry Ott also describes this concept in Reference 1.

partition circuits on PCBFigure 3. An example of how to partition circuit functions on a board.

Knowing now that low frequency signal returns tend to spread out more, we can see that any analog or low-frequency circuitry must be separated from digital, power conversion, or motor controller circuits. Likewise, sensitive RF receiver circuits, such as GPS, cellular, or Wi-Fi devices must also be kept separate from digital, power conversion, or motor controller circuitry, as well.

[Continue reading on EDN US: Signal routing and other guidelines]

Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.