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24 DESIGN007 MAGAZINE I MAY 2021 designed with a characteristic impedance of 50 to 100 ohms. Understanding PCB design is all about two important concepts: all currents flow in loops and high frequency signals are propagated as electromagnetic waves in transmission lines. ese two concepts are closely related and coupled to one another. Currents Flow in Loops Circuit theory suggests that current flows in loops from source to load and back to the source. In many cases of product failure, the return path has not been well defined and in some cases, the path is broken. e problem circuit designers oen miss is defining the return path of a high frequency signal back to the source. If you think about it, we don't even draw these return paths on the schematic diagram—we just show it as a series of various "ground" symbols. So, what is "high frequency"? Basically, it is anything higher than 50 to 100 kHz. For fre- quencies less than this, the return current will tend to follow the shortest path back to the source (path of least resistance). For frequen- cies above this, the return current tends to fol- low directly under the signal trace and back to the source (path of least impedance) as shown in Figure 1. To reduce EMI, we need to minimize the area of these loops. Undefined return paths oen result in large current loops from source to load and back to source. ese large current loops start to look like loop antennas, coupling noise currents to "antenna-like" structures, such as cables, in your product or system. Where some board designs go wrong is when high dV/dt return signals, such as those from low frequency DC/DC switch mode con- verters or high di/dt signals from digital logic and clock return signals get comingled with I/O circuit return currents, sensitive RF mod- ules (especially receivers), or sensitive analog return currents. Just be aware of the impor- tance of designing defined signal and power supply return paths. at's why the use of solid return planes under high frequency signals and then segregating digital, power, and analog cir- cuitry on your board is so important. How Signals Move At frequencies greater than about 50 to 100 kHz, digital signals start to propagate as elec- tromagnetic waves in transmission lines. A high-frequency signal (Figure 2) propagates along a transmission line (circuit trace over return plane, for example), and the wave front induces a conduction current in the cop- per trace and back along the return plane. Of Figure 1: Simulation showing the return path (in green) at low and high frequencies. (Image source: Keysight Technologies) a b

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