Issue link: https://iconnect007.uberflip.com/i/1533085
pour on one or both sides of a trace effec- tively creates a coplanar waveguide, which can reduce the impedance at the coupling point on the trace. If a standalone trace is routed to 60 Ω, the difference in impedance for the scenario of creating a coplanar waveguide represents a maximum variation of 9.2% in Zo (Figure 3): Standalone trace: Zo = 60 Ω Ground pour on both sides: Zo = 55.26 Ω Ground pours may be effective on high- impedance, analog two-layer boards but do not significantly reduce crosstalk on low- impedance digital multilayer boards since the traces are closely coupled to the plane. Also, ground pours have the disadvantage of altering the impedance of traces that run adjacent to a ground pour area, causing reflections, and are therefore not recommended for use in the digi- tal domain. 500 µm (20 mil) spacing should be used to avoid this issue. High-speed PCB design is not as simple as sending a signal from the driver to the receiver over a transmission line. One should also consider the presence and interaction of the power distribution network (PDN) and how and where the return displacement current flows. A logic schematic diagram masks details crucial to the operation of unintentional signal pathways vital to the understanding of signal performance, crosstalk, and electromagnetic emissions. e PCB designer needs to be able to visualize the connectivity of the return cur- rent flow in order to avoid large loop areas that increase series inductance, degrade signal integrity, and elevate crosstalk and electro- magnetic radiation. Ground impedance is the root cause of nearly all signal and power integrity issues; maintain- ing low ground impedance is essential for both. While a continuous ground reference plane effectively achieves this, it becomes more chal- lenging with the addition of plane layers in a multilayer PCB. A ground plane serves as a reliable signal return, provided it remains con- tinuous under the signal path. One must also understand the importance of referencing and how to control the return displacement current flow of a signal. Each signal layer should be adjacent to, and closely coupled to, a reference plane, which creates a clear, uninterrupted return path. As the layer count increases, this concept becomes easier to implement but decisions regarding return current paths become more challenging. e return current of a high-speed, fast-rise time digital signal will always follow the path of least inductance which is directly beneath the signal path. However, discontinuities tend to divert the return current, increasing the loop area, inductance, and delay, which is not desir- able. e best way to identify the discontinui- 32 DESIGN007 MAGAZINE I MARCH 2025