# Book Excerpt: Signal Integrity by Example, Chapter 3

The following is an excerpt from The Printed Circuit Designer's Guide to... Signal Integrity by Example, written by Fadi Deek of Mentor, a Siemens Business. Deek explores how to reach effective design solutions and make strong engineering tradeoffs through analysis techniques, best design principles, and software tools to achieve accurate simulations and measurements.

Chapter 3: Crosstalk

Root Cause
Crosstalk is another major issue to investigate during signal integrity analysis of a design. Typically, a third of the design’s noise budget is allocated to noise coming from crosstalk. In order to solve crosstalk issues, it is very important to understand the root cause. Coupling between two transmission lines occurs due to fringe electric fields and magnetic fields. When a signal propagates on a transmission line, it will generate fields as shown in Figure 3-1.

The E-fields, in blue, are lines emanating from the signal and return paths on which the signal is propagating and couples to all surrounding metal. That E-field will induce a voltage on any conductors lying inside the field. Similarly, the signal will also generate H-fields that will induce currents on the surrounding metal.

The coupling mechanism can also be described using mutual inductances and capacitances. A signal return path loop has a loop inductance. Any two loops in close proximity will have a loop mutual inductance between them. A signal carrying a time-varying current, di/dt, will couple from one loop to the other through this mutual inductance. Also, the same signal will have a time-varying voltage, dV/dt, and that will capacitively couple to neighboring traces.

Based on this, it is important to keep in mind that as a signal propagates down a trace, the coupling takes place at the location of the transitioning edge, where the dV/dt and the dI/dt are. As a signal propagates, the edge will have a spatial extent along the interconnect. Whether it is a falling or a rising edge, the time varying fields exist where that edge is. The steady state part of the signal does not contribute to coupling since it contains no time varying voltages or currents.

It is very important to mention that once a signal couples onto a trace as noise, the noise will split and propagate in both directions as shown in Figure 3-2.

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