High Speed PCB Design
In this High Speed PCB Design Guide get an overview of everything you need to know, including transmission lines, terminations, and controlling backward crosstalk.
TOPICS IN THIS SOLUTION
At its most basic a transmission is a pair of conductors used to deliver energy in the form of an electromagnetic field. Most of us are familiar with the wires that lead to our houses to deliver the power needed to operate our lights and appliances. In the context of PCB design, it is a signal in a signal layer on top of a plane or between two planes.
Figure 1. Types of PCB Transmission Lines
Knowing how to create and manage electromagnetic fields is the key to succeeding in high speed electronics.
The words crosstalk and coupling are used to describe the injection of electromagnetic energy from one transmission line to another running nearby. In printed circuit boards crosstalk is usually two traces running side by side in the same layer or one over the top of the other in adjacent layers. This coupled energy appears as noise on the victim trace and can cause malfunctions if the amplitude is too large. This section will describe how this noise is transferred from trace to trace and methods for preventing it from happening.
Figure 1 is a diagram showing two transmission lines traveling side by side. The upper transmission line is shown switching and the lower one is inactive. Notice that there are two waveforms along side the victim line. One is at the end of the lines where the driver is on the driven line and the other is at the opposite end or far end. Note that the waveshapes are different. The waveform at the driver end of the victim line is usually called backward cross talk or “near end cross talk”, “NEXT” and the waveform at the far end of the victim line is “forward cross talk or “far end cross talk”, “FEXT”.
Figure 1. Two Transmission Lines Side by Side Interacting
Exactly what these two waveforms wilt look like depends on what is on the four ends of the transmission lines. The possibilities are: a short circuit, a termination or an open circuit. Reference 1 at the end of this unit describes in detail how these end terminations affect the signals seen on the victim line. From that paper it will be observed that the worst case is when the far ends of both lines are open circuits and the near end of the victim line is a short circuit. That happens to be how most CMOS circuits operate. Under these conditions, the waveforms seen on the victim line will look very much like those shown in Figure 1.
When transmission lines run side by side the coupling mechanism is dominated by the magnetic component of the electromagnetic field. In over and under routing the electric field will dominate.
Several methods have been proposed for controlling backward crosstalk. Among these are:
- Restricting length that transmission lines run side by side
- Inserting “guard traces” between the two transmission lines
- Rows of “ground” vias on both sides of a sensitive signal
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