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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. Learn how this noise is transferred from trace to trace and methods for preventing it from happening. 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. A smart collection of books, magazines, electronics kits, robots, microcontrollers, tools, supplies, and more LONG LASTING NOISE PRANK: With 3 batteries plus 3 spares included, you can keep the chirping going for hours on end. But what really sets this prank apart is its on/off switch for prolonged use. That's right, you can turn it on and let it chirp away for as long as you want, until your victim is ready to crawl up the walls in frustration.
The proper method for controlling crosstalk in side-by-side routing is separation only. Rows of “Ground” Vias
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Many rules of thumb have recommended inserting “guard traces” between transmission lines as a method for controlling crosstalk. If this works, why does it work? And if it works is there any downside to using this method? The “standard practice” in many companies is to route with 5 mil line and 5 mils spaces. Referring to Figure 4, if a PCB were routed to these rules and the height above the nearest plane was 5 mils (also ) the crosstalk would be about 8%. If this were determined to be excessive and a guard trace were added, what would that involve? To make room for the guard trace a 5 mil space and a 5 mil trace need to be added. Now, the edge to edge separations is 15 mils instead of 5 mils and the crosstalk is less than 1%. It was not the guard trace that caused this decrease. It was the separation. Figure 7 is a screen showing how the separation between traces is specified as well as the trace width and height above the plane. It should be noted that trace width has no bearing on crosstalk, only edge-to-edge separation and height above the nearest plane are involved once transmission lines have been routed beyond the “critical length”. Figure 4 is titled “Offcenter” Stripline. This means that the transmission lines are between two planes but are not centered between the two planes. This is typical of PCBs that have two signal layers between a pair of planes. Notice that crosstalk decreases substantially as the height above the nearest plane is reduced. It also decreases even more rapidly as the traces are moved apart from each other. Figure 5 is a plot showing these values for micro-stripline, signal layers that are on the outside of a PCB.
Downsides to adding guard traces are: This makes routing much more difficult. The guard trade is not a barrier. It is a resonant circuit that may enhance crosstalk by creating a band pass filter. When over and under routing is done, where one transmission line is in one layer and the other is in the layer above or below, coupling is dominated by the electric field much as if a small capacitor had been connected between the two transmission lines. The coupled waveforms have that appearance. With the fast edges of modern logic, the amount of energy coupled grows so fast with the overlap between two traces that it exceeds allowable limits with very short runs. 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. Right the First Time, a Practical Handbook on High Speed PCB and System Design, Volumes 1 & 2,” Zasio and Ritchey, Speeding Edge 2003 and 2006.Membership connects and supports the people and projects that shape our future and supports the learning The current unit is at my office, sitting, mostly quietly, behind my PC. Lots of equipment around, so with the 4-45 minute delay between beeps and other electronic noises in the environment I wasn't sure whether anyone would notice. Figure 8 is a set of waveforms that result when the driven line switches from a logic 1 to a logic 0. The red waveform is the signal at the driver on the driven line and the purple waveform is the signal at the receiver on the driven line. The flat yellow line is the output of the victim line which is at a logic 0 and the waveform with the bump on it is the receiver end of the victim line.
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