Design Ideas

February 17, 1997

Trace-dimension control reduces crosstalk

Raymond Cosimano, Cosco Technologies, Endicott, NY

If you assemble a high-performance system using multiple single-board computers and a common backplane, you can encounter unacceptable levels of coupled noise, or crosstalk. This problem can plague such high-speed systems as real-time image-recognition systems or real-time video servers. Crosstalk, which is unwanted voltage capacitively coupled from a switching line to an adjacent quiet line, can cause false switching and random failures that are difficult and time-consuming to troubleshoot. You can reduce crosstalk by increasing the distance between the active and quiet lines, reducing the distance that the two traces run in parallel, slowing the rise time, and moving the traces closer to a reference plane.

Reducing the distance that the traces run in parallel limits the number of single-board computers or adapters you can use on the backplane. Degrading the switching signal's rise time hurts throughput. Reducing the width of the lines you use in the backplane moves the traces farther apart and increases the characteristic impedance, according to the following expression:

where w is the width of the signal line, t is the thickness of the line, and b is the distance between the upper and lower power planes of the sandwich.

You can compensate for the in-creased impedance by moving the power or reference planes closer to the signal traces. So, it's evident that using finer line width improves two of the controlling parameters of coupled noise. However, you face a trade-off. Finer lines can lead to higher cost. When you design the backplanes, you must work with your supplier to select the optimum design for your application. Figure 1 shows designs using 5- and 12-mil lines. Figure 2 shows the coupled noise in a 3V-dc quiet line, for a 70V system with 30-mil trace separations. The lowest crosstalk occurs in a system using 5-mil trace widths and 35.2-mil power-plane separation. Note also that a 10-layer stackup at the ground and signal layers reduces the ground bounce that typically occurs with a large number of simultaneous switching events in high-performance applications. (DI #1992)

Figure 1

To reap the benefits of smaller trace widths, you must reduce the separation of the power planes.

Figure 2

Reducing signal-trace widths dramatically reduces coupled noise.