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Q: Crosstalk is getting worse, after we had it under control. It’s not just transmission lines anymore. It’s almost laughable, but it’s costing us money. What can be done?
Rick Hartley: Crosstalk is getting worse, due to the many ICs of today with very fast rise and fall time outputs. Crosstalk, like many problems, is not related to clock frequency, rather to the rate of change of energy in transmission lines, due to most of today’s ICs having rise times faster than 1 nsec, with some even faster than 500 psec. It only takes an inch of parallelism between two transmission lines to cause maximum crosstalk coupling between the lines when signals have 500 psec edges.
There are several techniques that can vastly reduce crosstalk:
- Always route signal lines directly above a plane, preferably a ground plane.
- Try to space critical lines (ones with really fast edges) at least 1X the distance to the nearest plane for inner layer routes, and 2X distance to the plane on outer layers.
- Whenever possible, series terminate transmission lines that are fast enough to be an aggressor. Series termination vastly improves crosstalk, as well as ground bounce, EMI, power bus switching noise and ringing (vswr/return loss).
- Only use “guard traces” when they can be attached to a ground plane with vias spaced every one-tenth wavelength of the highest frequency (0.5/rise time). Do not overdo this. Vias closer than one-tenth wavelength simply take up space and make routing on other layers harder. When signal traces can be spaced far enough apart to allow for a guard trace that is properly attached to a ground plane, you can likely eliminate the guard trace, since the aggressor and victim lines will be very far apart, typically six to eight times the distance to the nearest plane.
To ensure that the above concepts have achieved the intended goals, use a 3D field solver or other high-end simulator to estimate crosstalk prior to building the board. The techniques mentioned here will not eliminate crosstalk, rather will reduce it to more acceptable levels.
To read this entire article, which appeared in the July 2021 issue of Design007 Magazine, click here.