Next public classes: Essential Principles of Signal Integrity, Advanced Signal Integrity Design, Multi Giga Bit Design: Sept 29- Oct 7, 2009 in San Jose, CA

Next No Myths Allowed Webinar, “Stack-up Design for Differential Pairs”, presented free on Sept  16, 1 pm EDT.

Last month’s pop quiz question related to the impact from coupling on the differential impedance of symmetric stripline differential pairs.

If two coupled symmetric stripline traces start out with a differential impedance of 100 Ohms, and they are brought closer together, what happens to the differential impedance?

The general answer is that increasing the coupling will always decrease the differential impedance; but by how much? The only way to really know is with a 2D field solver. Because I’ve looked at similar problems a lot, I know the answer is somewhere between a 10% and 20% reduction in the differential impedance, but I can’t pin it down any finer without putting in the numbers with a field solver.

My favorite and simplest to use 2D field solver is Si9000 from Polar Instruments. I set up the problem as shown in the figure to the left and swept the spacing to look at the impact on the differential impedance.

In this example, the line width is 5 mils, half ounce copper and the dielectric thickness above and below is 6.7 mils, with a Dk of 4.2. When the spacing is 30 mils, 6x the line width, the differential impedance is 100 Ohms.

As we decrease the spacing, the differential impedance decreases, as we expect. When the spacing is 5 mils, equal to the line width, the differential impedance has dropped to 86 Ohms. This is a 14% reduction in the differential impedance.

We had 325 answers to this quiz on our web site. More than half of you got the right answer, which was either 10% or 20%.

If you want more details about this topic, be sure to check out the next No Myths Allowed Webinar. “Stack up design for differential pairs.” coming up on Sept 16, 2009, or our live class, Essential Principles of Signal Integrity.

Be sure to visit our home page to answer the new pop quiz!

See you in cyberspace.