What I Learned this Month

Did you miss one of our recent live training events? Now you can take them as online classes. Check out the online curriculums at www.beTheSignal.com

Over the years, I have written over 75 columns for Printed Circuit Design and Fabrication Magazine. Many of them are posted on our web site.

The most popular column I wrote, as judged by the emails I got back from those who enjoyed it, was the one last month, “Bogatin’s Top 10 Rules.“

I shudder to recall that I have been working in our industry for more than 30 years, and teaching electrical design topics for almost 25 of these years. Using the definition of an expert as “one who has made all the mistakes possible,” I am well on my way to becoming an expert in the field.

I have always tried to learn from my mistakes and in the interests of helping to generalize the mistakes I’ve made and some of the ways others can avoid making these same mistakes, I’ve compiled a list of rules to follow that apply not just to signal integrity, but many other design projects.

With permission, I’ve re-printed a copy of my PCD&F column and posted it on my web site. To download your copy, just create a free account on our web site and you can download this column, or any one of the other more than 150 items available to members.

I hope you enjoy.

Don’t miss this unique opportunity to attend the Controlling Transmission Line Loss Boot Camp in Singapore, Taipei and Tokyo in October 2010. Check www.beTheSignal.com for registration and details.

I am not perfect. I make mistakes. And I always appreciate it when others with a keener eye than I, are able to find these mistakes and point them out. In fact, I think such behavior should be rewarded.

In the last ten years, I have given out a special No Myths Allowed mug to the person who is first to point out any errors in any of my books, papers, handouts or any other publication. Luckily, there has only been one case of a technical error, which I wrote about in my Printed Circuit and Design Column, but many typos of various sorts.

If I missed a period and you were the first to find it, you got a mug. If I had the wrong capitalization in a title or sentence, and you were the first to point this out, you got a mug. If I misspelled a word, and you were the first to point it out, you got a mug. Much to my chagrin, I have given out over 150 mugs.

I have written seven books and hundreds of columns and feature articles. It never ceases to amaze me that I can read the same piece three times and find different errors each time. Where were the errors the first time I read the piece and why did I not see them? If I read it a fourth time, will I find more errors?

While I always try to do as good a job as I can in preparing the content I publish, sometimes a fresh pair of eyes is the only way to really find my mistakes.

To all of you who are members of the Mug Club and earned your No Myths Allowed Mug…thank yu.

Don’t miss our first ever, one-day boot camp on Differential Pair Design. Oct 4, 2010, Santa Clara, CA. Check www.beTheSignal.com for details.

So you think that by designing a differential pair transmission line as tightly coupled, you will reduce the channel to channel cross talk? In principle, this is exactly correct. But, by how much will tightly coupling a pair reduced cross talk? Is this really a strong motivation for tightly coupled differential pairs?

Of course, the most common answer to all signal integrity questions is “…it depends” and the only way to answer “it depends” questions is by putting in the numbers. When dealing with uniform differential pairs such as microstrips or striplines in circuit boards, the right tool to use to analyze channel to channel cross talk is a 2-D field solver.

When you put in the numbers, you find that for microstrip, in which far end cross talk can be a huge issue, using tightly coupled differential pairs gains you about 5 dB lower cross talk than using uncoupled pairs. Tightly coupled microstrips is a very good idea. Of course, if you are worried about far end cross talk, don’t use microstrip, route your traces in stripline.

In stripline, the near end cross talk is reduced by less than 1 dB by going to tightly coupled traces. While there are important reasons to consider tightly coupled differential pairs in uniform transmission lines, reducing channel to channel cross talk is not a strong driver.

Except when the interconnect paths are not uniform transmission lines. If the return path is screwed up, as in going through vias, or using flex or in a connector or an IC package, using tightly coupled differential pairs, will have a significant impact in reducing channel to channel cross talk.

Estimating the cross talk in these geometries is tough and often requires a 3D field solver. I recently worked with Bill Martins and Madhavan Swaminathan to explore channel to channel cross talk in via transitions using SPHINX, their tool from E-System Design.

The problem we analyzed is the transition of two widely spaced differential channels passing through a power and ground plane cavity and back to the surface again.  The figure to the left shows the path of one channel. The second channel would be adjacent to it.

We looked at two configurations of vias: each of the four vias in the first transition on 25 mil centers and then on 100 mil centers.  We would expect that the tighter the coupling between the two vias, in each channel, the lower the channel to channel cross talk.

The geometry was set up and the 8 port S-parameters were simulated up to 20 GHz using SPHINX.  I then used Agilent’s PLTS to re-display the simulated, differential responses in the time domain.

The differential TDR response shows the location of the via transitions as high impedance peaks, compared to the uniform lines with no via transitions.

In addition, in the bottom traces, you can see the near end channel to channel cross talk of about 1% occurring at the via transitions, and reduced noise when the vias are closely spaced. When the two vias that make up each differential pair are spaced on 100 mil centers, they inject quite a bit of differential noise in the power and ground plane cavity and we see this as residual noise picked up between the two channels long after the edge has passed by.

Whenever you have an interconnect with a screwed up return path, it’s important to first engineer this region as short as possible, and second to use as tight a coupling in the differential pair as is practical. These two features will minimize the discontinuity of the region and enable the highest bandwidth possible.

To learn more about this topic and others related to differential pair design, check out my Differential Pair Boot Camp, presented in partnership with PCBDesign007, Oct 4, 2010 in Santa Clara. You can learn more about the signal integrity training programs we offer by visiting our web site: www.beTheSignal.com.