Eric Bogatin's Blog: What I Learned This Month

I’ve assembled a few of the signal integrity experts I turn to when I have a technical question to join me in an “Ask the Experts” panel at DesignCon 2012.

On Wed Feb 1 at 2:00-2:45 pm in the ChipHead Theater, I will moderate a panel discussion, “Ask the experts, anything goes”. Included on the panel of experts are:

• Scott McMorrow, Teraspeed
• Bruce Archambeault, IBM
• Jim Nadolny, Samtec
• Yuriy Shlepnev, Simberian
• Ravi Kollipara, Rambus
• Jianmin Zhang, Cisco
• Jason Miller, Oracle

I have the best job in the world. I get to solicit questions from the audience and pose them to the panel. As an experiment, I will also accept questions by twitter.

On twitter, send your questions to @beTheSignal and use #SIDoctorIsIn.

If you heard a design guideline that just didn’t sound right, if you just released a design to fab and are staying up at night worrying about an iffy design feature, if you need the correct answer to settle an argument with your design team, come to this once in a lifetime opportunity to ask the world’s top signal integrity experts, personally.

You are guaranteed to learn something new and important from this exciting panel discussion.

• Should decoupling capacitors go on top of the board, or on the bottom of the board?
• Should DC blocking caps go near the TX or the RX?
• Should meander lines have a few long loops or many short loops?
• Which is better, tight or loose coupled differential pairs?
• Does a microstrip transmission line really cause EMC failures?
• What are the three most commons sources of failure in DDR3 designs?
• Where does the return current in a common signal go when it transitions from a circuit board to an unshielded twisted pair cable

Of course, the answer is always, “…it depends”, but learn from the experts, on what it depends.

See you there!

One of the many hats I wear is as an Adjunct Professor at Printed Circuit University.  This is the brainchild of Pete Waddell, Dean of PCU, publisher of Printed Circuit Design and Fabrication Magazine and president of UP Media Group. His goal is to create a one-stop-shop for training on all aspects of printed circuit board technology, including fabrication, processing, materials, assembly and design.

Many of the signal integrity online lectures I created over the last ten years and online versions of my popular curricula, both introductory and advanced, were contributed to PCU and are currently available to registered students at PCU.

As an experiment in online education, Pete and his team, lead by Mike Buetow, editor of Printed Circuit Design and Fabrication Magazine, invited me to hold office hours at PCU. I see this as a chance to sort of meet up with any past, present or future students over a cup of coffee and chat about signal integrity.

When I teach classes I always enjoy listening to and answering questions from students. I learn what is important that I may not have included in my lecture and what is confusing. I use the questions I hear as feedback to constantly evolve and improve my classes. And, I confess, it still gives me a distinct pleasure to see a student achieve that “ah ha!” moment of insight when a complex principle suddenly crystalizes into clarity.

So, come join me for a cup of coffee and a chat about signal integrity, at my first ever office hours. Any questions are fair game. I’m looking forward sharing some of my hard earned insights into the practical effects of high speed signals interacting with interconnects and learning a few new things from you.

You must be a member of the PCU community to participate, but membership is free. Be sure to visit the PCU site before Jan 26 at 2 pm EST to set up your free account if you want to join us on time! If you miss the live office hours, the chat will be recorded and available as an archive. Check it out!

As an added bonus, if you can’t make it, tweet me your question before my office hours, I’ll answer them during the live event and it will be posted in the archive for you to read.

Tweet me @beTheSignal.com and use #SIDoctorIsIn.

You can’t miss this one of a kind event at DesignCon 2012, scheduled for Tues, Jan 30, 2012 at 1:00 to 1:45 in the Chip Head Theater.

In 45 minutes, I will walk you through How Return Loss Gets its Ripples.

If you have every looked at a plot of the return loss of almost any interconnect you’ve seen the distinctive features of ripples. The return loss goes through peaks and dips. Sometimes you see these ripples in the insertion loss, sometimes not. It doesn’t matter if you look at measured or simulated data. Why is this?

What is the feature of the interconnect that causes this S-parameter feature? Why does it have the spacing it has or the magnitude of the peaks? What can you read from the front screen about the interconnect from the peaks and dips?

All will be made clear in this 45 minute quick presentation.

As a special experiment, I will be taking questions by tweets. Send your questions on this topic to @beTheSignal and use hashtag #ReturnLossRipples

See you there!

The perfect cases for every luddite to carry their iPod, camera and laptop, available from Retro to Go.

I am not a Luddite. I embrace technology: from my iPhone, iPad and hi end PC to the latest, state of the art, high-end, simulation software I use every day. But I’ve only barely dipped my toe in social media, up to now.

When the pressure to write a regular blog became too great to resist, I became a blogger and for the last four years, post regular on my signal integrity blog at beTheSignal.com/blog. I’ve slowly learned the distinction of content and my voice in a blog, as compared to a column, feature article or book.

Blogs, I’ve learned, are for content you might want to “chat about” when you talk to a colleague you haven’t seen for a while. It’s a more informal, casual style, with a short, single topic focus.

I use my signal integrity blog on beTheSignal to post my comments on articles or books I’ve recently read, or a cool web site I saw. If I had an interesting technical discussion with someone, I write it up, to share it with a broader community. If I learned something new from these activities, I think others involved in high speed digital design or signal integrity, might find the information of value as well.

But that’s been the extent of my involvement in social media. We tried creating a Facebook page for beTheSignal, but we never figured out how to successfully utilize it- for our benefit or for our customers.

I am a big fan of Leo Laporte and his most popular show, This Week in Tech (TWiT). A few years ago, he’d focus on PCs, then the theme migrated to cell phones, then it was Facebook and for the last year, it’s been Twitter.

As twitter grew in popularity and influence, I got more and more worried- what was I missing?

Even with many of my younger students telling me I should have a Twitter account, I hesitated, reluctant to dive in, especially after crashing and burning on Facebook.

But now, my new LeCroy cousins, Hilary Lustig (@HilaryLustig) and Jeremy Graef,  twisted my arm and dragged me kicking and screaming into the 21st century, convincing me I need to Tweet. But with them to back me up and guide me along this new path, I decided to take the plunge.

I can now say, I am a Twitter. My handle is @beTheSignal.com.

I have great plans for my tweets- and I promise, they are only going to be good stuff, well worth the ten second interruption these tweets create.

If you care about signal integrity, high speed electronics and technology, you can follow me into the 21st century @beTheSignal.

The first thing I think of when I think of the New Year is DesignCon. Every year for the last 20, at the end of the year, I’ve been in the middle of wrapping up preparation for the upcoming DesignCon. I started my involvement with DesignCon before it was DesignCon, back in the days when it was the HP High Speed Digital Symposium.

Each year, the conference is like a giant class reunion party. I get to hang out with folks I see once a year, with whom I’ve shared adventures, last-minute deadline-driven adrenaline-pumped cramming-sessions and sweating in front of a large audience filled with folks far more expert than I, challenging what I just spent a year working on. Gee, this sounds a lot like my orals exam in graduate school…

Not a single DesignCon goes by that I don’t learn something significant that I never thought about before and at which I generate more partnerships and projects to carry me easily through the year to the next DesignCon.

At this year’s DesignCon, in addition to participating in two papers:

13-TP6:   A Practical Approach for Using Circuit Board Qualification Test Results to Accurately Simulate High Speed Serial Link Performance 

9-TP5: A Robust Method for Addressing 12 Gbps Interoperability for High-Loss and Crosstalk-Aggressed Channels

I am trying two new activities.

On Tues at 1-1:45 pm in the Theater, I will be doing a “speed training” session. In 45 minutes every attendee will learn how return loss gets its ripples. This is the most important feature of return loss and tells a lot about the interconnect. You may even see a leopard or two….

On Wed at 2:00-2:45pm in the Theater, I will moderate a panel discussion, “Ask the experts, anything goes”. I asked a few of the folks I go to with my SI questions to hang around and field questions from the audience. They can be as basic as what is inductance, to as complicated as, where does the return current in a common signal go when it transitions from a circuit board to an unshielded twisted pair cable. Anything goes….

If you want to have a fun time, join me at these events at DesignCon. For your convenience, I posted all the links to my events on my home page as well: www.beTheSignal.com. When I am not attending an event, find me at the LeCroy booth, #101.

See you there!

December 2011 was a busy travel month for me. I gave a series of classes in India and Israel to packed rooms. Here I am in the S-parameters for SI class in Herzilya, Israel.

At this point in the class, I was showing how a simple approximation can be used to estimate the insertion loss in single-ended or differential channels based on the line width and the material properties.

The insertion loss per length, in dB/inch, for a 50 Ohm single-ended or 100 Ohm differential line is roughly:

S21 per length = 1/w x sqrt(f) + 2.3 x Df x sqrt(Dk) x f    dB/inch

with w in mils and f in GHz

For example, at 1 GHz, if the line width is 8 mils in FR4, the insertion loss per length is about

S21 per length = 1/8 x sqrt(1) + 2.3 x 0.02 x sqrt(4) x f dB/inch = 0.12 + 0.09 = 0.2 dB/in

The plot on the screen at this point in the lecture is a comparison of the measured insertion loss per inch from two different lines and this simple approximation. The results are pretty close.

This approximation is not meant to be a substitute to a good field solver, but it  helps give a rough idea of what to expect in real interconnects. It’s real value is that “…sometimes an OK answer NOW! is better than a good answer late.”

For more details on our other classes, check out our web site at www.beTheSignal.com

I’m not willing to say print is dead, but if it doesn’t evolve, it may be left in the digital dust. I am finding more and more multi media resources on the web. As someone who has created quite a bit of multi media, I know it takes more time, and more takes, to create a video piece than a a written piece.

I see three types of video pieces being generated and posted online these days: straight up marketing commercials or infomercials, interviews with experts and tutorials. I recently came across the EngineeringTV portal with more than 1200 videos spanning a wide range of engineering topics such as embedded computing, rf, medical and aerospace applications.

If a picture is worth a thousand words, a 9 minute video might be worth a thousand pictures. This is especially true when the video is a demo rather than just a recorded presentation. Even just a recorded lecture is useful when you can see the cursor moving on the slides. It literally is like attending the live lecture.

The biggest drawback to most of the online video content out there is that most posted videos are just infomercials. The signal to noise ratio of an infomercial is generally not very high.

However, here are few videos on the EngineeringTV site that are useful to take a look at. Here are a few recommendations;

Hercules, a petawatt laser (just like a laser pointer, but bigger)

2011 salary survey of electrical engineers

A very nice series of tutorials on transmission line simulation using AWR tools.

Part 1 of a 3 part series from AWR (from which the image above was taken.

S-parameters have become the defacto standard to describe the electrical properties of interconnects. While historically, the formalism around them was developed in the frequency domain, they can also be used to describe the time domain behavior of interconnects.

The illustration above, taken from a recent paper Alan Blankman and I wrote for an Australian publication, introduces the features behind S-parameters and how they can be measured or simulated. If you want to launch your path into the world of S-parameters, you might want to check out the article we wrote, and maybe the S-parameter class I teach.

It’s not cats playing piano or raccoons with their heads in peanut butter jars, but some of the videos on the EDN video vault can be as entertaining. Here is a growing collection of short videos contributed by experts and journalists that relate to electronics design and tricks and tips that might be useful to us practicing engineers and geeks at heart. Here is a short list of recommended videos to give you a feel. Check them all out here.

MakerBot’s MK 7 3D Printer 

A memorial to Bob Pease and his office, just as messy as it was 50 years ago.

SPARQ overview video

The Molex high-speed ride from send to end, through on-ramps and backplanes and off ramps

A number of video app notes by the late, great Jim Williams, my analog guru from whom who I learned analog design as a young undergraduate at MIT.

Hope for a new generation of engineers: we need more Samuel Majors for the future of the US.

In my Differential Pair Design class, I emphasize the three trends in all high speed serial links:

1. all interfaces are differential channels
2. all interfaces are currently at multi gigabit data rates
3. all interfaces have a roadmap to go to ever higher data rates

In a recent article in EE Times, Dr. Claude Gauthier, of MoSys, presented a very nice chart, shown above, which illustrates these trends perfectly.

In his article, “Overcoming 40G/100G SerDes design and implementation challenges”, he offers three end user applications which drive the “need for speed”:

1. internet consumer applications such as Facebook, YouTube and IP-TV
2. data intensive solutions: weather modeling, genome research, and financial analysis
3. the rise of cloud computing and storage

These three applications will demand ever higher data rates for all of our electronic products.

Gauthier goes on in his article to provide an analysis of the PLL performance in one of his SerDes designs. He suggests that in a 10 Gbps channel, to meet the 1 part in 10^15 BER specifications, “the PLL must exhibit ultra low jitter, on the order of sub-600 fsec.”

If you are in the multi Gigabit regime, check out his article.