Eric Bogatin's Blog: What I Learned This Month

Our next No Myths Allowed webinar, May 6, 1 pm EDT, “S-Parameters, Signal Integrity and You”. Details and registration available at www.beTheSignal.com

At DesignCon 2009, the paper I co-authored with Bert Simonovich, Mike Resso and Sanjeev Gupta, won a best paper award. It was titled “Practical Analysis of Backplane Vias.”

You can download a copy of the paper, which is BTS107, from our web site. I also expanded on this topic in the April 2009 issue of Signal Integrity Insights, available on our web site.

Bert had constructed a number of via and signal trace structures in a 26 layer board and did 4-port VNA measurements of the channels which included two sets of vias and a uniform stripline interconnect between them. We all collaborated in analyzing the measurements and building simple topology based circuit models to describe them.

Two aspects of this project surprised me, both relating to how complex structures can often have very simple descriptions even to bandwidths exceeding 10 GHz.

The stripline structures were simple uniform lines, with pre-preg on one side and core laminate on the other. Using the Park Nelco dielectric calculator, Bert dialed in the glass weave construction and the resin type and the calculator spit out the Dk and Df values.

Using the built in 2D boundary element field solver in ADS, we matched the measured stripline performance with the predictions based on the material properties. (For an analysis of the accuracy of the ADS and Polar Instruments field solver based transmission line analysis tools, see the Signal Integrity Insights issue for Jan, 2009).

The agreement between the predicted Dk and what we measured was within about 1-2%. If you know the laminate composition, the materials vendors can provide very accurate Dk values. Often, the reason the actual Dk value provided by a fab house is off from what is measured is because the construction is not tracked, or a general number is provided rather than the as-fabricated construction.

Of course the dissipation factor value, 0.008, was way off from what we measured. The extracted value from the measurement was actually 0.02. I routinely find the as-measured Df in completed boards is always higher than the specified values. I have heard many other folks also report this same behavior.

The second startling result from this project is how a relatively complex structure like a differential via, going through 26 layers, with variable length through part of via stub, and with an adjacent return via can be described with so simple a model as a differential pair for the top and bottom segments.

In fact, the value of the differential impedance was very closely matched by the simple analytical estimate of twin rods. However complicated the actual physical structure might be, long vias in backplanes can behave as simple electrical structures and simple circuit models can be used to predict their behavior.

OF course, to get the simple model just from the desing, before you “build it and test it”, with the complication of clearance hole size, non function pads, thick power planes, and capture pads, a 3D field solver is an essential design tool. But, once you have the S-parameter performance, you can fit a simple differential pair model to it and use the model as a scalable tool in circuit simulations. This model allows you to simulate the behavior of any via between any layers, just given the layer position. This way, you don’t have to run 26 different 3D solutions.

This is further confirmation that Einstein’s principle, “Everything should be made as simple as possible, but not too simple,” applies very well to signal integrity analysis.

Our next No Myths Allowed webinar, May 6, 1 pm EDT, “S-Parameters, Signal Integrity and You”. Details and registration available at www.beTheSignal.com

Mike Resso, my long time collaborator of more than 15 years, and I have recently finished a book published by the IEC, “Signal Integrity Characterization Techniques“.

This book focuses on TDR and VNA measurement techniques and the use of S-parameters for signal integrity applications. We pulled together about 25 papers presented at DesignCon which Mike and I have authored or co-authored and supplemented them with five application notes I wrote for Agilent.

Unlike the papers, which are typically at an advanced level, these application notes start with the basics and explain, step by step how to interpret measurements in either the time or frequency domain and as either single-ended or differential signals.

If you deal with TDR or VNA measurements or need to know about S-parameters, this is a great resource. It will get you up to speed and enable you to understand the sophisticated measurements and analysis techniques used throughout the industry by the leaders in signal integrity.

We hope you enjoy it. Additional articles on this topic can also be found on my web site.

Mark your calendars for our next No Myths Allowed webinar, scheduled for Wed, May 6, 2009 at 1 pm EDT. Like all of our webinars in this series, it will last about 45 minutes with 15 minutes for Q&A and be well worth your time.

This one is entitled, “S-Parameters, Signal Integrity and You.” In 45 minutes, we will introduce the most important features of S-parameters, starting at the very beginning and exploring some of their features and why they are becoming the defacto standard to describe the high frequency behavior of interconnects.

I’ve started a list of the questions we will address in our brief 45 minutes. If you have another question important to you not on the list, drop me a note and I will consider adding it to the webinar. Here is what we will cover:

• What are S-parameters?
• Why should I care?
• Where do they come from?
• How are they simulated?
• How are they measured?
• How accurate are they?
• How do I look at them?
• What’s the deal with differential S-parameters?
• How do I measure differential S-parameters?
• What can I learn about an interconnect from them?
• What can I do with them?
• What are some of the common pitfalls I should watch out for?
• What are some of the resources I can leverage to get more value from S-parameters?

You must sign up by May 4th, in order to attend. We will send out an email note with the access information the day before the webinar. The webinar will be recorded and posted to our website, along with all our other video recordings, and available to all paid subscribers the day after the presentation.

Hope to see you in cyberspace!

Our next No Myths Allowed webinar, May 6, 1 pm EDT, “S-Parameters, Signal Integrity and You”. Details and registration available at www.beTheSignal.com

Fiber weave induced skew in high speed serial links is a serious problem at 5 Gbps and above, but a new option from Dielectric Solutions may dramatically reduce this problem. Their solution, NovaSpeed 1080,  is a new low Dk glass fiber woven into a flatter fabric.

At IPC Expo last week, I had a chance to chat with John Kuhn, VP of Technology at Dielectric Solutions. You can watch my interview with him which I did for Real Time with IPC. This is a topic I’ve written on a number of times in the past. Here’s what I learned from John.

Glass weave induced skew arises when signals in the two lines that make up a differential pair travel at different speeds due to local variations in the dielectric constant they see. The dielectric constant variation is due to the higher dielectric constant of the glass weave, typically 6-7, compared to the resin, typically 3-3.5, and its bunching into fiber bundles. This is illustrated in the figure above.

If one line in a pair happens to be closer to a fiber bundle than its partner line, it will see a higher dielectric constant and travel slower than its partner. By the time the two signals come out the end of equal length lines, the slow path will be delayed compared to the faster path and there will be time delay skew between them.

This skew causes an asymmetry between the signals in the two lines and converts some of the differential signal into common signal, distorting the rise time of the differential signal, causing ISI, collapse of the eye and deterministic jitter.

In FR4 systems the typical glass weave skew varies depending on the weave pitch, the line to line pitch and the glass weave. While it may run to a worse case of 10 psec/inch, it may typically be 2-4 psec/inch. By its nature, the effect of glass weave skew is statistical.

The spec for the worst case acceptable skew is usually about 10% of a unit interval. At 5 Gbps, in PCIe Gen II, for example, the unit interval is 200 psec and the maximum acceptable total skew is less than 20 psec. This means that it is easily possible for the two lines that make up a differential pair to violate a skew spec after running only 5 inches or less.

It doesn’t mean no pairs will work longer than 5 inches. Weave induce skew is a statistical problem and depends on the random alignment of the signal track precisely over the worst case of the glass weave tracks. But, if you build enough boards with enough lines, some of them are bound to show excessive skew, and show a high bit error rate.

Fiber skew is one of the hardest problems to debug. Its signature is poor eye opening in one channel while an adjacent channel is just fine. Or, all the paddles cards in a collection might work but not in all combinations with a specific backplane. If you see these problems, suspect fiber weave induced skew as the root cause.

While there are a number of design based solution, a new technology solution is available that should be added to your tool box. Dielectric Solutions has introduced a new type of glass fabric which dramatically reduces the fiber weave skew effect. Their solution comes from two innovations.

They formulate their own glass, melt it down, extrude it into glass fiber and weave the fiber in a fabric. This gives them complete control over the entire glass fabric process. They’ve formulated a new low Dk glass with a dielectric constant of about 4.5, compared to the standard E glass with a dielectric constant of 6.8.

Second, they weave this fiber into a flat fabric so there is little lateral variation in the glass density. This combination means significant reduction in the glass weave skew. While FR4 and E glass combinations might show a maximum skew of 10 psec/inch, the worst case skew with the low Dk spread glass fabric is less than 1.6 psec/inch. This means much more margin in current systems, and enabling higher bandwidth systems when fiber skew sets the limit.

In the quest for higher bit rates and longer runs, the NovaSpeed fabric from Dielectric Solutions is an exciting alternative that can dramatically improve performance. One less problem for the high speed signal integrity engineer to worry about.

Hope to see you in Cyberspace at our next webinar!

I give about 20 lectures around the world each year and get a chance to meet many of the folks I correspond with by email. If you missed the live presentation I give, you now have a chance to view a recording of the lecture.

All our recorded lectures are available to paid subscribers. Feel free to browse the list on our web site. As I give new lectures, I will try to record them and post them as well. We have about 12 recorded lectures on the web site now, and hope to have more than 20 by the end of the year.

If you are not a paid subscriber, you can still download a copy of the handouts to many of my lectures. Browse the list of lectures on our web site.

There is no substitute to being there, but when travel is restricted, sometimes its just not possible to directly participate. Now you have an alternative.

I hope to see you there!

Our next No Myths Allowed Webinar: “S-Parameters, Signal Integrity and You”, May 6, 2009 1 pm EDT. Details are at www.beTheSignal.com

Last month’s pop quiz was:

“To build a transparent differential via, what is the most important feature to engineer?”

Here are the results from 180 participants.  While the most common answer to all signal integrity questions is “it depends”, it’s not always the best answer. In the case of transparent vias, the limitation is really the via stub. The correct answer is the fourth one, minimize the stub length, which 23% of you correctly answered.

All of the other factors are important. but the one with the biggest impact and which limits the bit rate of signals transmitted down the interconnect, is the length of the via stub. As a rough rule of thumb, the maximum stub length, in mils, that can be used in an interconnect system to transmit a bit rate, BR, in Gbps, is roughly:

Len < 300 mils/BR.

For the details on this and other properties of differential pairs, check out the last No Myths Allowed webinar, “NMA-800 Practical Differential Pair Design”. The handouts are available for download, and if you missed the live webinar, you can view the recording from our web site, www.beTheSignal.com.

And don’t miss our next No Myths Allowed webinar, “S-Parameters, Signal Integrity and You” on May 6 at 1 pm PDT.  I’ll see you there!