What I Learned this Month

“At 25 Gbps, everything makes a difference,” Dave Dunham told the crowd gathered for the 2nd Front Range Signal Integrity Seminar Series held in Longmont, CO on May 9, 2013.

Dave, the Director of Signal Integrity Engineering at Molex, outline the process Molex uses in designing connectors for ultra high speed, where simultaneous mechanical and electrical requirements push the envelop of what is practical.

He outlined a five step process and walked through a few examples.

The first step is establish a few simple figures of merit or rules of thumb. For example, usually the design goal is at the Nyquist frequency of the application data rate. The specs for return loss are typically less than –12 dB and for near end cross talk, less than –40 dB.

The second step is generating concept mechanical and electrical models. These are the basis for stress-strain curves and initial electrical performance.

The third step is feedback from all the team- mold engineering, stamping, assembly tooling, plating and marketing. This cycle of concept design- multi-disciplinary review feedback and re-design, continues a few cycles until a near final design converges.

In the fourth step, the final design is released. This is the best approximation to what will deliver the performance, reliability and manufacturability requirements. When it costs more than $100k for a mold to test out a design, simulation analysis tools are leveraged to explore virtual prototypes, rather than using the build it and test it approach.

This means it’s important to have confidence in the analysis tools that they will accurately predict the measured performance of a part once built. A design of experiments (DOE) study of the virtual prototype is a key element. This identifies the most important design variables and where attention needs to be focused for robust manufacturing.

The fifth and final step is verifying the design and creating the deliverables. For many customers, the board design is just as important in determining the connector performance as the connector itself. Dave’s team provides engineering support to help customer optimize the board design based on the specific details of the connectors.

If you would like to hear the details of how Dave implements this process for the highest performing connectors in the Molex portfolio, you can watch the complete recording and download a copy of his slides on the www.beTheSignal.com web site.

While  you are there, you can also watch the recording of Jeff Loyer’s presentation from March 7, 2013.

If you are in Longmont, join us for the live Front Range Signal Integrity Seminar Series. All these events are always free. I hope to see you there!

With the decline of print media, it’s sometimes hard to find quality feature articles that have high value technical content on topics related to signal integrity.

The experts who write many of these papers are still out there, but they are posting their white papers, application notes and presentations on their own web sites. I’ve started collecting links to these experts’ pages as a resource to identify top quality papers on signal integrity. You can find the list here.

This is a new landing page on my blog web site with a list of the experts, their web sites and some of the companies with a large selection of webinars and application notes available for viewing.

If you think I missed a good resource, please drop me a note offline and I will be happy to consider adding it to my list.

It’s easy to measure S-parameters. It’s challenging to measure them without introducing artifacts and interpret the measurements correctly.

Our newest class, Hands on Workshop for S-Parameter Measurements (HOW-SPM), was created to teach these valuable measurement and analysis skills. We just finished our first class and it was a wonderful success.

This class was limited to only 12 students and filled up in the first 2 weeks after it was posted. Our next class on March 1, will also fill quickly.

Pairs of students shared a SPARQ, Signal Integrity Network Analyzer from Teledyne LeCroy, operating up to 40 GHz. All the principles and techniques we taught applied to ANY VNA. We limited attendance to 12 students total, so that everyone would get stick time and individual attention from our expert instructors. During the course of the day, everyone measured a variety of samples, each illustrating different measurement and analysis principles.

Resistors were used to illustrate how the measurement from a DMM at DC translated into the same resistance as obtained with a 40 GHz VNA- in the frequency and time domains.

A range of quality level 50-Ohm cables were measured to show the wide difference in bandwidth of good and bad cables, and the role of the connectors. We used port re-normalization to change the port impedances to view 75 Ohm cables and de-embedded the launches to show the impact at low frequency from the the port impedance, and the influence at high frequency from the connectors.

Don DeGroot of CNN Labs, designed and built a really cool test vehicle for the class. This board will be available for sale by CCN Labs in early 2013.

We looked at non-uniform and uniform lines with good and bad launches, in microstrip and stripline, as single ended and as differential. It was obvious looking at the first few measurements how strong a role the launches played above about 2 GHz. By de-embedding them low return loss measurements could be obtained up to 15 GHz, the limit to the $4 SMAs we used on the board.

One of the special exercises we did was to export the measurements from the test board as touchstone files and bring them into an analysis tool to extract the Dk and Df over frequency. All students saw that the Dk was strongly affected by the launches above about 2 GHz. This is one example where de-embedding is so important and why we focused an entire module just on how to perform fast and accurate de-embedding, and equally important, how to verify the quality of the de-embed file.

If you want to improve your VNA measurements, you really need to check out this class. Our next one is on March 1, in Longmont, CO. It will fill quickly so you’ll want to sign up early. Hope to see you there!

Every now and then, as I walk the floor of a trade show, a product or booth really catches my attention. At this show, I was stopped in my tracks when I saw the EMI Avengers, in battle with Evil EMI. Leading the Avengers was Eriko Yamato, as Wonder Woman.

Once she got my attention, her gentle, persistent tug would not let me go until I learned about the latest product Tech-Dream added to their distribution list, EM-ISight. This is a new near field scanning tool which moves a robotic arm in 3D around the surface of a functioning board, “sniffing” the near field at frequencies from 10 kHz to 40 GHz.

Hotspots in the local electromagnetic field at any frequency can be mapped over the product surface and even superimposed over a photo of the product to identify potential high field regions.

Of course, as Eriko points out, it’s always important to not confuse the near field with the far field. Sometimes a local near field hotspot is just a local hot spot and does not contribute to radiated emissions.

But a tool like the EM-ISight will give you a new window into the currents flowing on your board. And more information is always a good thing.

Measurements are the anchor to reality. There is no substitute for building something and measuring it. However, a good simulator can often get you to an acceptable answer quickly by helping you answer “…it depends” questions and explore design space.

We often refer to the results of a simulator as a “virtual prototype”.

Of course, whenever you use a simulator, it’s important to practice “safe simulation”. I describe this as my rule #9, from my list of ten rules:

rule #9: Never do a simulation or measurement without first anticipating the result. If you are wrong, there is either something wrong with your intuition or with the set up of the tool or with the tool itself. Either way, by exploring the difference, you will learn something important. If you are correct, you get a nice warm feeling that you might actually understand something.

With this perspective, I think it is critical for every electrical engineer to have a simulator handy and use it to quickly answer “it depends” questions. A numerical simulator is just one of the three analysis tools every engineer should use. All three: rules of thumb, approximations and numerical simulation tools, are equally important and each have their own balance between accuracy and effort to get an answer.

Unfortunately, many questions involving impedance, reflections or transmission lines are too complicated to get even a reasonably accurate answer using rules of thumb or approximations and numerical simulation is essential.

I’ve explored many free versions of SPICE and have found QUCS to be the best tool for signal integrity problems. This is why I use it in all of my classes and feature it in some of our hands on labs.

At the 2012 IEEE EMC Symposium, I gave a presentation introducing QUCS and created five simple, yet valuable examples of QUCS circuits. These circuit files, as well as an executable version of QUCS, are available for free download from my web site.

If you want to accelerate up the signal integrity learning curve, I strongly suggest you take a look at QUCS. Of course, it doesn’t do everything you may want, like HyperLynx or ADS or Simbeor, might do, but it’s free!

QUCS should be one of the important tools in every engineer’s tool box to help you get to the correct answer, faster.

Even though we’ve provided signal integrity training classes for more than 20 years, we are constantly evolving them based on the changing needs of the industry and feedback from our customers, our students.

Our most successful experiment has been the introduction of hands on labs to all of our classes. These are based on three different tools which we’ve arranged to give to all of our students.

QUCS is the easiest to use SPICE-like circuit simulator I’ve ever found. We provide the software and all the lab exercises using this tool.

Mentor’s HyperLynx is the most popular advanced circuit simulator with integrated field solver. Recently Mentor has announced cloud access to HyperLynx. Through special arrangement, we give our students access to their cloud server and the special labs we’ve created that run on this server.

LeCroy’s SI Studio is a powerful signal analyzer which has an S-parameter viewer, a signal synthesizer, a channel simulator and clock recovery algorithms which can implement FFE, DFE and CTLE algorithms. Additional analysis tools can be applied to extract useful parameters from the eye, such as jitter decomposition.

By special arrangement with Mom and Dad at LeCroy, we give a fully licensed version of SI Studio to all students who attend our classes. This software tool has a $5,000 list price.

Starting in 2012, all students to any of our classes will receive:

• A color copy of each slide in the handouts
• An e-book copy of Signal and Power Integrity Simplified viewable on ANY device
• A copy of the QUCS software and the QUCS hands on labs
• Access to the Mentor Graphics cloud server and the HyperLynx hands on labs
• A licensed copy of the LeCroy SI Studio software and the SI Studio hands on labs
• Breakfast, snacks and lunch
• A certificate of completion

Check out the schedule of classes and all the course details on our web site.

The EEWeb, an online web portal for electrical engineers, has been interviewing engineers about who they are, what got them started in engineering and how do they practice what they do.  There are more than 50 different interviews posted and more coming up all the time.

In particular, I recommend you check out these three as some of my favorites:

http://www.eeweb.com/spotlight/interview-with-william-arthur

http://www.eeweb.com/spotlight/interview-with-steve-weir

http://www.eeweb.com/spotlight/interview-with-jessica-barber

And if you are still up for more, check out the interview I gave:

http://www.eeweb.com/spotlight/interview-with-dr.-eric-bogatin

You might also want to browse around on their she EEWeb site and check out the online impedance calculator- just be aware, it’s equation based, not a field solver!

I love my iPad and my iPhone. I am constantly looking for apps that can help me get to a signal integrity answer faster. I found a few and here’s one I use now and then and you might find useful.

While I am a big fan of 2D field solvers, like the simple to use and accurate Polar Instruments SI9000, I am also a big fan of the motto,

“Sometimes an OK answer NOW! is better than a great answer late.”

I don’t always have my computer with me to load up and run SI9000, but I always have my iPhone.

Agilent has created a simple, equation-based calculator for seven different cross sections. The PCBCalc app runs on an iPhone or iPad. It probably runs on other operating systems, but I only use the iOS.

One cool feature I like, in addition to the fact I have an excuse to use my iPhone for signal integrity analysis, is that I can type values for all of the parameters except one and then touch the icon from that parameter and it will back calculate the value of the missing parameter.

I set up a simple differential pair circuit, and asked for what value of line width would give me 100 Ohms diff impedance. I typed in all the parameters except the value for w, then tapped the w button and it gave me back 3.97 mils.

It’s important to keep in mind, this is not a field solver; it is an approximation. You should not use this iPhone app to sign off on a design, but if you want to do a quick estimate, or want to do a sanity check, or if you are on an airplane and want to explore design space a little to get a feel for how big an impact there is on differential impedance from increasing the separation in a differential stripline, this is a pretty cool app every signal integrity engineer should have on their iPhone.

If you have your own favorite iPhone or iPad app for a signal integrity application, drop me a note and I might report on it in my blog.

“Measure and simulate twice, build once,” is the motto of Moss Bay EDA, the distribution arm for software developed at IBM and with other engineers, for engineers. This is also the motto of most high-end OEM companies that have come to recognize the value of simulation and measurement in getting to the correct answer faster.

Moss Bay EDA’s flagship product, EMSAT, is a rule checker that evaluates a board layout file against 16 customizable rules developed exclusively by IBM. These rules check for violations of common EMC guidelines any one of which could contribute to a compliance test failure. “It’s like having an EMC Expert sitting on your shoulder reviewing your board,” Gene Garat, founder of Moss Bay EDA says.

While many EDA companies are talking about providing a cloud computing platform for their tools, Moss Bay EDA is trying a different approach. For those interested in “kicking the tires” before buying their own copy of EMSAT, Moss Bay EDA offers a pay-as-you-go-approach. The software is downloaded and runs on your local computer, but the license resides in the cloud. Periodically the local software checks the license on the cloud and tracks usage.

A fixed fee covers all the automated PCB EMC design review necessary for a single board file. This is a low cost entry path for a small company wanting to move farther up the experience curve and add an EMC design review in their design flow. With their extensive contacts in the EMC industry, Moss Bay EDA also offers recommendations for expert EMC consultants who can not only fix a current board, but also customize EMSAT design rules for all future boards.

A website begun by Mike Violette of Washington Laboratories, Ltd offers some relief for those suffering from information overload. He and his team of “Larry King” like-interviewers, including Coco Bean, talk with the top players in the signal integrity and EMC world. They also cover trade shows and other live events relevant to our industry.

They were at the recent 2011 IEEE EMC Conf in Long Beach in full force, creating 30 short video interviews of many of the key companies showing off new products at the trade show.

If you have time for only one video, you have to watch Coco Beam rapping “Interference on My Mind”.