What I Learned this Month

My latest feature article posted on the Test and Measurement World web site and the EDN Online web site, offers an example of how to characterize a high density interface.

In this  project, I worked with Gordon Vinther at Ardent Concepts. They have a pretty cool interface, the Omniprobe-R, that enables contacting an array of micro coax cables to any footprint, like a BGA.

This can be used to either test an active BGA in a load board test application or when it is attached to a product board.

Their interposer technology can also interface between an array of coax cables and a high density array of pads on a circuit board. This sort of technology is essential when testing motherboards with high speed serial links.

In our paper, we looked at the high speed performance of two cable interfaces connected together with their compliant interposer. The 4-port measurements were done to 20 GHz using a Teledyne LeCroy SPARQ. We walk through these measurements and show how to interpret the results and display them in a way to get immediately useful information.

The punch line is that the Ardent interposer is pretty darn transparent. Check out the feature article for the details.

If you want to learn more about interpreting S-parameter measurements, you’ll want to check out the next S-Parameters for SI 2-day class we have coming up in late Feb 2013. Hope to see you there!

There are two clear trends in all high speed interfaces, such as PCIe, SAS, Infiniband and even DDR memory: data rates are currently in the gigabit regime and there is a roadmap that requires the current data rates to operate at even higher data rates in the next generation.

As we all know, as data rate increases, signal integrity effects get worse and luck goes down.

If you want to get some insight into the design strategies and tactics to manage the transition to the next higher data rate, you’ll want to watch this webinar I moderated in late Aug, 2012. This topic is one of the themes in our new class: Advanced Gigabit Channel Design (AGCD). For more information, check out our web site.

In my role as contributing editor for EDN, in this webinar, I moderated a panel of three industry experts: Jim Nadolny of Samtec, Brad Griffin of Cadence and Allen Tung of NXP. I posed them three questions about the problems, strategies and tactics of higher data rate system design and we discussed the answers.

There were a number of key points that came out. Here is a brief teaser.

Brad Griffin: for shorter time to the correct design, it’s critical to integrate analysis as part of the layout and design flow. This includes not just reflections, cross talk and losses, but also “power aware” analysis.

Allen Tung: USB 3.0 operating at 5 Gbps will see significant eye closure due to the losses in the boards and cables in typical applications. One way of opening eyes and minimizing the impact from the interconnects is adding a re-driver or repeater chip in the signal path, typically placed at the edge of the board. This does not require any changes to the rest of the circuit.

Jim Nadolny: As a good rule of thumb, the eye will probably be sufficiently open with no equalization if the insertion loss at the Nyquist is no more than –7 dB. There should be about 20 dB SNR so the acceptable cross talk should be less than about –25 to –30 dB in this case. Using pre-emphasis only, you can recover an acceptable eye with about –12 dB insertion loss at the Nyquist. And with FIR, CTLE and DFE, you can recover an acceptable eye with about –25 dB insertion loss at the Nyquist.

You can read a longer review of this webinar by Richard Goering, posted here. And, you can view the entire webinar, recorded and posted here.

If you have suggestions for future webinars, drop me a line!

In the Dec 1993 issue of Surface Mount Technology Magazine, I published an invited article on “Packaging to the Year 2000.” A few years ago, SMT Magazine was taken over by Iconnect007 and is available online.

But my article was published before the days of the internet and was never posted to the web. I just posted a pdf copy on my web site, where you can download a copy.

In a past life, I specialized in semiconductor packaging technology, working in printed circuit fabrication processes, monitoring and control of board fab process, multi chip module technology development and BGA implementation. And I wrote a number of books on interconnect technology.

I wrote this article to paint a picture of where I thought packaging technology would end up in the 21st century, about 7 years out at the time.

I came across it recently when cleaning out some files and was surprised at how timely many of my comments were. Here is a sampling of my comments and predictions, which I think apply just as well today, twenty years later.

“There will be no universal packaging solutions for every customer. Rather the industry will be comprised of niche markets.”

“With such technologies on hand, many of the products we use today- day-timers, maps newspapers, TVs, libraries, video stores- will also become extinct.”

“As dice become peripheral-pad-limited, companies other than IBM will be inclined towards the adoptions of area array pads and flip chip assembly processes to support single and multi chip packages.”

“What is needed is cost-effective evolution of current technologies… Execution rather than creation will be the guiding principle to the year 2000.”

“… two principles will be required to survive:

• to be successful, vendors must understand the specific constraints of each customer and understand what is of value. There will be a different answer with each customer.
• The corollary is to be careful when projecting one customer’s reasons for adopting a technology on another’s”

“To understand what is important and worth paying for, is also to understand what is not important and not worth paying for.”

For the entire article, and many others, visit www.beTheSignal.com, and check out the Signal Integrity Library.

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.

An example of a ground loop which causes voltage noise between two instruments due to magnetic field pick up. Courtesy of P. M. Bellan, inventor of the Loop Slooth.

If you have a pesky 60 Hz hum in your low noise electronics or some high frequency noise on a signal line which appears when a motor near by turns off or on, you may have a ground loop problem. A simple device from Paul Bellan, called the Loop Slooth, may help you find and eliminate the ground loops in your product.

Prof. Bellan is Professor of Applied Physics specializing in plasma physics at Caltech by day (and many nights), and moonlights as the inventor of the Loop Slooth. He was often faced with the problem of spurious noise on very low level signals from plasma diagnostic sensors close to very noisy plasma chambers. The problem was more often than not ground loops connecting instruments and sensors which picked up the large transient magnetic fields in the lab which turned them into induced voltages.

He developed a simple device to “tone out” which cables were part of the loop which was usually enough information need to fix the problem. When he realized other researchers probably had similar ground loop problems and could benefit from his sniffer, he turned it into the Loop Slooth.

It has two units. The “exciter” generates a 100 kHz “tone” on part of the ground loop using an induction coil that clamps around an identified ground line. This works on the same principle as a current probe, in reverse. The AC current in the exciter coil generates a changing magnetic field around the cable that’s inside the coil, which induces a current in the cable- the ground line and the other wires.

The second unit is the detector with a sharply tuned circuit for 100 kHz. The detector uses as a Rogowski coil which is a clever way of making a toroid with a gap so that suspect cables can be slipped right into the toroid.

The Rogowski coil, shown in the figure from Wikipedia, is an open toroid with both ends of the coil coming out the same terminal. The voltage between the ends of the coil is very sensitive to the changing flux through the inside of the toroid which is due to the current passing through the center of the toroid.

The detector box has a meter and an LED that lights when it picks up significant 100 kHz tone. This indicates that one of the cables that’s been slipped inside the detector coil is part of the ground loop. With the clamp on the exciter and the open toroid, Bellan says, a cabling system can be debugged as it stands, with no changes or disconnections necessary.

While you may be careful in laying out the wiring in your system and think you have minimized ground loops, as soon as you have multiple instruments plugged into multiple power plugs, with cable shields connected together, you have ground loops.

Bellan suggests a good habit is to always plug instruments that will share cable returns, into adjacent plugs, to minimize the loop area of their returns, and to twist the coax cables together to reduce the area through which stray magnetic fields can pass and induce voltages. As a final practice, ferrites around the power cables will increase the impedance of the ground loop and reduce the induced current.

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”.

I attended the 2011 IEEE EMC Symposium in Long Beach and encountered a number of exciting companies, people and products. The Canadian company EMSCAN is one of these.

Their EMxpert can measure, plot and analyze the current distribution in circuit boards at specific frequencies. It uses an array of high bandwidth magnetic field sensors on a flatbed which picks up the near field magnetic field from a circuit board.  When the board is turned on, return currents in the planes generate local magnetic fields with frequency components based on the signal’s spectrum and a spatial distribution and magnitude based on the current distribution.

When you superimpose on the map of the current distribution, the component outlines on the board you get an immediate visual clue about where currents are actually flowing on the board. Usually the currents on the bottom of the board that can be sensed outside the bottom are due to the power and ground distribution, but may also relate to the aggregate of signal returns. You might use this information to consider adding more capacitors to reduce the local currents and reduce the near field radiated emissions.

The near field external magnetic fields are not always a direct indication of far field radiated emissions, but it offers a new window into what’s going on with a functional board. 

Each magnetic field pick-up-loop sensor can measure the spectral response of the current.  Depending on the specific signal, you will see frequency components at harmonics of the signal.

You can then sit at one frequency and scan across the array to measure the peak amplitude of the magnetic field, at that frequency. This maps the current distribution at that frequency in the board that leaks through to the bottom. 

On my visit to the EMSCAN booth, I recorded this short video demonstration performed by Cedric Caudron, an applications engineer with EMSCAN in which he walks through the process of looking at the spectrum, the current distribution, then adding a capacitor and showing that the current distribution in the planes is reduced by adding the capacitor. It’s pretty darn cool.

Next No Myths Allowed Webinar: Frequency Dependent Material Properties, so what?, Thurs, Feb 25, 2010, 1 pm EST. Free, but you must pre-register here.

Spring Institute of Signal Integrity Classes, April thru May 2010, San Jose, more info and online registration here.

One of my fun activities at DesignCon is getting to conduct 10-minute interviews with signal integrity stars. These are filmed by RealTime with DesignCon and posted on their website.

I did about 10 different interviews and other reporters with RealTime did another 20 or so interviews. If you missed DesignCon, or just want to see some of the new products, materials, tools and design solutions you might have missed, you’ll want to check out the posted interviews.

In particular, here are some of my favorites you’ll want to be sure to view:
Colin Warwick, Agilent, talking about their new 3D display. In addition to looking like a couple of really cool SI Dudes, we were able to see the results from a full wave EM field solver of current flow in a via field. With the LCD shutter glasses, and interleaved left-right screen being displayed on the monitor, it really did look like the vias were standing out in front of the screen. This 3D capability is embedded in Momentum and EMPro., able to show currents, fields and voltages.

 Joel Peiffer, 3M, talking about C-ply. 3M can provide thinner than 25 micron thick C-Ply laminates, sandwiched between copper planes. The dielectric is ground up Barium Titanate filled epoxy offering a Dk of 16-20. This is great for power and ground planes. The breakdown strength of the I mil core is more than 100 v. While Joel said these materials are in production and he has customers shipping their product with C-Ply, I could not get him to reveal any customer names. He just hinted that the early adopters are cell phone manufactures, and 10-15% of all cell phones are shipped with C-Ply.

Don DeGroot, CCN-I, talking about pcb materials measurements in his company. Don came from NIST, where he worked for 12 years as a researcher in the rf test group. He’s spun his experience in precision measurements into a company that performs contract materials measurements. He talked about the transitioning of NIST engineering techniques into a commercial business and how he does practical materials characterization.

Todd Westerhoff, SiSoft, talking about what’s new from SiSoft. At DesignCon 2010, SiSoft engineers gave 3 papers. One was on “When Shorter isn’t Better.” Todd described some problems where reflections in short tracks can cause problems, especially with resonances at specific lengths. If the traces are long enough some of these resonances may damp out and not be a problem. The danger, he says, in applying design rules is you may miss these length specific problems. This is why he advocates doing a post layout analysis.

To catch all of the RealTime interviews, check out their web site.

DesignCon 2010 is right around the corner. It will be a busy time for all. As you set your schedule for the fours days of the show, be sure to add these events to your list:

Visit beTheSignal.com at booth #319. You’ll want to pick up a mug, an Appendix A -pocket guide to signal integrity design guidelines and, of course, some chocolate! Stop by and meet Susan and Laura. And I may have copies of my science fiction book, Shadow Engineer, on sale.

Monday, Feb 1, in the Theatre, I will present a 3 hour education forum, “Practical Magic: Signal Integrity Problems Disappear with the Right Tools“. My Agilent buddies and I will be showing about a dozen demos of some really cool hardware and software tools that I think should be in every signal integrity engineer’s tool box. check out my youtube video!

Tues, 8:30 am, I will present a paper “Frequency Dependent Material Properties: So What?”, with Don DeGroot, Sanjeev Gupta and Colin Warwick. If you are wondering about all the hype associated with “causal material properties” and want to know how does this apply to me, you’ll want to check out this talk.

Tues, 9:20, my colleague, Paul Huray, will present, “Impact of Copper Surface Texture on Loss, a Model that Works.” There’s a lot of buzz these days about rms roughness of copper. Come hear Prof Huray explain it’s really the surface texture of the copper, not just the rms roughness, that affects the extra losses from rough copper. You may find, as I discovered, that “everything you know about current and signal propagation is wrong.” Come hear the right way of thinking about how signals really propagate on interconnects.

Tues, 3:45 pm, I will participate on a panel discussion, “Science Fiction…is it really fiction?” This has got to be one of the more fun events at DesignCon, at least for me. I get to share the panelist table with Gentry Lee, famed co-author with Aurthur C. Clark of the Rama series, among other books, and noted scientist at JPL. We will talk a little about our visions of the future and open up the floor to discussion. Rumor has it, some of us might have books available for a book signing!

Wed 8:30 am. If you missed my education forum on Monday afternoon, you can catch it again on Wed morning.

I’m exhausted already, just writing about the exciting happenings at DesignCon 2010. See you there!

I recently had a chance to don a pair of LCD shutter glasses and stare into a synthesized 3D image that popped out of the screen at me. Cascading colors flowed around obstacles. I could move my head around and see how the pattern of colors moved in and around the objects in the foreground.

No, this wasn’t a scene I witnessed in Avatar, which I also viewed in 3D IMAX, it was a demo of Agilent’s new 3D glasses incorporated in an upgrade to their popular Momentum field solver suite. I had a chance to sample the new 3D vision system at the FPGA Camp in San Jose on Nov 11, 2009. Wow! Pretty darn cool!

Tightly coupled into the Agilent’s ADS simulation environment are Momentum, which does 2.5 D full wave simulations and EMPro which does 3D full wave simulation. While both of these tools can show 3D perspectives of the static or dynamic, electric or magnetic fields or currents in and around conductors, the simulations seem to come alive when viewed in true 3D.

To make this possible, Agilent has teamed with Nvidia to leverage their high end GPUs for the visual processing. The 3D images are generated by projecting on the monitor an image for just the right eye, while synchronized with the opened right eye shutter on the LCD glasses, and then projecting the image for the left eye.

The frame rate is high enough so that you don’t perceive the flicker, but see the screen in true 3D, giving the sense of having the object, and its field distribution, projecting in front of the screen. I suppose the next step is to incorporate a 3D mouse pointer and be able to move it around to interact with the 3D environment.

If you want to learn more about this novel imaging feature, be sure to check out the webinar Agilent is providing on Jan 21, 2009. You can sign up at this link.

I can’t wait to find the right demos to use for my upcoming live classes. One of these days soon, if I hand you a pair of LCD glasses when you walk into the room, you’ll know what to expect.