What I Learned this Month

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.

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.

I presented a paper at this year’s DesignCon on the topic of causal models for materials. Whenever you go from a frequency domain description of a function into the time domain, you have to worry about causality.

Causality means a response can’t happen before the stimulus. This is the essence of cause and effect. The problem is that sine waves, and arbitrary combinations of sine waves in the time domain, are not usually causal. They extend back to minus infinity in time.

To make combinations of sine waves appear causal and be useful to describe time domain responses of real systems, we need to conspire the real and imaginary parts of the frequency domain response so all the waves at t < 0 cancel out leaving just stuff that happens after t = 0.

Any dielectric properties, described by the real part of the dielectric constant, Dk, and the ratio of the imaginary to the real, Df, must be causal, and so must have this relationship between their real and imaginary parts. This is described by the Kramers-Kronig relationship.

This means that “to know the real part of the dielectric constant is to know the imaginary part.” If we make the assumption that the real part decreases with the log of the frequency, then we can build a simple analytical expression for the dissipation factor Df.

We find that if the Dk drops with the log of the frequency, the Df will be roughly proportional to the slope of Dk and log F. This means the higher the dissipation factor the more dispersion.

It’s the dispersion that causes increased rise time degradation, above and beyond what we expect from just the losses. If your simulator does not include this frequency dependent dielectric constant, you will under estimate the rise time and actual performance may be worse than you predict.

If you care about high speed serial links and use FR4 like materials, you should care about frequency dependent material properties. In which case, here are three places for more information:

The webinar I am doing on Feb 25, 2010, will review this topic in great detail, and in a way that anyone can understand.

You can download a copy of the paper I wrote with my partners which was presented at DesignCon.

You can download a copy of the slides from this DesignCon presentation.

I am turning the DesignCon presentation into a video lecture. This will be posted on the web site in the next few weeks. Stay tuned!