The Pulse: Vias, Modeling, and Signal Integrity

Editor Andy Shaughnessy recently sprung a request on me: Would I like to write an item on vias? A complex and tricky subject, I thought. But after some reflection it seemed like a good idea…so, here goes.

I was just about to leave for PCB West in September, and my connecting flight developed a fault. Always better when the plane breaks on the ground than in the air, but inconvenient nevertheless. If you want to be certain to catch your long-haul flight, says the ground agent, you must go via London Gatwick.

I’ll cut a long story short, but I had 10 minutes to decide. Should I, on the one hand, get to London Heathrow via London Gatwick, or take my original connection which maybe will, maybe won’t, get me to Heathrow in time for my connection? I bit the bullet, had my bags offloaded and started the process of looping through security and booking a new flight on an alternate airline only 45 minutes before departure whilst at the same time trying to book an overnight hotel whilst going through security to the new flight.

In the process, I became one of those annoying people who is on the phone at the gate and up the jetway. I did try to book online but that’s a mare too when you are in a rush. Still, I got the last room available close to the desired terminal.

What’s all that got to do signal integrity? Well, it’s a lot easier to go somewhere by a direct route than via somewhere else. The same is true for signals. The best a designer can do is to ensure the via offers the least inconvenience to the signal; otherwise it’s going to end up at its destination a bit grumpy and battered out of shape, just like I was when I boarded the aircraft for the long-haul flight the following morning. Though a few glasses of shiraz later, my mental state had equalised. But signals aren’t always so lucky.

Before delving down into the electrical via, I would like you to have a think about the following scenario on a transmission line. Bear with me: Understanding the following will help make more sense of the via scenario. The second thing I would like you to bear in mind simultaneously is that good modeling can’t fix a bad design. The model can tell you where a design is weak, but if you have committed your design to product, the model can only tell you how it behaves. Some less experienced designers seem to fall into the trap of thinking a better model will fix something that doesn’t work; it won’t. It will only reassure you that the design was bad in the first place.

To read this entire article, which appeared in the November 2016 issue, click here.

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2016

The Pulse: Vias, Modeling, and Signal Integrity

12-05-2016

Remember that good modeling can’t fix a bad design. The model can tell you where a design is weak, but if you have committed your design to product, the model can only tell you how it behaves. Some less experienced designers seem to think a better model will fix something that doesn’t work; it won’t. It will only reassure you that the design was bad in the first place.

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2015

Impedance Control, Revisited

06-10-2015

The positives for new fabricators and designers lie in the fact that, even though impedance control may be new to them, there is a wealth of information available. Some of this information is common sense and some is a little counterintuitive. So, this month I’d like to go back to the fundamentals, and even if you are an experienced hand at the subject, it can be worth revisiting the basics from time to time.

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I3: Incident, Instantaneous, Impedance

03-11-2015

In my December 2013 column, I discussed “rooting out the root cause” and how sometimes, the real root cause is hidden when digging for the solution to a problem. In that column, I described how sometimes in an attempt to better correlate measured impedance with modelled impedance, fabricators were tempted to “goal seek” the dielectric constant to reduce the gap between predicted and measured impedance.

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The Road Ahead: 2015 and Beyond

02-12-2015

Editor Andy Shaughnessy kindly offered me the chance to write a column looking ahead at 2015. Predicting the future—that’s a tall order. But as I write this in mid-December, the news feeds trumpet that the USA is opening a new chapter in ties with Cuba, and that the price of oil is at $55 and headed south. Really? Did anyone see that coming? What happened to “peak oil?”

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2014

Tolerant of Tolerance?

03-30-2014

Wouldn’t life be great if everything fit together perfectly? There would be no need for tolerance. However, for that to be the case, everything would need to be ideal and without variation...

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2013

Rooting Out the Root Cause

08-31-2013

When your measured trace impedance is significantly different from the calculated/modeled trace impedance, be careful before jumping to conclusions.

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The Pulse: Changing, Yet Changeless

01-16-2013

Like the whack-a-mole game where the moles keep popping up at random after being knocked back into their holes, the same old questions about technical hurdles surrounding signal integrity continue to surface as technology advances.

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2012

Repeatability, Reproducibility and Rising Frequency: The R3 Predicament

08-29-2012

One of the more popular editions of The Pulse in 2011 was the article "Transmission Lines - a Voyage From DC." Starting again from DC and working through the frequency bands, Martyn Gaudion looks at what is realistic to achieve and where economic compromises may need to be made.

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2011

Transmission Lines – a Voyage From Dc – No, Not Washington ...Part 2

08-01-2011

In the second part of this two-part article we continue on our voyage through a transmission line from DC onwards and upwards through the frequency spectrum, step by step exploring the characteristics from very low to ultra high frequencies.

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Transmission Lines – a Voyage From DC – No, Not Washington, Part 1

07-01-2011

In this two-part article I'd like to join you on a voyage through a transmission line from DC onwards and upwards through the frequency spectrum. In Part 1 we trace the impedance from infinity at DC to the GHz region where it reaches the steady state value of its characteristic impedance.

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Crosshatching Compromise

06-16-2011

Sometimes engineering results in some uncomfortable compromises; this is often the case with PCBs as the mathematical methods used by the modelling tools are based on "ideal" physical properties of materials rather than the actual physical materials in use.

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Correlation, Communication, Calibration

05-31-2011

At ElectroTest Expo at Bletchley Park, UK, Martyn Gaudion noticed the extent to which some technologies change, while the overall concepts do not. Prospective customers still ask exactly the same questions as they did 50 years ago: “What’s the bandwidth? Will it work in my application? How accurate?” Followed by the predictable, “How much does it cost?”

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When Is a 10ghz Transmission Line Not a 10ghz Transmission Line?

03-13-2011

'Just as in life, in electronics the only certainty is uncertainty.' -- John Allen Paulos

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Regional Differences – a Voyage of Glass Reinforcement

01-13-2011

Why bulk Er is not the same as local Er

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2010

The Pulse: Laminates Losses and Line Length, Part II

12-20-2010

In the last edition of "The Pulse," we began a discussion on how a modern field solver can help choose the most cost-effective material for a high-frequency application. Last month we looked briefly at the effects of line length and dielectric losses and this month we focus on copper losses; all three are primary drivers for losses.

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The Pulse: Laminates Losses and Line Length, Part I

12-01-2010

The EE creating the "platform spec" and the PCB fabricator responsible for its realisation face an array of materials with a mix of choices: From ease of processing to reliability requirements and signal integrity. For then next two months, "The Pulse" will focus on signal integrity, describing how to use field solvers to select the best materials when trading cost versus SI performance.

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Signal Integrity – the ‘S’ Words

10-01-2010

Three words, or rather, phrases are in the process of entering the vernacular of the PCB industry, albeit one phrase is already familiar, but taking on a different meaning. All start with S and all relate in one way or another to signal integrity.

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All Set to Measure Differential Insertion Loss?

09-13-2010

This column discusses the gradual adaptation necessary for PCB fabricators as more and more silicon families drive the industry toward the requirement for in house measurement of insertion loss.

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Zen and the Art of Accurate Impedance Measurement* – With Apologies to Prisi

08-12-2010

In his 1974 philosophical novel "Zen and the art of Motorcycle maintenance” Robert M. Prisig contrasts his regular and ongoing daily approach to motorcycle maintenance with his friend's alternate view of leaving well alone between annual service center based maintenance. What has this got to do with accurate impedance measurement you may ask? Please read on to discover more…

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New Column: The Pulse

07-14-2010

Polar Instruments CEO Martyn Gaudion will be exploring a number of themes. A major SI topic that is set to grow is the emergence of new silicon families designed to push traditional materials into the multi-gigahertz arena. These new chipsets lift transmission speeds up to a point where signal losses rather than reflections become the predominant concern from an SI perspective.

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