The Pulse: Modelled, Measured, Mindful—Closing the SI Loop

“Which is correct—the modelled or the measured result?” A colleague posed this question to Dr. Eric Bogatin at a Polar conference many years ago. To my colleague’s evident surprise, Dr. Bogatin replied, “Neither.” Read on to find out more.

To a mechanical engineer used to laser precision in mechanical measurements, the world of high-speed electronics can seem somewhat alien. Likewise, to an electronics engineer who has inhabited a low-frequency world in a previous life and is suddenly exposed to high-speed digital, the world of ultra-high-speed serial communications can seem uncomfortably imprecise. DC voltages can be measured to many significant digits with a high degree of precision. Mechanical dimensioning in the laser age seems, and is, incredibly precise. But the world of high-speed digital signalling is less about absolute ones and zeros and more about massaging pulse shapes to squeeze them at the highest possible data rate down channels determined to squash and erode their carefully shaped waveforms.

In this woolly world where high-speed signals enter a transmission line with a well-defined shape and emerge at the receiving end eroded and distorted—and at the limits of interpretation by the receiver—it is well worth running simulation to look at the various levers that can be figuratively pulled to help the pulse arrive in a reasonable shape. At speeds up to 2 or 3 GHz, it usually suffices to ensure the transmission line impedance matches the driver and receiver. And a field solver makes light work of the calculation—a little juggling with line width and dielectric substrate height will have your signals arriving in good shape. But push the frequency higher, and other factors come into play. At this point, it makes good sense to run multiple simulations and ultimately test against measurements.

Whilst on the subject of loss tangent, for many PCB fabricators, it is one of those “mystery” characteristics that isn’t easy to visualise or measure. The simplest way of thinking of loss tangent is to look at it as the ability (albeit undesired) to turn precious RF energy into heat. It’s excellent if you are designing microwave ovens, but not so helpful if you are attempting to transmit small amplitude high-speed signals from point A to B along a PCB transmission line. Because it is a tricky thing to measure, it’s no surprise that there are a variety of measurement methods and some more appropriate to some applications than others.

Split post resonator methods, for example, are ideal for bulk measurement of loss tangent when manufacturing base materials. When choosing a value of loss tangent for use in signal integrity applications, you will get best results if you use a value derived by using transmission line techniques. The loss tangent in a data sheet may have been measured in a variety of ways depending on application and frequency of measurement (most data sheets note this), but if in doubt, ask.

To read this entire column, which appeared in the June 2019 issue of Design007 Magazine, click here.

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2019

The Pulse: Modelled, Measured, Mindful—Closing the SI Loop

07-18-2019

In this woolly world where high-speed signals enter a transmission line with a well-defined shape and emerge at the receiving end eroded and distorted—and at the limits of interpretation by the receiver—it is well worth running simulation to look at the various levers that can be figuratively pulled to help the pulse arrive in a reasonable shape. At speeds up to 2 or 3 GHz, it usually suffices to ensure the transmission line impedance matches the driver and receiver. And a field solver makes light work of the calculation. But push the frequency higher, and other factors come into play.

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2018

The Pulse: The Rough Road to Revelation

03-07-2018

Several years ago, an unsuspecting French yachtsman moored his yacht to the railings of the local harbour. For a very nervous full tide cycle, he awaited to see if the cleats would pull out of the glass fiber hull. Fortunately, the glass held. A yachtsman at high tide isn’t too worried about whether the seabed is rough or smooth, but at low tide, the concern about a sandy or rocky seabed is altogether different. With PCBs, the move to low-loss laminates exposes a similar situation.

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2017

The Pulse: Tangential Thoughts--Loss Tangent Values

12-06-2017

Numbers are fascinating things, and the way they are presented can influence our thinking far more than we would like to admit, with $15.99 seeming like a much better deal than $16. Likewise, a salary of $60,000 sounds better than one of $0.061 million, even though the latter is a larger number. Our brain has been programmed to suppress the importance of numbers to the right of the decimal point. Such is the case with the loss tangent of materials. It is a tiny number and so to our minds looks insignificant, but it has a directly proportional effect on the energy loss suffered by a dielectric.

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2016

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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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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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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