The Pulse: Field Solver Finesse for Modelling Transmission Lines

When I-Connect007 asked me to contribute to this issue on field solvers, I wondered what more could be added to this subject. But as a supplier and developer of field solvers, Polar is still asked some of the same questions—both by experienced customers who are perhaps exposed to a new scenario and, as is most welcome, by new entrants to the industry.

I will start by saying that all field solvers are accurate; they solve Maxwell’s equations by one or another of the available mathematical methods. When all are fed with the same data, all should generate very similar results, and any differences observed will be orders of magnitude less than the variations in the PCB transmission lines caused by the composite nature of PCB substrate dielectrics and the variations of the plating and etching processes.

However, field solver is a very generic term for a range of tools in this application for predicting the behavior of PCB transmission lines. It is important to remember that some areas of field solvers are not actually field solving. Calculating the loss owing to surface roughness of copper is a prime example of this. The surface nature of plated copper is so complex that full field solving would be impractical, so most commercial “solvers” will overlay the core field-solving function with empirical techniques: Hammerstad, Groisse, Huray, and Cannonball-Huray, to name just a few. These empirical extensions extend the capability of the field solver into modelling parameters that are:

  • Vital to model insertion loss
  • Impractical to field solve given the complexity of the surface profile and the available compute power in the hands of even the best equipped SI engineers

Feeding the solver with the correct dimensions is vitally important, as no tool will give an accurate result if fed with incorrect start parameters. Customers frequently ask whether the transmission line structure height should include the trace thickness in the total height. This is easy to answer if you are working “backward from a microsection” but if you are imagining the finished PCB from a simulation, then it’s less obvious.

Martyn_July_Fig1_cap.jpgMy question in Figure 1 is set as a puzzle as it is one of the most frequently asked items regarding transmission line modelling. The H1 dimension represents the core thickness, and the H2 dimension is that from the top of the core to the foil. You can see H2 is denoting prepreg as the trapezoid is pressed into the Er2 region. By how much? Well, if you work from a microsection, the dimensions will be clear to see. But when modelling with a solver before the board is built, you must make a prediction as to how much the T1 will impact the H2 dimension.

Think about it: If there is a lot of copper on the signal layer, most of the T1 will need to be added to the pressed thickness of the prepreg; if the routing density is low, then far less. This is where a stackup tool comes in handy, as a good stackup tool will virtually press the prepregs and consider the Cu density on the signal layer to calculate the optimum value for H2 to feed into the solver. This is the point I was stressing earlier: Commercial PCB transmission line field solvers must possess a variety of tools and capabilities over and above the core solver engine to feed it with good mechanical data to solve.

Martyn_July_Fig2_cap.jpg
As you look at Figure 2, consider the question posed by Figure 1. By using Speedstack to pre-process the material data, the pressed height of H2 and the impact of T1 have been calculated to feed into the solver engine. The left-hand side of the image shows the raw prepreg thicknesses. The signal trace (shown in blue) has a 5-mil core below and two sheets of 3-mil prepreg above. Another core is placed above the two prepregs at the top extent of the highlighted area. Now look at the structure on the right-hand side of the picture. This is an offset stripline located with the signal on the blue layer on the LHS.

As you would expect, H1 = 5 mil—the lower core thickness—but H2 does not equal 6 mil (3 mil plus 3 mil prepreg). H is calculated as 6.28 mils, the combined thickness of the structure when the two sheets of prepreg are pressed into the copper distributed on the signal layer. Taking care of this type of pre-processing is the key to obtaining accurate predictions from your solver.

Martyn_July_Fig3_cap.jpg
Viewing the stack in Figure 2 as a schematic view with fixed size layers for materials is ideal from a planning perspective, but once the stack is complete and pressed, a proportional view makes it plain to see if you have added incorrect materials in error—in the blink of an eye.

Conclusion
I have used a small sample of the techniques you need to deploy to ensure that a field solver engine is fed with accurate data. Quality material data from the supplier is also key in this process, but the key takeaway is that, when looking at the appropriate field solver for your requirements, you must always remember that all solvers are accurate; it is the pre-processing of the data being fed to the solver which unlocks its full potential. This is true for lossless lines up to around 2GHz on through to ultra high-speed lines where insertion loss needs serious consideration too. You should also take care that measurement data is validated, but that is the topic of another article—already partially covered in my April 2022 column in Design007 Magazine“Using Touchstone Files to Build Measurement Confidence.”

This column originally appeared in the July 2022 issue of Design007 Magazine.

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2022

The Pulse: Field Solver Finesse for Modelling Transmission Lines

07-28-2022

When I-Connect007 asked me to contribute for this issue on field solvers, I wondered what more could be added to this extensively discussed subject, but as a supplier and developer of field solvers, Polar still gets asked the same questions both by experienced customers who are perhaps exposed to a new scenario and, as is most welcome, by new entrants to the industry.

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The Pulse: Using Touchstone Files to Build Measurement Confidence

04-21-2022

Measuring PCB insertion loss can be time consuming, and the probes and cables tend to be significantly more costly (and delicate) than those used for characteristic impedance measurement. Nonetheless, given the high capital investment required for test systems, cables, and probes—and the design of the test vehicles themselves—wouldn’t it be nice if you could have a way of looking at your expected results before you put a test probe to a PCB?

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2021

The Pulse: Fake Fudged Facts—Using Software to Get the Right High-Speed Answer

10-21-2021

In the science of high-speed signalling, the signals obey the laws of physics, so when a design won’t work or meet a specification, no amount of psychological persuasion will smooth the signals path from source to load. Wouldn’t life be different if by speaking nicely—or shouting—at an underperforming circuit that it springs to life.

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The Pulse: PCB Design Education—What ‘They’ Don’t Tell You

08-17-2021

For a new designer entering this space for the first time it can be quite an eye opener (no wordplay intended) to discover just how many different disciplines are involved in turning a good design into a fit for purpose PCB.

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The Pulse: Simulating Stackup and Signal Integrity

04-22-2021

Civil engineer Isambard Kingdom Brunel set a high bar for simulation and modelling—to reduce the number of prototypes and predict the safety margins for structural loads.

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2020

The Pulse: Don’t Ignore DC Trace Resistance

12-16-2020

Time flies! But the laws of physics don’t. Martyn Gaudion focuses on how important it is becoming to take DC trace resistance into account when measuring and specifying thin copper traces.

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The Pulse: Application Notes—Advice for Authors

07-27-2020

Application notes are the key to shedding light on new topics or new products and software tools in an easily digestible form. As both a consumer and an author many application notes, Martyn Gaudion explores various types and how to approach them.

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The Pulse: Communicating Materials From Design to PCB Fabrication

05-12-2020

Designer and fabricator communication—especially for high-speed PCBs—should be a bidirectional “thing.” It is so easy for a designer to say, “Just build this,” and hand over a challenging design to a fabricator who could have performed better with some preliminary conversation or dialog before placing the order. Martyn Gaudion explores communicating materials from PCB design to fabrication.

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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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