The Pulse: Using Touchstone Files to Build Measurement Confidence

Measuring PCB insertion loss can be time consuming(when compared with impedance testing), 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?

I am not talking about simply simulating the result, rather recreating the raw files that can be used to drive the insertion loss test software and feeding the test software with data “as if” it was idealised measurement data.

Wouldn’t it be handy if you could prepare simulated raw files “as if” from a VNA to feed into your de-embedding software to ensure that it processes and the results from the test software are what you expect?

If you have followed this column for some time, you will recall the advice on measurement I have shared before, courtesy of Eric Bogatin:

“To make a good RF measurement, you need some engineering intuition to get a gut feel for what the measurement should be, and you also should model the expected results with a field solver before making the measurement itself.”

My column proposes an extra step made possible using Touchstone files.

Touchstone files are an industry standard ASCII file used across the EDA community for the recording and sharing of network parameter data, such as s-parameters (and more). They are the common output of vector network analysers, and of simulation tools. Originated by EEs in 1984, the file format has lived through a variety of owners and is now under the umbrella of Keysight Technologies. The files themselves have become a de-facto industry standard with their simplicity and lack of variation leading to very widespread usage.

Most fabricators are familiar with impedance measurement and can possibly make impedance measurements with their hands behind their back and their eyes closed. For insertion loss measurements, the list of who can do that is arguably much smaller. One of the reasons is the process of modeling and measurement is inherently more complex, even with attendant software to help, so a non-SI specialist in the fab industry needs all the confidence tools available to have trust in a measurement.

Let’s look at the process difference from modeled to measured. First for impedance:

  1. Model expected Zo result by entering trace geometry and base material characteristics with a field solver.
  2. Create suitable test vehicle, typically a four- to six-inch coupon with via connection at each end. The via characteristics are not super critical.
  3. Test the coupon trace with a suitable TDR or impedance test system.
  4. Compare 1 with 3. Provided you made what you thought you made, you will have a reasonable chance of good correlation.

Now to the process for insertion loss. The principle is the same but because of the much higher (and lower) frequency content there are some extra steps which can lead to disappointment if you don’t plan.

  1. Model the expected insertion loss by entering trace geometry, coupon length, and add in loss tangent and appropriate surface roughness to the characteristics.
  2. Create a suitable test vehicle, typically a pair of differential traces with identical cross-section but one pair shorter than the other; five-inch and 10-inch traces are not uncommon. Most ultra-high-speed architectures are differential, so a pair of differential traces is required. The vias and landing pads need to be specifically designed to an OEM specification or to the advice of the probe manufacturer you choose. Via design is much more critical than with impedance.
  3. Measure the s-parameters of the two traces over the desired frequency band.
  4. Post process the s-parameter pair data. Most methods require the test data to be processed by mathematically removing any residual via effects by looking at the difference between the long and short line. This is a complex process: One algorithm for this is Delta-L 4.0, but many other methods exist.
  5. The post processing software will give you the dB loss per unit length of the trace under test which you can compare with my first point.

Gaudion_April_Fig1_cap.jpg
What Could Go Wrong?
Quite a lot actually. The VNA operator needs to ensure that the VNA is calibrated to the particular cable setup (and possibly the probe as well). Though Delta-L 4.0 does a pretty good job of removing probe effects, there is still a multi-step process to initially calibrate the VNA before measurement starts with a given setup. The probes deployed are precision RF measurement probes, and either need placing with a probe station, or need to have the coupon design tooled with location holes to ensure the operator is not at risk of misaligning. Also, the differential probes need to be connected to both ends of the short line and gather one set of s-parameters and both ends of the long line. Contrast this with a differential impedance measurement which requires a precalibrated TDR and one differential probe. There are far fewer steps.

I often get asked why a measurement isn’t as expected; without being physically present, even via a weblink, that’s a tough call. What can be done to rule out as many things as possible and build confidence in the complete system? Interestingly, as soon as the user is confident that the system is okay then good measurements become simpler to achieve.

This is where the Touchstone files come in handy. Simply by exporting a pair of 4 port .s4p Touchstone files for the coupon under test from the modeling software, these two files can be fed to the long line/short line post-processing software. With Polar Atlas you can set the input to the Delta-L to be Touchstone rather than the VNA. This simple step allows you to send idealised results for the two coupons direct into the test software. Post-process the results and verify that with “ideal” data the long line/short line post processing works as expected. Once that is proven, the operator can go back, recalibrate the VNA cable setup, and make the measurement again with increased confidence of a quality result.

Gaudion_April_Fig2_cap.jpg
What kind of situations can this help with? A first time or inexperienced VNA user can be faced with a variety of unfamiliar situations during insertion loss measurement: The VNA needs to be correctly warmed up and stabilised, a calibration procedure needs to be run once the cables are connected with the appropriate artefacts, and care must be taken for all the ports to be connected in the correct order to the correct artefact. With practice, “muscle memory” makes this process easy but first-time users need to take care, and it’s best to do this in a quiet situation without interruption.

Connectors all need to be torqued to the specified level, and whilst this is recommended on TDR measurements, in practice you can get away with manual connections on a TDR. With a VNA, the need to torque connectors is far more critical. Simple errors, like connecting the VNA probes to the short line when you should be probing the long line, are easy to fall into. The ability to dry run the measurement with accurate simulated s-parameters removes a level of doubt and raises an operator’s confidence that good measurements are possible and, indeed, with practice are repeatable and normal.

In conclusion, the ability to insert trusted s-parameter transmission line data directly into the insertion loss processing software in place of raw measurement data raises operator trust in the system when using with “real” data from a VNA.

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

See this additional content from Polar:

Back

2022

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?

View Story
Back

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.

View Story

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.

View Story

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.

View Story
Back

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.

View Story

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.

View Story

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.

View Story
Back

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.

View Story
Back

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.

View Story
Back

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.

View Story
Back

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.

View Story
Back

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.

View Story

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.

View Story
Back

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...

View Story
Back

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.

View Story

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.

View Story
Back

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.

View Story
Back

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.

View Story

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.

View Story

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.

View Story

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?”

View Story

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

View Story

Regional Differences – a Voyage of Glass Reinforcement

01-13-2011

Why bulk Er is not the same as local Er

View Story
Back

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.

View Story

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.

View Story

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.

View Story

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.

View Story

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…

View Story

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.

View Story
Copyright © 2022 I-Connect007. All rights reserved.