“Bogatin’s Practical Guide to Transmission Line Design and Characterization for Signal Integrity Applications” – A Review

Everything you ever wanted to know about PCB transmission lines – and more – in a digestible format with just the right amount of math to back up the illuminating practical illustrations.


Ed.: Martyn Gaudion is managing director of Polar Instruments Ltd. He began his career at Tektronix in test engineering on high-bandwidth portable oscilloscopes. He joined Polar in 1990, where he was responsible for the design and development of the Toneohm 950, Polar’s multilayer PCB short circuit locator. He became CEO in 2010. He also develops tailored content for the Polar YouTube channel. He reviewed this book for PCD&F.

Hot off the virtual press – a copy of Dr. Eric Bogatin’s new guide to transmission line design appeared in my Artech eBook account.

Do we really need another transmission line book? That’s what Dr. Bogatin asks right at the outset. After reading this new tome from virtual cover to cover, yes we do. This is a thoroughly practical book an peppered with links to Bogatins’s brief informative video explanations which expand and add dynamic content in a way that printed matter alone cannot.

Whether you are a recent graduate who wants a more practical insight to the behavior of transmission lines after doing all the hard work of the pure math side of study, or an experienced electrical engineer moving into the high speed arena – or even a PCB technologist or fabricator wanting an insight into all the mysterious terminology that surrounds the subject – this is a resource book for you. It is equally valuable whether you are dipping into chapters of specific interest, or taking a deep breath and reading from (virtual) cover to cover.

In my day job I spend most of my time helping customers who are new to transmission lines ensure that they document and design them correctly for fabrication, and I confess over the years much is taken as given. By reading Bogatin’s new book I have gained insight into transmission line behavior that is very familiar but I didn’t know the why – and the why makes everything make more sense. 

It is staggering that the electrical behavior of a simple pair of copper traces with a sandwich of dielectric material can generate a book running to 600 pages without loss of interest, but this is exactly what Bogatin does with the subject. Along the way you will find out why you should always think of signal and return paths and not in terms of signal and ground. You will find that while the RF and digital design spaces may run at similar frequencies, the design considerations for both are poles apart. (No pun intended.) You will also discover that simulators and field solvers don’t design circuits – you do – and you best have an idea of what you intend to happen and the expected outcome before reaching for the simulator. Words are important, and Bogatin stresses that though digital and RF and EMC specialists all deal with high-speed signals – and a lot of the jargon is similar – there are often situations where technical terms overlap while their meanings don’t. Bogatin takes an important stance in defining and understanding the terminology to ensure you are understood when working across disciplines.

On measurement – there are many precision tools for measuring high speed signals and time and frequency domain information, all with accuracy beyond your dreams – but as with simulation – Bogatin cautions that unless you understand what you are measuring and how to design your test vehicle, any or all of that expensive equipment can lead you to the wrong answer. Time spent in the measurement section of the book is well invested and will enable you to extract the best possible measurements from whatever TDR/sampling oscilloscope/vector network analyzer you have to hand.

I personally like the examples where Bogatin mixes electronic timescales in nanoseconds with human relatable timescales (days) to bring tangible meaning to his explanations. I also like his informative section on why intuition in the frequency domain does not translate easily (at all?) to the time domain, and that while both are valid and useful you need to work with a degree of selective schizophrenia while working in these domains.

Last but not least, alongside the video links and examples are links to both evaluation versions of commercial tools and useful no cost utilities so you can run the simulations and experiment for yourself.

Martyn Gaudion, June 2020

Bogatin’s Practical Guide to Transmission Line Design and Characterization for Signal Integrity Applications

by Dr. Eric Bogatin

Available from Artech House

Post-Covid, Will Connections Be Scarcer?

A friend writes to say he’s been about the effects of “social distancing” policies. Will they have a permanent effect on our lives, he wondered. And will they impact events such as trade shows?

It wasn’t the first time I have been asked that of late. The subject also came up on a couple of podcasts I’ve had the good fortune to do, one with Judy Warner of Altium and the other with Mike Konrad of Aqueous Technologies.

My short answer is, I think there will be an impact, but it will swing toward more contact, not less. Indeed, after being cooped up for so long, I think people will crave human connections. Moreover, I don’t think it will have an effect on trade shows. In fact, I think this will reveal lots of holes/flaws in inter-/intra-company digital communications, which gives us all something to work on for the next quarantine (heaven forbid). 

We aren’t the only ones contemplating what happens next. The Boston Globe this week published a piece in which several self-styled business futurists and science-fiction writers expect the world will look like next fall/winter.

I can’t say I’m impressed with most of their responses, which if anything feel exaggerated for effect. But see for yourself.

Ralph Morrison, RIP

I don’t have all the details, but the industry icon Ralph Morrison passed away Aug. 2.

Ralph spent more than 50 years in the electronics industry. He was responsible for teaching thousands of engineers all about the fundamentals of how voltage and currents work. Up until about eight years ago, he was a constant presence at the PCB West design conferences.

He wrote several books, including Solving Interference Problems in Electronics, Grounding and Shielding Techniques in Instrumentation, and The Fields of Electronics: Understanding Electronics Using Basic Physics.

I didn’t know Ralph in his prime, but we developed an amiable relationship over the past 18 months as he did some writing for us. I truly enjoyed him — he had an economy with words that I will always strain to achieve.

Last year, he put together a series of articles for PCD&F: Voltage in the GHz World, Limiting Radiation from Logic Circuit Boards, Maxwell and Transmission Lines, and The Layout of Fast Logic on Printed Circuit Boards.

According to longtime friend Dan Beeker, a memorial service will take place in October. I will miss his emails and occasional phone calls, and most of all, his kind words about the role we editors play in helping designers and engineers better understand their craft.

What is a Centroid File?

Call it what you may, but surface mount assembly robots need a magic file to determine where to place your components and how to orient them. We call it a centroid. What is a centroid file and why is it important to your PCB assembler?

Many assemblers use automated equipment to place surface mount components on PCBs. One of the tools we use to rapidly program these machines is the centroid file (aka insertion, pick-and-place or XY file).

Some CAD packages automatically generate this file, some will not. Sometimes you may simply need to modify the file, and some assemblers can make minor changes to the file or create it for you for a small fee.

Ultimately, the centroid file describes the position and orientation of all surface mount components on the PCB. A centroid file includes: the reference designator, X and Y position, rotation and the side of board (top or bottom). Only SMT parts should be listed in the centroid file the basic format for the centroid file is a comma delimited (.csv) file with data in the following order: RefDes, Layer, LocationX, LocationY, Rotation.

Here’s a breakdown of the data:

RefDes
The reference designator that matches your BOM and PCB markation.

Layer
Either the word “top” or “bottom.” This is not necessarily the CAD layer designator. Just “top” for a part located on the top of the board and “bottom” for parts on the bottom side of the board. Top is often referred to as the component side and bottom the solder side by assemblers and fabricators.

Location
The “LocationX” and “LocationY” values describe the part’s offset from the board origin. The location values require that the part origin be centered in the part. The board XY origin of 0,0 is in the lower left corner of the board. The 0,0 origin for the bottom of the board is in the lower left corner, looking at the top of the board, though the board. Preferred units are in inches (0.0000″).

Rotation
Rotation goes counterclockwise for all parts on top and clockwise for parts on the bottom. In both cases, this is from the perspective of looking at the top of the board. For bottom side parts, it is looking through the board, still from the perspective of looking at the top of the board.

LED & Diode Markation Guidelines for PCBs

Have you ever had an LED or other diode placed backwards? PCB assemblers work hard to place every component from the largest, highest pin-count logic chip down to the smallest passive components and micro wafer-scale BGAs correctly every single time. A key element of that accuracy is our understanding of your board and the component markings.

If you use surface mount diodes or LEDs, you probably understand the challenges involved in correctly and consistently indicating diode polarity. LEDs are usually cathode negative, while zeners and uni-directional TVS diodes can be cathode positive. Barrier diodes can be either orientation. It all depends on whether the diode is a rectifier, an LED, a uni-directional TVS, part of a daisy-chain and a host of other considerations.

When you start looking at the CAD libraries, you not only have all the differences from that manufacturer, you may also have different markation schemes from each CAD package developer and from each library builder.

Guidelines for diode polarity mark silk-screening — the diode symbol, “K” for cathode or “A” for anode. To ensure the best accuracy, we recommend extra care in marking diodes to remove any ambiguity.

The preferred method is to place the diode schematic symbol in the silkscreen. You may also place a “K” for cathode adjacent to the cathode. “K” is used because “C” could imply that the spot wants a capacitor. An “A” adjacent to the anode on the board works too, though it’s less common. If you are producing a board without silkscreen, put the mark in the copper layer or submit a clear assembly drawing with the other board files.

Relying on +, – or _ are not definitive in what they indicate and are not recommended. For example, a “+” or “-“ sign isn’t good enough, because it’s not always true that current flows through a diode from the anode to the cathode. For the common barrier diode or rectifier, it’s a pretty safe bet. However, with a zener diode or TVS, it’s not necessarily true. That is why marking a diode on your PCB with the plus sign (+) is not good practice.

Ever Use a TI X2SON Packaged Part?

The name stands for extra small outline no-lead. It’s a newish package from Texas Instruments. In my experience, TI is one of the better companies insofar as testing and documenting manufacturability is concerned. The datasheet for this device is no exception.

The TI part is the five-lead thing above the grain of Jasmine rice, surrounded by a few 01005 ceramic capacitors. I’m selling the capacitors for $500 each. (Just kidding.)

The part is 0.8 x 0.8mm, with the five leads. TI suggests either a 4 mil (0.102mm) trace coming out of the center pad, or a 4 mil via in the pad (the via must be filled and plated at the fabricator ) to escape the center pad. They also do a nice job of detailing out the solder paste stencil layer, as in the following image:

You’ll most likely need a custom CAD footprint for one of these. Either very carefully do it yourself, or go to a solid source like SnapEDA. If they don’t already have it in their library, they’ll make it for you.

These small packages aren’t going away. We’re only going to see more of them. They may seem intimidating, but with a good footprint and a competent manufacturer, they aren’t so bad.

Duane Benson
“A ruler of follows”? That makes no sense.
How about ” a rule of followers”?

http://blog.screamingcircuits.com

Shocker at Sanmina

Jure Sola spent 26 years atop Sanmina as chairman, president and eventually CEO. His replacement lasted less than 12 months.

In a stunning announcement, Sanmina today announced the resignation of Bob Eulau as chief executive. The move is effective immediately.

Eulau was handpicked to replace Sola as chief executive of Sanmina, a move that took effect last October. At the time, Sola gushed over his successor’s abilities, stating “Bob has a deep understanding of Sanmina’s strategy, customer focus, technology offerings and day-to-day execution. I am confident we’ve selected a strong leader. Bob’s wealth of experience and strong leadership are invaluable to the strategic direction of Sanmina and are precisely what Sanmina needs for a successful future.”

What changed in a year? Often, quick changes like this are tied to financial issues or disagreements with the board over direction. Sanmina was quick to reaffirm financial guidance for its current quarter, and is on pace to surpass last fiscal year’s revenue total. At the low end of guidance, the EMS firm will top $7.1 billion, about 3% more than the prior fiscal year. In its most recent earnings call, Eulau forecast increasing margins and yield improvements.

Michael Clarke, another Sanmina alum and a current board member, will take over come Oct. 1.

PCB Chat: Flexible Circuits

We have a new podcast available, this one with Mark Finstad and Nick Koop. Alert readers will recognize them as the authors of our popular “Flexperts” column on flexible circuit design and manufacturing. They discuss real-life flex circuit struggles and how to overcome them in this week’s chat.

Also, listeners who want to learn more about flex circuit design and manufacturing are encouraged to come listen to Mark and Nick on Thursday, Sept. 13, from 9-11 at the Santa Clara Convention Center during PCB West.