Ferrite Cores For Low-Frequency EMI Cable Suppression

Posted March 7, 2014 by Henry Ott
Categories: EMC Components, Troubleshooting

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Ferrite cores (chokes) provide an inexpensive, and effective, way of coupling high-frequency resistance into a cable in order to reduce the  common-mode current, and hence the radiation (or pickup) from the cable. They are commonly used  on mouse, keyboard, video, and other peripheral cables connected to personal computers, as well as on power supply cables when a device is powered from an external transformer (wall-wart) or power supply. The ferrite core acts as a one-turn common-mode choke, and can be effective in reducing the conducted and/or radiated emission from the cable, as well as suppressing high-frequency pick-up in the cable. Basically ferrites can be thought of as high-frequency resistors, with little or no impedance at low-frequencies or dc. Ferrite cores are most effective in providing attenuation of  unwanted noise signals above 10 MHz. The figure below shows a ferrite choke on a USB cable.

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For low-frequency cable emission problems, typically below 10 MHz, ferrite chokes have not been very useful, since their impedance is too low, at these frequencies, to be effective. I have always wished  for a similar, simple low-frequency solution to cable emission/susceptibility problems. My wish finally has been granted.

Recently Fair-Rite Products Corp. introduced a new low-frequency, Type 75, ferrite material optimized for EMI suppression in the 200 kHz to 30 MHz frequency range. This material has an impedance peak in the 1 to 2 MHz range. Information on the new Type 75 ferrite cores is included in Fair-Rite’s  17th edition catalog.

For example,  part number 2675540002 (9/16″ OD,  1/4″ ID , and 1-1/8″ long core) has a peak impedance of  160 Ω at about 1.6 MHz as shown below.

2675540002-1

Type 75 ferrite cores can be especially useful in reducing emission problems in the 500 kHz to 10 MHz frequency range, across which the impedance of the above ferrite is greater than 80 Ω..

The impedance of  ferrite cores can be further increased by using multiple turns. However, this also increases the inter-winding capacitance and degrades the high-frequency performance of the choke. Since the Type 75 material is intended for low-frequency use, this increase in capacitance is less of an issue, and two to five turns can be used with very little, if any, detrimental effect. The same 2675540002 core discussed above has an impedance of 1,400 Ω at 1.3 MHz when three turns are used, and an impedance of almost 4,000 Ω when five turns are used, see below. A three turn choke using this core will have an impedance greater than 500 Ω from 300 kHz to 20 MHz.

2675540002-2

Even at 150 kHz, the low-end of the FCC/CISPR conducted emission measurement range, the three turn configuration has an impedance of approximately 250 Ω, and the five turn configuration has an impedance of approximately 700 Ω. Therefore, by using multiple turns high impedances can now be obtained at these low-frequencies.

Both smaller and larger cores are available from Fair-Rite. Type 75 cores ranging from 3/8″ to 1-1/4″ OD (7/32″ to 3/4″ ID) are listed in the Fair-Rite catalog. At present only solid cores are available, but snap-on split cores should also be available later this year. Snap-on cores are convenient for troubleshooting, and can be easily applied as an after-the-fact fix to cables. Since the manganese-zinc (MnZn) Type 75 cores are slightly conductive (resistivity, ρ = 3×10^2 Ω-cm), care should be taken that they do not touch any live electrical terminals.

For more information  on this new low-frequency ferrite material, check out Fair-Rite’s Low-Frequency Suppression Flyer, a pdf copy of which is available  at:

http://www.fair-rite.com/newfair/pdf/LowFreqSuppression.pdf

Give the new Fair-Rite Type 75 material a try for your low-frequency emission/susceptibility cable problems. You might be pleasantly suppressed with the results.

EMC Training Seminar in the Pacific Northwest

Posted February 28, 2014 by Henry Ott
Categories: EMC Seminars, EMC Training, Uncategorized

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Only one week left to receive a $200 discount on a three-day Electromagnetic Compatibility Engineering  seminar presented by Henry Ott  in partnership with IN Compliance magazine.

Henry Ott Consultants in partnership with IN Compliance magazine  will be presenting a three-day public Electromagnetic Compatibility Engineering seminar and EMC Exhibition in Seattle, Washington on April 22-24, 2014.

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This three-day intensive seminar covers the practical aspects of noise and interference control in electronic systems and provides a working knowledge of EMC principles.  Emphasis is on the cost-effective design of analog & digital systems. Commercial,  industrial, automotive, and military aspects of EMC are emphasized.  The amount and complexity of mathematics are kept to a minimum, and ideas are illustrated with examples of actual case histories. Participants will obtain the knowledge necessary to design electronic equipment that is compatible with the electromagnetic environment, and is in compliance with national and international EMC regulations.

Topics covered include:

  • Cabling
  • Grounding Principles
  • Digital Layout and Grounding
  • High-Speed Digital Decoupling
  • Differential-Mode Emission
  • Common-Mode Filtering
  • Digital Transmission Lines
  • Mixed-Signal PCB Grounding and Layout
  • RF and Transient Immunity
  • Conducted Emission
  • Shielding

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In addition to class notes, participants will receive a copy of my  872 page, award-winning book Electromagnetic Compatibility Engineering, John Wiley & Sons, 2009.

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On Wednesday April 23, 2014 we will have a number of EMC companies exhibiting their products and services during the continental breakfast, morning and afternoon breaks, an extended lunch, and an evening reception immediately after class. Attendees will have an opportunity to visit and connect with some of the industries’ top suppliers including, test laboratories, test equipment companies, and EMC component manufactures.

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Some of the companies exhibiting will be:

  • LCR Electronics (Power Line Filters)
  • Advanced Test Equipment Rentals
  • Fair-Rite Products
  • Rigol Technologies (Oscilloscopes and Spectrum Analyzers)
  • Fischer Custom Communications (Current Probes etc.)
  • Amplifier Research
  • Northwest EMC
  • MAJR Products (Shielding products)
  • TUV Rheinland
  • Haefely Hiptronics
  • HV Technologies
  • Intermark USA

Registration is limited – so sign up early. Register by March 7, 2014 and take advantage of a $200 early registration discount. Don’t miss this great opportunity, click here for more information. Hope to see you in Seattle!

2013 IEEE EMC Society Symposium

Posted April 29, 2013 by Henry Ott
Categories: EMC Seminars, EMC Training, News

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This year the 2013 IEEE International Symposium on EMC will be held at the Colorado Convention Center in Denver, CO from August 5-9, 2013. The Symposium is the largest gathering of EMC professionals in the world. There will be in excess of 190 technical papers presented, plus workshops, tutorials, social events, and over 90 exhibitors will be present to showcase their services and wares. The theme for this year’s Symposium is Winds of Change.

I will again be chairing the all day Fundamentals of EMC Tutorial on Monday August 5, 2013.  This tutorial session is intended to educate new EMC engineers and technicians, the future leaders of our industry and society, as well as those looking for a refresher on EMC fundamentals. I think that managers and non-EMC specialists will also benefit from the session, by giving them an awareness and understanding of some of the basic principles of EMC. The material presented will be “down-to-earth,” practical, and with a minimum of mathematics. Historically this is the most widely attended session at the Symposium, with close to 200 attendees.

The subjects to be presented at the tutorial, and their presenters are listed below:

1) The Secret Lives of Capacitors, Inductors, and Resistors (“Things Your Professors Didn’t Tell You!”), presented by Mark Steffka, General Motors, USA

2) Grounding: The Grounds for EMC Design, presented by Elya B. Joffe, KTM Project Engineering, Israel

3) Dealing with EMI in Cables, presented  by William Kimmel, Kimmel-Gerke Associates, USA

4) The Ten Best Ways to Maximize the Emission From Your Product, presented by Henry W. Ott, Henry Ott Consultants, USA

5) Basic EMC Measurements, Andy Marvin, University of York, UK

The above collection of international authors are all well-respected in the EMC community, know their subject well,  and have the ability to effectively communicate their knowledge to others.

In addition, on Friday morning August 9, 2013 I will participate in a workshop on Grounding – Concepts, Physics and Myths. This workshop, chaired by Bruce Archambeault,  will consist of three presentations followed by a panel discussion. The presentations will be as follows:

1) I will discuss Basic Grounding Concepts and Myths as well as the Ubiquitous Ground Loop.

2) Bruce Archambeault (IBM, Research Triangle Park, NC) will discuss Return Current Paths Versus Ground.

3) Bob Scully (NASA, Houston, TX) will discuss What Does Ground Mean in Outer Space?

EMC2013Logo

The 2013 Symposium will include presentations on many topics to enhance your understanding of EMC, sharpen your design skills, and perfect your skill in dealing with EM phenomena. Join your colleagues in the mile high city for a week of learning, collaboration and interaction with your industry peers.

For more information on the symposium, see the symposium website www.emc2013.org.

I hope to see you all in Denver in August.

Workbench EMC Measurements

Posted July 6, 2012 by Henry Ott
Categories: Test Equipment, Troubleshooting

Tags:

I have just posted a new article on “Workbench EMC Measurements” to the Tech Tips section of my webpage (www.hottconsultants.com). Workbench EMC measurements are simple, inexpensive, precompliance tests that a product designer can perform, in his/her own laboratory, early in the development phase of a product in order to obtain an indication of the EMC performance of the product.

The link to the article is:

www.hottconsultants.com/techtips/Workbench_EMC_Measurements.pdf

Digital Logic Return Current Flow

Posted March 3, 2012 by Henry Ott
Categories: Theory

The lowest impedance (inductance) signal return path for high-frequency current is in a plane directly adjacent to the signal trace. What one wants is the smallest signal current loop area possible. In order to evaluate a PCB layout/stackup and determine if this objective is achieved or not, one must first know exactly how the actual return current flows on the PCB.

Many designers/engineers, however, are confused about how and where digital logic return currents flow, and what is the source of the digital logic current. In my EMC Seminars, I receive many questions relating to this subject, typical question are:

  1. Is it better to have a digital logic trace adjacent to a ground plane, or adjacent to a power plane?
  2. What about a trace between a power and ground plane?
  3. Would it be better to have the trace between two ground planes, or possibly two power planes?

To answer these questions we must know two things:

  • What is the source of the current?
  • What is the path taken by the current in returning to the source?

First let me state that the logic gate is not the source of the current, the logic gate only acts as a switch. Only the transient (switching) current is important, and the transient current is not dependent on the existence of the load at the end of the line. The source of the current is either:

  • The driver’s decoupling capacitor, or
  • The parasitic signal trace to ground/power plane capacitance in parallel with the load capacitance

The return current path is a function of:

  • The trace configuration, microstrip or stripline
  • The adjacent plane or planes (power and/or ground), and
  • The logic transition (low-to-high, or high-to-low)

There are ten possible combinations of the above.

To answer the question “what is the return current path?”, I created the following Power Point presentation. Click here to view the presentation.

Power Point Presentation - Digital Logic Current Flow

From the ten examples presented in the above presentation, it can be concluded that it makes no difference whatsoever if the adjacent plane, or planes, are ground or power. In all cases the current returns directly to the source through a small loop. In none of the cases does the current have to go out of its way, or flow through a large loop, in order to return to the source.

Therefore, the answer to the original three questions is that it does not matter if the return path is the ground plane or the power plane, all configurations are equally acceptable.

We can, however, conclude that stripline is always better than microstrip since two current loops exist, and in one loop the current flow is clockwise and in the other loop the current flow is counterclockwise. Therefore, the radiation from the two loops tend to cancel each other.

With respect to the question of what is the source of the current, this is summarized in the table on slide 13 of the presentation.

Note: That the power plane discussed in the above presentation is the same voltage as that supplying the driver. The case of referencing a power plane not the same as the driver voltage is a different subject–possibly the topic of a future blog.

Reference: Ott, H. W., “Electromagnetic Compatibility Engineering,” Section 10.7, Digital Logic Current Flow, John Wiley & Sons, 2009.

Three Book EMC/Signal Integrity Library

Posted January 29, 2012 by Henry Ott
Categories: Book Rviews, EMC Training, Theory

Would you like to have a mini-library on the subject of EMC and Signal Integrity that covers the frequency range from DC to light. The following three books, all highly recommended, will accomplish that. All the books are very readable, with a minimum of mathematics, and as a bonus there is very little overlap between the material contained in each of the books. I refer to all three of these books regularly.

Three Book Mini EMC Library

Electromagnetic Compatibility Engineering, by Henry W. Ott, published by John Wiley & Sons in 2009 has won the PROSE Award from the Association of American Publishers for the best book published in 2009 in the category of Engineering and Technology. It is the most comprehensive book on the subject and is the first EMC book that you should read. In addition to the core subjects of cabling, grounding, balancing, filtering, and shielding that made my previous book, Noise Reduction Techniques in Electronic Systems, an international success (translated into six other languages), this new book includes additional coverage of equipment and systems grounding, switching power supplies and variable speed motor drives, digital circuit power distribution and decoupling, PCB layout and stackup, mixed-signal PCB layout, rf and transient immunity, power line disturbances, and simple pre-compliance EMC measurements. Written in an easy to read and understandable style, it’s full of real-life practical examples. This book contains eighteen chapters, six appendices, and 872 pages.

High-Speed Digital Design, by Howard Johnson and Marty Graham, published by Prentice Hall in 1993 picks up where Electromagnetic Compatibility Engineering leaves off, with respect to high-speed digital circuits. Despite its title, this is more an EMC book than a classical digital design book. Its subtitle, “A Handbook of Black Magic,” gives away its true nature. Besides containing much practical information on the EMC aspects of high-speed digital design, the book also contains useful information on various measurement techniques that can be used to measure such things as power-ground plane impedance, metastable states, etc. Subjects include transmission lines, terminations, vias, clock distribution, clock oscillators, ribbon cables and connectors, layer stackup, and power distribution. This book is considered by many as a “crossover book” covering both EMC and signal integrity. Everything in this book is easy to read and practical. A “must have” book if you are designing high-speed digital circuits! This book contains twelve chapters, three appendices, and 447 pages.

High-Speed Digital System Design, by Hall, Hall, and McCall, published by John Wiley and Sons in 2000 continues where the Johnson and Graham book ends. This is a signal integrity book! It is very well written with a good balance between theory and practical applications. The first sentence of the book sets the tone of the book; it reads, The speed of light is just too slow. Subjects include transmission line considerations, crosstalk, IC package and pin-out considerations, power delivery and decoupling, non-ideal current return paths, simultaneous switching noise, timing and skew considerations, and radiation. The book also includes a chapter on design methodologies useful in the design of high-speed systems with a large number of variables. As does the Johnson and Graham book, this book  contains a chapter on high-speed measurement techniques. The information contained in this book is useful at both the IC and PCB level. This book contains eleven chapters, six appendices, and 347 pages.

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Want to expand your library by adding two additional books that fall on either end of the spectrum from the above three core books? Then consider the following two books:

Two Additional EMC Books

EDN Designer’s Guide to Electromagnetic Compatibility, Second Edition, by Bill Kimmel and Daryl Gerke, 2000 (originally published by EDN magazine in 1994), is a practical nuts and bolts approach to EMI, written for the non-EMI expert. One hundred pages of down-to-earth practical advice on EMC, without any equations or mathematics, by two authors who work regularly in the EMC trenches. The paperback book is available on the authors’ web site.

Introduction to Electromagnetic Compatibility, Second Edition, by Clayton Paul, published by John Wiley and Sons in 2006 is more theoretical than any of the other books, as it was intended to be used for a university course in EMC. A good book to get more detail on the theory behind EMC. Lots of useful information, especially the chapter on crosstalk, a subject that Dr. Paul is considered to be a leading expert. This book contains eleven chapters, four appendices, and 1016 pages.

So there you have it, three, (or possibly five) books that cover 90% of what you need to know about Electromagnetic Compatibility.

Electromagnetic Compatibility Engineering, Digital Edition

Posted January 7, 2012 by Henry Ott
Categories: Book Rviews

Tags: , ,

2012 is officially here, and I would like to wish everyone a happy and healthy New Year.

Electromagnetic Compatibility EngineeringEver since my new book “Electromagnetic Compatibility Engineering” was published in August 2009, I have wished for an iPhone/iPad compatible digital version so that I could easily carry the book around with me on my various business trips. At almost 900 pages, the print edition was not the most convenient book to carry on trips. Well for 2012 my wish has been granted.

Digital versions of “Electromagnetic Compatibility Engineering” are now available for the Apple iPad, iPhone, and iPod touch, as well as for the Kindle. The iPad/iPhone version is available from Apple’s iBook store, and the Kindle version is available from Amazon.com.

An e-book version, for your laptop or desktop computer, has been available since 2009 from the publisher (John Wiley). To see the range of digital versions available visit http://www.wiley.com/WileyCDA/WileyTitle/productCd-1118210654.html and look under Other Available Formats.

More information, as well as an errata sheet, on the book is available on my website http://www.hottconsultants.com/EMCE_book_files/emce_book.html.

Best wishes to one and all for the New Year.


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