EEWeb Pulse - Volume 31

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Issue 31 January 31, 2012

Michael Hoff A123 Systems

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TA B L E O F C O N T E N T S TABLE OF CONTENTS

4

Michael Hoff A123 SYSTEMS Interview with Michael Hoff - Director of Applications Engineering

Featured Products More on Software Clock Handling

BY DAVE LACEY WITH XMOS

A look at global clock recovery, distributed clock synchronization and event synchronization.

Measurement Uncertainty and CISPR 11

10 12 16

BY RYAN URNESS WITH LS RESEARCH Considering measurement uncertainty in the fourth edition of CISPR 11.

RTZ - Return to Zero Comic

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19

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INTERVIEW

How did you get into electronics/engineering and when did you start?

I got started in electronics when I was in elementary school working on my toy train set. It started as a single loop, then I expanded it over time to have multiple loops with switches, switchbacks, elevated sections, mountains, tunnels and towns. I wired a control panel for town and street lighting and track control. After nearly electrocuting myself more than once, I knew that this field was for me. But seriously, the ability to enable motion, vision and action with electrical power, even way back then, gripped my imagination with thoughts, dreams and ideas that I can still remember now.

Going into college, I briefly considered mechanical engineering, but in the end, I was hooked by the idea and mysteries of harnessing the mathematical properties of those tiny electrons.

Can you tell us about your work history/journey to becoming the Director of Applications Engineering at A123 Systems, Inc.?

I was the second engineer hired at

American Power Conversion back in the 1980s just before the PC boom took hold of the world. Along with the rest of the company, I learned what it takes to grow from a startup company under leaking roofs and under-sized warehouses to a multi-billion dollar industry player with multiple global factories and engineering centers. I morphed from a junior engineering assistant to lead the advanced product research center of the largest division there.

Michael Hoff - Director of Applications Engineering; Energy Solutions Group

Before seventh grade, I took out every book in the local library on electronics to slowly try to understand things like the difference between current and voltage, a relay from a tube, and an NPN from a PNP. To my parents’ chagrin, I took apart every broken appliance we had in the house to see how the electronics worked.

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FEATURED INTERVIEW

Michael H off A123 Systems

INTERVIEW

When I stumbled on A123 with another possible battery technology, I took a visit to see if they had the capabilities that they promised. One of the APC founders and I asked them some hard questions, and we were satisfied that they had the best battery we had ever seen. Soon after, Dave Vieau, A123’s CEO, asked me to help them build their system’s capability because they only had cell and electrochemical design expertise in-house at the time.

I joined A123 more than five years ago to launch the systems engineering group, design several products and product lines, build the safety and compliance group, and now head the applications engineering team—the “AE team.”

In this latter role, we take the customer’s needs and desires and turn them into a product concept complete with estimated costs, budget, resources and timelines from contract to fulfillment. The exposure to all kinds of markets, applications, geographies and customers makes the job very interesting. For example, we might be designing and building a small 4-cell pack for a roving robot, while at the same time imagineering a large grid-based farm of battery containers containing half a million cells.

Do you have any tricks up your sleeve?

I love to tell young engineers about

the conundrum faced by both an engineer and a scientist in a long corridor. They are asked to split the distance with each step in order to reach the prize at the other end of the corridor. The brilliant scientist quickly realizes that the objective will never be achievable because his velocity converges to zero. But the engineer, after mulling on it for a moment (like engineers like to do), decided that for all practical purposes, she can get close enough to reach out and grab the prize and take it home. I use this principle to accomplish things that most people would give up on before they even attempt it. I consider the most important things first, and worry about the details when I have to.

What has been your favorite project?

My favorite projects are ones where I have broad involvement. At many large companies, engineers’ involvement is compartmentalized according to their job title. This can limit their exposure to, and passion for, projects to which they contribute.

As the lead engineer on the Smart-UPS 400 project at APC I was involved with all aspects of the project. I conceived the physical shape, and designed the circuits, PCB, magnetics, ASICs and mechanical hardware. Then, after building and programming the production test fixtures, and releasing it to production, I travelled to Europe to announce it at a sales show in Hanover, Germany.

At A123, I also contribute in multiple aspects of the programs in which I’m involved. Here engineers are

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expected to contribute in multiple disciplines of the design, not just those defined by their job titles.

The opportunity to gain this broad experience not only benefits the company, but the individual as well.

The ownership and pride that one feels at A123 from contributing to a product’s success is second to none. That’s the beauty of this place: When you rise to the challenge, you are rewarded with even more challenges!

Do you have any note-worthy engineering experiences?

I have my hands in four released patents, and three more in the works. I have spoken at four major conferences and written several papers for the public. I have released multiple products from UPS to battery packs, but one of the proudest moments in my career occurred recently when a young student—not sure where he wanted to end up—happened to intern for me. After his time here, he wrote and thanked me for my time and said that because of his intern experience with me he was going to settle down and target an engineering degree for his college years.

What are you currently working on?

Currently my team and I have just finished imagineering a roughly 10MW, 4MWh energy storage system that will be used to enable the installation of a large wind farm on a small island power grid.

Because the wind farm could produce a large percentage of the island’s power needs in one quick

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FEATURED INTERVIEW

In that capacity, I directed our team to investigate alternative energy storage media like advanced leadacid, NiMH, lithium ion, fuel cells, flywheels, capacitors and even plasma toroids.

INTERVIEW

What are your main goals and responsibilities as the Director of Applications Engineering?

Even though we are a mediumsized company, in many ways we still act like a small one. As director of my department, I strive to meet our customer expectations and schedules, but more often than not I end up drawing one-line diagrams, simple mechanical layouts, filling out the spreadsheets, writing specifications, and reviewing the same from the rest of the team. That often means working late hours with the team and getting in early to do the rest of the stuff that has to get done. My goal as a manager

has always been to make everyone know they are a vital part of the team, and to make the team look like it contains more people than it really does.

My favorite projects are ones where I have broad involvement.

At many large companies, engineers’ involvement is compartmentalized according to their job title. This can limit their exposure to, and passion for, projects to which they contribute.

We saw that you are working on a team that is responsible for front-end product concept development, can you tell us more about this team?

Our team consists of members with mechanical, electrical, systems, materials, and modeling expertise.

We take rough exterior dimensions and electrical specifications and turn them into PowerPoint concepts—sometimes with enough details from which to actually build a prototype—which we do as well.

One time a customer requested 100

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such prototypes to test on its famed formidable “wall of power.” We designed the circuits, mechanics, and firmware, and basically “printed” the prototypes in our fast prototyping lab in the back.

Other times, in lieu of a prototype, customers will require a simulation to understand how a product we develop will behave in their environment. One cannot just install a 32MW power system on the power grid and “play” with various modes of operation without creating havoc.

Such is the biggest opportunity for our team’s modeling experts.

Can you tell us more about A123 Systems, Inc. and the technology it is developing?

When I first arrived, A123 was solely in the power tool business. Black and Decker had pulled us by our bootstraps to take our fledgling but promising technology from the lab to production and provide it with its first lithium ion power tool line. It was a success and I have several tools in my home workshop to prove it! Since then we have branched out into multiple industries such as Smart-grid, telecom, datacenter, industrial, medical, heavy duty truck, automotive, aviation, , hobby, you name it. The technology is a game changer in some industries.

Without this technology, some of our customers’ applications just can’t exist economically.

The core of the technology is a highpower cell design. Everything from the external shell to the composition of the active material is selected to maximize the power and efficiency at the core of every system we develop. The beautiful thing about the materials we use in the cell is

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FEATURED INTERVIEW

wind gust, one can imagine how a blustery day would affect the power quality for all the nearby customers.

Their line voltage would swing rapidly in one direction or another as the wind stops and starts, and the small island’s traditional fuel-based power generators would try in vain to control their output in response to the rapidly fluctuating power influxes. An A123 energy storage system can absorb the power influxes and gradually transition the turbines’ power on or off the grid to allow the traditional fossilbased generation time to respond to the changing needs. The energy storage system acts like a big shock absorber on the grid system to keep it from bouncing around like a big network of interconnected springs.

Without our system, our customers would not be able to install the wind turbine on the island to save their rate payers millions of dollars and protect the environment by minimizing vast quantities of wasted carbon in the atmosphere.

INTERVIEW

What are some of the major changes the company has seen since its start in 2001?

Change is the only constant A123 experiences. A123 was started on the promise of the famous Hamaker force shown in Figure 1.

Our founders conceived of a way to use this principle to build a much higher energy cell by using the natural forces of nano particles to self-assemble in the smallest geometries possible—the smaller the geometries, the more energy that can fit into a smaller volume.

Well, this idea worked in the lab, but it was difficult to reproduce on a production scale. Looking at years and years of research ahead of them, and a dwindling venturebacked budget, they decided to change course and develop a high powered battery instead. Wow! They went 180° around from a high

E2 m + 3 hv 8 2 E2

energy, off-the-wall design to a highly reproducible, high powered design, and still using nanomaterials to accomplish their goals.

Since I joined, A123 has grown to follow the opportunities in the power-hungry Smart-grid, telecom, datacenter, industrial, medical, heavy duty truck, automotive, aviation and hobby markets around the world. This has involved a tremendous amount of change, growth, morphing, and drive to expand our horizons.

What are some new technologies A123 Systems, Inc. is working on?

Since we made that shift years ago from energy to power, we still haven’t forgotten the nagging need for more energy in a smaller box. Our cofounder, Yet-Ming Chiang, came up with a concept for how to apply our chemistry in a whole new way to achieve higher energy density at a much lower cost. The work is still in the lab, but it has promise. To capitalize on its possibility, while not risking the focus and energy of our core business efforts, A123 spun off a separate company, called 24M, with funding to get it started in the investigation of this approach to building a battery.

Can you tell us about A123 Systems’ new battery developments?

Some of A123 cell’s key strengths make them difficult to manage. For example, the voltage of the cell is

Our company is heading in three major directions: Smartgrid, commercial and automotive. Each one has promise, and a dire need for the capabilities that our core cell and associated systems can provide for them.

FEATURED INTERVIEW

that they are much less volatile, and therefore safer, than any other lithium ion battery chemistry on the market. It also turns out that the very technology that enables high power also allows the battery to cycle tens of thousands of times. This is unheard of compared to the usual battery technologies today. Those three attributes are where we get our mantra: “Power, safety and life.”

extremely flat with respect to time as it discharges into a load. This presents an acute challenge for monitoring the battery system to determine how much charge is left in storage. Imagine driving in your car with the fuel gauge reading half a tank when2 it suddenly drops2 to 2 2 (n1 -anmile ) E E 2 ) (nbefore 3 - n 2you 3 3 hv 3 - E2 1 - E2 reading empty m+ mc A123 = kT c 2 2 1 /2 2 2 1/ 2 2 2 1 /2 2 4 8 2 (n1 + n2 run "(nis1 just E1 + E2 E3 + E2 + none n3 of+gas! n2 ) This ) (out 2 ) of+ (n 3 + n our control challenges but one that (n12 - n22) (n32 - n22) we are making headway on, one project at a time. (n12 + n22) 1/2 (n32 + n22) 1/2 "(n12 + n22) 1/2 + (n32 + n22) 1/2}

Figure 1

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Another challenge relates to the incredible power of our cells.

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INTERVIEW

What direction do you see your business heading in the next few years?

Our company is heading in three major directions: Smart-grid, commercial and automotive. Each one has promise, and a dire need for the capabilities that our core cell and associated systems can provide for them. Making transportation and the power grid more efficient, and making commercial products last longer, will significantly reduce the world’s need for oil and natural resources. Because of that, I really think that we can make a difference, globally, in each of these directions.

What can we expect to see from A123 Systems, Inc. in the near future?

One of the biggest complaints from many of our customers, especially the smaller ones, is that our technology is not accessible to them.

The product development teams are up to their ears in large projects for large OEM accounts building custom products for their specific needs. To make our technology more accessible, we have come up with a scalable architecture that allows us to sell standardized modules that can be used in a wide variety of applications without extensive system design activities and engineering resources. Our

first example is a model to replace what some people think is the world’s most common sealed-lead acid battery format: the 12V7.

a technology that saves customers money, and at the same time help to clean up the environment, we, the customer and the world all share a big win-win-win. ■

We plan to expand this offering to other form factors to allow customers to take advantage of the superior power, safety and life of our cells but not have to design a complex battery management system or wait a year or more for a custom-designed pack.

What challenges do you foresee in our industry?

The biggest challenge to us and our industry is the economy. Even though our products may be superior and can even create new markets, businesses and individuals are not spending until they know that they can follow through with their payments. Fortunately, there are several markets in our business where the application of our technology can actually save (and even make) our customers money in the end. The biggest challenge is to demonstrate such savings to the investors. The economy is the biggest challenge to everyone in the technology business. But as in A123’s case, it can also be the biggest opportunity. If one can build

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FEATURED INTERVIEW

Imagine 83,000 of them in one energy storage unit contributing to an external or internal short circuit! The numbers associated with the short circuit current levels are mind-boggling. Have you ever built a 200,000 amp switch and measured such a current? That’s an experience worth remembering.

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F E AT U R E D P R O D U C T S VDD

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INA196

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VMON

I12V

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TEMP12V

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/EN

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LDO1

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UCD90124A WDI from main processor

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INA196

WARN_OC_0.8V_ OR_12V

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SYSTEM RESET

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OTHER SEQUENCER DONE (CASCADE INPUT)

PWM

2MHz

I0.8V

Vmarg

Closed Loop Margining

4- wire Fan

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I2C/ PMBUS

JTAG

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TACH GND

DC Fan

The UCD90124A is a 12-rail PMBus/I2C addressable power-supply sequencer and monitor. The device integrates a 12-bit ADC for monitoring up to 12 power-supply voltage inputs. Twenty-six GPIO pins can be used for power supply enables, power-on reset signals, external interrupts, cascading, or other system functions. Twelve of these pins offer PWM functionality. Using these pins, the UCD90124A offers support for fan control, margining, and general-purpose PWM functions. Specific power states can be achieved using the Pin-Selected Rail States feature.

This feature allows with the use of up to 3 GPIs to enable and disable any rail. This is useful for implementing system low-power modes and the Advanced Configuration and Power Interface (ACPI) specification that is used for hardware devices. For more information, please click here.

Small-Footprint Flash LEDs Avago Technologies, a leading supplier of analog interface components for communications, industrial and consumer applications, announced two series of compact LEDs that reduce space requirements for designing auto-focus auxiliary flash functionality into digital cameras.

The new ASMT-FJ70 and ASMT-FG70 devices are available in one of the market’s thinnest, smallest-footprint packages for LEDs with the brightness needed for auto-focus functionality in dark settings. The ASMT-Fx70 LEDs are available in a tiny, environmentally-friendly 3.6 (L) by 3.2 (W) by 3.4 (H) mm surface-mount package that helps meet market demand for thinner digital cameras. The LEDs use a clear, non-diffused lens to provide high luminous intensity within a narrow radiation pattern, providing smooth, consistent optical performance for precise auto-focus functionality. The ASMT-FJ70 devices are orange and the ASMT-FG70 devices are the industry’s first green auxiliary flash LEDs in this size range.For more information, please click here.

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MORE ON:

Software Clock Handling Dave Lacey Technical Director of Software Tools

In a previous article I went over the basics of handling clocks in software. In this article the theme will continue as I look at global clock recovery, distributed clock synchronization and event synchronization.

Global clock recovery Suppose there are several agents in a complex system.

Each one of these will have its own reference clock that gives it a sense of time. An agent’s notion of time is given by how many reference clock ticks have passed since startup.

To synchronize actions and share clocks, the agents need to have a common, global sense of time. This is easy with a bus system like USB or Firewire since a bus tends to carry its own clock. However, for distributed, packet-based interconnects, things get harder. The IEEE standards 1588, 1588v2 and 802.1as and the IETF NTP standard are designed to specifically address the problem of distributed global clock recovery.

The first task of the system is to elect one of the agents as the master. It is this agent’s clock that will become the

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common global clock. IEEE 1588 and variants use a peer-to-peer leadership election protocol that tries to elect the highest quality clock amongst the agents. It is guaranteed to elect one and only one master.

Once the master is elected, each agent needs to know the following information: • Is my clock going slower/faster than the global master clock? If so, by how much?

• What is the current time now according to the master clock?

The fundamental method used is to regularly transmit synchronization messages from the master to the other agents. The message contains the global time that the message was sent. The diagram in Figure 1 shows this happening.

The slave knows the outgoing timestamps performed by the master (Tm1, Tm2 and Tm3) as well as the inbound

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TECHNICAL ARTICLE Tm1

Tm2

sync

Tm3

sync

TECHNICAL ARTICLE

MASTER

Time

sync

transmit time

SLAVE

Ts1

Ts2

Time

Ts3

Figure 1

timestamps performed by the slave (Ts1, Ts2 and Ts3).

Given these timestamps it can work out the ratio between the master clock and its own local clock. For example, the following calculation performs a measure of the ratio between the two clocks: rate_ratio = (Ts2 – Ts1) / (Tm2 – Tm1) Multiple measures of this value can be filtered to average out variations in the transit time. Tracking the value over time will also track any drift in the two clocks involved.

To fully convert between global time and local time the slave also needs to know its time offset from the master, which means it needs to know the transit time of the sync messages. To do this, the master and slave perform a two way exchange of messages (Figure 2).

Assume that the transit time is symmetrical: 2 * transit_time = (Ts1 – Tm1) + (Tm2 – Ts2) So, transit_time = 0.5 * ((Tm2 – Tm1) + (Ts2 – Ts2)) Care is needed here, since the timestamps Tm2 and Tm1 are taken with reference to a different clock than Ts2 and Ts1. So the slave needs to make the calculation:

In reality, the time synchronization protocols like 802.1as are a bit more complicated than what is portrayed here, but the basic idea is the same.

Event synchronization Once global clock synchronization is in place, the local agents can convert global time into local time: local_time = rate_ratio * global_time + offset This means that two agents can synchronize their actions.

Each gets given a global time to perform an event. The agents convert this global time to local time and then schedule the event to happen at that time.

Of course, the events will not happen at exactly the same time. There will be some synchronization error. This error will comprise of the following: • Error due to the global clock recovery.

• Error due to variability in the agent’s ability to trigger an event at a particular local time.

• Error due to variation in the agent’s local clock since the last synchronization.

transit_time = 0.5 * ((Tm2 – Tm1) + (Ts2 – Ts2) / rate_ratio)

The magnitude of these errors will depend on the communication technology, the synchronization protocol, the accuracy of the message time-stamping between agents and the local event synchronization within the agent.

Knowing this transit time allows the slave to work out its offset from the master clock. It now has all the information it needs to convert between global and local time and vice versa.

In the situation where communication is over Ethernet using IEEE 802.1as as a time synchronization protocol and the agent is an XMOS XCore device, the synchronization error can be less than 150ns.

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TECHNICAL ARTICLE Tm1

Tm2

msg1

SLAVE

Ts1

TECHNICAL ARTICLE

MASTER

Time

msg2

Time

Ts2

Figure 2

Recovery of other clocks based on a global clock Sometimes there is a need to recover a secondary clock using global time as a reference. This is the case in the IEEE AVB 1722 standard where a media clock needs to be recovered to transmit audio or video from one endpoint to another.

In this case, the clock is recovered using a control loop as described in my previous article on software clocking.

The difference in this case is how the error is detected.

The agent transmitting the clock can also transmit the expected time that certain clock edges should occur.

When the slave recovers and outputs a clock, it can track when that edge actually occurs and get a delta between the expected and actual time. The existence of the global clock allows the agents to have a common timebase to calculate the delta.

This delta gives the error term to feed into a P-I-D control loop. However, there is one further “gotcha” to be aware of. The aim is to recover the clock period—and that makes the delta between the expected and actual edge time the integrated error and not the proportional error. This means that the proportional error is the rate of change of the delta. Once this is understood, the control loop can be tuned in a standard manner.

About the Author Dr. David Lacey works as Technical Director of Software Tools at XMOS Ltd. With over ten years of research and development in programming tools and compilation technology, he now works on the development tools for XMOS devices. As well as tools development, he has worked on application development for parallel and embedded microprocessors including work in areas such as math libraries, networking, financial simulation, and audio processing. XMOS Website. ■

actual expected delta

Figure 3

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Get the Datasheet and Order Samples http://www.intersil.com

Dual 14-Bit, 250/200/125 MSPS JESD204B High Speed Serial Output ADC ISLA224S

Features

The ISLA224S is a series of low-power, high-performance, dual-channel 14-bit, analog-to-digital converters. Designed with FemtoCharge™ technology on a standard CMOS process, the series supports sampling rates of up to 250MSPS. The ISLA224S is part of a pin-compatible family of 12- and 14-bit dual-channel A/Ds with maximum sample rates ranging from 125MSPS to 250MSPS and shares the same analog core as Intersil's proven ISLA224P series of ADCs. The family minimizes power consumption while providing state-of-the art dynamic performance, offering an optimal performance-vspower trade-off.

• JESD204A/B High Speed Data Interface - JESD204A Compliant - JESD204B Device Subclass 0 Compliant - JESD204B Device Subclass 2 Compatible - Up to 3 JESD204 Output Lanes Running up to 4.375Gbps - Highly Configurable JESD204 Transmitter • Multiple Chip Time Alignment and Deterministic Latency Support (JESD204B Device Subclass 2) • SPI Programmable Debugging Features and Test Patterns • 48-pin QFN 7mmx7mm Package

Differentiating the ISLA224S from the ISLA224P is its highly configurable, JESD204B-compliant, high speed serial output link. The link offers data rates up to 4.375Gbps per lane and multiple packing modes. It can be configured to use two or three lanes to transmit the conversion data, allowing for flexibility in the receiver design. The SERDES transmitter also provides deterministic latency and multi-chip time alignment support to satisfy an application's complex synchronization requirements.

A serial peripheral interface (SPI) port allows for extensive configurability of the JESD204B transmitter including access to its built-in link and transport-layer test patterns. The SPI port also provides control for numerous additional features including the fine gain and offset adjustments of the two ADC cores as well as the programmable clock divider, enabling 2x and 4x harmonic clocking.

The ISLA224S is available in a space-saving 7mmx7mm 48 Ld QFN package. The package features a thermal pad for improved thermal performance and is specified over the full industrial temperature range (-40°C to +85°C).

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• SNR @ 250/200/125MSPS 73.2/74.1/75.1 dBFS fIN = 30MHz 70.7/70.7/70.6 dBFS fIN = 363MHz • SFDR @ 250/200/125MSPS 82/91/94 dBc fIN = 30MHz 77/79/67 dBc fIN = 363MHz • Total Power Consumption: 989mW @ 250MSPS

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Radar and Satellite Antenna Array Processing Broadband Communications and Microwave Receivers High-Performance Data Acquisition Communications Test Equipment High-Speed Medical Imaging

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ISLA224S25

14

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12

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12

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FIGURE 1. SERDES DATA EYE AT 4.375Gbps

December 20, 2011 FN7911.0

Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2011 All Rights Reserved. All other trademarks mentioned are the property of their respective owners.

Measurement Uncertainty and

CISPR 11 Ryan Urness

Laboratory Manager

Measurement uncertainty, as described in ISO/IEC Guide 98, is a “parameter, associated with the result of a measurement, [which] characterizes the dispersion of values that could reasonably be attributed to the measurand.”

With respect to EMC measurements performed for compliance testing to the requirements of the European Union, the standard for modeling uncertainty calculations is CISPR 16-4-2.

Measurement Uncertainty in CISPR 11 4th Ed. The 2006 release of Amendment 2 to the fourth edition of CISPR 11 added CISPR 16-4-2 as a normative reference; and measurement uncertainty was considered, through

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the inclusion of section 11.4, as an assessment of conformity of equipment. The amendment in section 11.4 reads as follows: • The results of measurements of emissions from ISM equipment shall reference the measurement instrumentation uncertainty considerations contained in CISPR 16-4-2.

• Determining compliance with the limits in this standard shall be based on the results of the compliance measurement, not taking into account measurement instrumentation uncertainty. However, the measurement uncertainty of the measurement instrumentation and its associated connections between the various instruments in the measurement chain shall be calculated, and both the measurement results and the calculated uncertainty shall appear in the test report.

• Through the institution of this amendment, measurement uncertainty of the site and equipment used in determining compliance needed only to be included in the documentation, and were not

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Measurement Uncertainty in CISPR 11 5th Ed. The 2009 release of the fifth edition of CISPR 11 includes a significant repositioning of the measurement uncertainty requirements. The new requirements for measurement uncertainty are specified in the assessment of conformity section 12.5: • Determining compliance with the limits in this standard shall be based on the results of the compliance measurements, taking into account the considerations on measurement instrumentation uncertainty. • Where applicable, measurement instrumentation uncertainty shall be treated as specified in CISPR 16-4-2.

With the inclusion of this statement, it is apparent that the competency of calculations of a laboratory’s measurement instrumentation could have a significant impact on its ability to demonstrate compliance to the limits outlined within the standard.

Modeling Uncertainty with CISPR 16-4-2 Although CISPR 11 states that determining compliance requires taking into account measurement uncertainty, the standard does not expressly provide direction in how to account for it. The conservative approach to determining conformity would account for at least a margin of compliance to the limits greater than the measurement uncertainty of the equipment used to perform the measurements. Unfortunately, often the

largest contribution to a measurement uncertainty budget is the measurement receiver itself, and is of significant enough dispersion that this method of compliance assessment is often impractical. As a result, CISPR 16-4-2 may be used to determine conformity assessment, as long as the uncertainty is calculated as specified within the standard.

It is required that instrument uncertainty is evaluated and takes into consideration these quantities: the standard uncertainty u(xi) in dB and the sensitivity coefficient ci shall be evaluated, taking into account the estimate xi for each listed quantity. The combined standard uncertainty uc(y) can then be calculated using the following equation:

/ c u (x ) 2 i

uc (y) =

2

Once the combined standard uncertainty has been determined, the expanded measurement instrument uncertainty for the lab Ulab can be extracted through the following equation:

U lab = 2uc (y) Ulab yields an approximately 95% level of confidence for the near-normal distribution typical of most measurement results, and this is the value declared in the test report.

CISPR 11 Conformity Once Ulab is determined for the measurement instrumentation, it must be referenced against Table 1, which consists of Ucispr: the values of the expanded uncertainties evaluated to the associated quantities in section 4, and detailed in Annex A of CISPR 16-4-2.

Uolcpr

Measurement Conducted disturbance (mains port)

(9 kHz - 150 kHz) (150 kHz - 30 MHz)

4.0 dB 3.6 dB

Disturbance power

(30 MHz - 300 MHz)

4.5 dB

Radiated disturbance (electric field strength on an open area test site or alternative test site

(30 MHz - 1000 MHz)

5.2 dB

Other

i

i

Under Consideration

Figure 2: Values of Ucispr

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integrated as a consideration in determining final compliance to the limits outlined within the standard.

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• If Ulab is less than or equal to Ucispr, then measured disturbances do not need to be further corrected for measurement uncertainty, and can be compared against the limits expressed within; however, if Ulab is greater than Ucispr, then the difference between Ulab and Ucispr must be added to the disturbance measured as an uncertainty correction, and then compared against the limits expressed within.

Corollary Assessment It has been demonstrated that the accuracy of the laboratory’s method of determining measurement uncertainty is an essential component in assessing the conformity of a product to the limits and requirements of the 5th edition of CISPR 11.

LS Research includes its complete uncertainty budget used to determine the instrument expanded uncertainties as an appendix to every CISPR 11 test report. The Ulab values calculated for LS Research are below the Ucispr values outlined in Table 1 of CISPR 16-4-2, which requires no additional correction to measurements when determining compliance to CISPR 11.

References • ISO/IEC IS Guide 98-3:2008 Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995), p. 2.

• IEC CISPR 11:2003-03, Amendment 2 – Industrial

scientific and medical equipment (ISM) – Radiofrequency disturbance characteristics – Limits and methods of measurement, p. 7.

• IEC CISPR 11:2009-05 Industrial scientific and medical equipment (ISM) – Radio-frequency disturbance characteristics – Limits and methods of measurement, p. 39.

• Medler, Jens: “Uncertainty Contribution of the EMI Test Receiver in RF Disturbance Measurements,” 2010 Asia-Pacific International Symposium on Electromagnetic Compatibility, Beijing, 2010, p. 994.

• IEC CISPR 16-4-2:2003-11 Specification for radio disturbance and immunity measuring apparatus and methods – Part 4-2: Uncertainties in EMC measurements, p. 15.

• IEC CISPR 16-4-2:2003-11 Specification for radio disturbance and immunity measuring apparatus and methods – Part 4-2: Uncertainties in EMC measurements, pp.15-17.

About the Author Ryan Urness is the Laboratory Manager for LS Research.

Ryan has enjoyed over 15 years in product qualification, reliability performance, and regulatory compliance for dynamics, environmental, electronics, and EMC testing.

Mr. Urness specializes in subordinate control and data acquisition, process automation, and test system design.

Ryan holds a bachelor’s in engineering studies and organizational management, and was the first iNARTE certified laboratory auditor in the state of Wisconsin.

Ryan can be reached at rurness@lsr.com. ■

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Determining compliance to the CISPR 11 limits can then be determined as follows:

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