EMC Analysis During PCB Layout

Catch Issues Earlier

If your electronic product design fails EMC compliance testing for its target market, that product can’t be sold. That’s why EMC analysis is such an important step. In this article, Craig shows how implementing EMC analysis during the design phase provides an opportunity to avoid failing EMC compliance testing after fabrication.

By Craig Armenti,
Mentor, A Siemens Business

Electromagnetic Compatibility (EMC) is generally defined as the ability of a product to function in its environment without introducing electromagnetic disturbance. EMC compliance is a necessary condition for releasing products to market. Simply stated, if a product does not pass EMC compliance testing for the target market, the product cannot be sold. Regulatory bodies around the world define limits on the radiated and conducted emissions that a device is allowed to produce. Automotive and aerospace manufacturers can set even stricter standards for their suppliers. Design teams are well aware of the importance of ensuring their product is EMC compliant. All that said, many do not attempt to perform EMC analysis during design.

There is a perception that EMC analysis during PCB layout can be a time-consuming task that is challenging to set up and properly configure, with difficult-to-interpret results. Historically, the focus of analysis during design has been on Signal Integrity (SI) and Power Integrity (PI). Manual EMC “analysis” typically is performed post-fabrication, based on the results of testing the actual product. What is often overlooked is that implementing EMC analysis during the design phase provides an opportunity to avoid failing EMC compliance testing after fabrication.

Figure 1
EMC analysis implemented during PCB layout

The current generation of ECAD tools offers EMC analysis functionality that is easy to use, with well-documented rule checks that often include an explanation for each principle and advice on how to address issues. Implementing EMC analysis at appropriate points during PCB layout, prior to fabrication, can mitigate the need for redesign(s) that affect both product development cost and overall time to market (Figure 1).

EMC Simplified

EMC can be a confusing topic, especially for new engineers and designers or those not well versed in the subject matter. Furthermore, there is often confusion as to the difference between electromagnetic compatibility (EMC) and electromagnetic interference (EMI). Although this article is not intended to be an in-depth tutorial on EMC and EMI theory, a quick review of the definitions is appropriate.
As previously stated, EMC is generally defined as the ability of a product to function in its environment without introducing electromagnetic disturbance. Specifically, the product must:

• Tolerate a stated degree of interference
• Not generate more than a stated amount of interference
• Be self-compatible

EMI is generally defined as disturbance that affects an electrical circuit, due to either electromagnetic induction or electromagnetic radiation.

To further simplify the two definitions: EMC is how vulnerable the product is to the environment, and EMI is what the product introduces into the environment (Figure 2).

Figure 2
The four basic EMC/EMI coupling mechanisms relative to the source and victim

The complexity of the topic contributes to the perception that implementing EMC analysis during PCB layout can be a time-consuming task that is challenging to set up and properly configure, with results that are difficult to interpret. The alternative, however, foregoing automated in-design analysis and waiting to test the actual product post-fabrication, has the potential to be significantly more time consuming and costly. Although EMC test labs are not required to provide the average EMC testing pass rate, several studies suggest that the first time pass rate is approximately 50%. Furthermore, EMC compliance failure has been cited as the second cause for redesigns in the automotive industry. Given that an EMC failure will require one or more redesigns that affect both product development costs and overall time to market, performing EMC analysis during PCB layout (designing for EMC compliance) is essential.

Left-Shift to Layout

The term “left-shift” within the engineering space is often used to describe the act of moving (or shifting) a task that would normally occur toward a later phase of the design process, to occur also during an earlier phase. . …

Read the full article in the July 336 issue of Circuit Cellar

Mentor | www.mentor.com

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Module Combines Isolated Data Comms and Power in One Device

Murata Power Solutions has announced the introduction of the NMUSB2022PMC, a surface mount powered data isolator module that conveniently provides dual port USB data and power isolation from a single upstream port. When used in conjunction with a USB host, a single NMUSB202PMC module counts as two USB hubs for cascaded applications and provides full 5V/500mA power to each downstream port. 250 VRMS reinforced isolation provides safety, immunity to EMI and breaking of ground loops.

The is NMUSB2022PMC is fully compliant with USB 2.0 specification, which enables hassle-free, “plug and play” operation with any USB-compatible device. It can do automatic switching between full-speed (12 Mbps) and low speed (1.5 Mbps) operation. Operating temperature ranges from -40°C to +85°C.

The device enables USB isolation function with a single SMT component. Users may power any USB compatible device from the NMUSB module. The data isolation function included with the DC-DC module adds value and convenience to the user and also eases system approval for medical systems safety certifications. Applications include industrial control for isolating sensors and medical environments, where USB is becoming common for low cost sensing and communication but isolation is necessary for safety and noise immunity. It is also well suited for harsh environment data communication and sensor communications.

Murata Power Solutions | www.murata-ps.com

LED Lighting Controller for Automotive

Texas Instruments has introduced the first 3-channel high-side linear automotive light-emitting diode (LED) controller without internal MOSFETs that gives designers greater flexibility for their lighting designs. The TPS92830-Q1’s novel architecture enables higher power and better thermal dissipation than conventional LED controllers, and are particularly beneficial for automotive LED lighting applications that require high performance and reliability.

TPS92830-Q1Conventional LED drivers integrate the MOSFET, which limits designers’ ability to customize features. With that type of driver, designers often must make significant design modifications to achieve the desired system performance. The TPS92830-Q1 LED controller’s flexible on-board features give designers the freedom to select the best MOSFET for their system requirements. With this new approach, designers can more quickly and efficiently optimize their lighting power designs for automotive system requirements and desired dimming features.

The on-chip pulse-width modulation (PWM) generator or PWM input enables flexible dimming. Designers can use either the analog control or PWM to manage an output current of more than 150 mA per channel, to power automotive rear combination lamps and daytime running lights.

By pairing the LED controller with an external MOSFET, the designer can achieve the required high power output while distributing the power across the controller and MOSFET to avoid system overheating. By retaining linear architecture, the TPS92830-Q1 provides improved electromagnetic interference (EMI) and electromagnetic compatibility (EMC) performance.

Advanced protection and built-in open and short detection features help designers meet original equipment manufacturer (OEM) system reliability requirements. The output current derating feature protects the external MOSFET under high voltage conditions to ensure system reliability.

Support tools include the TPS92830-Q1 3-Channel High-Current Linear LED Controller evaluation module. Engineers can jump-start automotive lighting systems designs using the TPS92830-Q1 with the EMC Compliant Automotive Daytime Running Light and Position Light Reference Design with LED Thermal Protection. The TPS92830-Q1 is now available through the TI store and authorized distributors. Offered in a thin small outline package (TSOP), it is priced at US$1.96 in 1,000-unit quantities.

Texas Instruments | www.ti.com

Class I and II 500 W Configurable Medical Power Supplies

TDK has announced the XMS500 series of AC-DC power supplies, rated at 500 W output power, with a Class I and Class II (no earth ground connection) construction. The series conforms to curve B conducted and radiated emissions, with a 6 dB margin and has a low leakage current of less than 150 µA. The high operating efficiency and mechanical design enables the XMS500 to operate at full load with airflow rates of just 1 m/s, reducing audible noise. Applications include home healthcare, hospital, imaging and clinical diagnostic systems in addition to industrial, test and measurement and communications equipment.

xms500-pr-nov17-whiteDesigned as a configurable product, engineers can select from multiple standard options, to optimize both system performance and cost, without incurring development charges or minimum purchase quantities. The options include case styles (including an internal low speed fan), a choice of standby voltage, a 12 V fixed or variable speed fan supply, remote on/off, AC fail and single or dual input fuses. 12 V, 24 V, 3 6V or 48 V nominal outputs are offered, with other voltages upon request.

The XMS500 accepts a 90 VAC to 264 VAC input, withstanding 300Vac for five seconds and can operate in ambient temperatures of up to +70°C, derating linearly above 50°C by 2.5% per °C. The open frame version measures 102 mm x 180 mm x 37 mm and the U-channel 107 mm x 180 mm x 39.5 mm. The low profile allows installation in systems where space is limited.

All models are certified to IEC/EN/UL/CSA 60601-1 and IEC/EN/UL/CSA 60950-1 with CE marking for the Low Voltage, EMC and RoHS2 Directives. The series is compliant to EN 55011-B, EN 55032-B conducted and radiated emissions (for both Class I and II), EN 61000-3-2 harmonics, IEC 60601-1-2 Edition 4, and IEC 61000-4 immunity standards.

Input to output isolation is 4,000 VAC (2 x MoPP), input to ground isolation 1,500 VAC (1 x MoPP) and output to ground isolation is 1,500 VAC (1 x MoPP) for suitability in B and BF rated equipment.

TDK-Lambda Americas | www.us.tdk-lambda.com

DC-DC Converter Boasts 4 A, 60 V Internal Switch

Analog Devices, acquired last year by Linear Technology, has announced the Power by Linear LT8364, a current mode, 2 MHz step-up DC/DC converter with an internal 4 A, 60 V switch. It operates from an input voltage range of 2.8 V to 60 V, and is suitable for applications with input sources ranging from a single-cell Li-Ion battery to multi-cell battery stacks, automotive inputs, telecom power supplies and industrial power rails.

LT8364

The LT8364 can be configured as either a boost, SEPIC or an inverting converter. Its switching frequency can be programmed between 300 kHz and 2 MHz, enabling designers to minimize external component sizes and avoid critical frequency bands, such as AM radio. Furthermore, it offers over 90% efficiency while switching at 2 MHz. Burst Mode operation reduces quiescent current to only 9 μA while keeping output ripple below 15 mVp-p. The combination of a small 4 mm x 3 mm DFN or high voltage MSOP-16E package and tiny externals ensures a highly compact footprint while minimizing solution cost.

The LT8364’s 100 mΩ power switch delivers efficiencies of over 95%. It also offers spread spectrum frequency modulation to minimize EMI concerns. A single feedback pin sets the output voltage whether the output is positive or negative, minimizing pin count. Other features include external synchronization, programmable UVLO, frequency foldback and programmable soft-start.

The LT8364EDE is available in a 4 mm x 3 mm DFN-12 package and the LT8364EMSE is available in a high voltage MSOP-16E (4 pins removed for high voltage spacing). Industrial temperature (–40°C to 125°C) versions (LT8364IDE and LT8364IMSE), and high temperature (–40°C to 150°C) versions (LT8364HDE and LT8364HMSE) are also available.

Summary of Features: LT8364

  •     Wide Input Voltage Range: 2.8 V to 60 V
  •     Internal 4 A, 60 V Power Switch
  •     Ultralow Quiescent Current & Low Ripple Burst Mode Operation: IQ = 9 μ A
  •     BIAS Pin for Higher Efficiency
  •     Positive or Negative Output Voltage Programming with a Single Feedback Pin
  •     Programmable, Synchronizable Frequency: 300 kHz to 2 MHz
  •     Optional Spread Spectrum Frequency Modulation for Low EMI
  •     Thermally Enhanced 12-Lead 4 mm × 3 mm DFN & High Voltage Pin Spacing Version of 16-Lead MSOP packages

Pricing for the LT8364 in 1,100s starts at $3.25.

Linear Technology | www.linear.com

January Circuit Cellar: Sneak Preview

The January issue of Circuit Cellar magazine is coming soon. And it’s got a robust selection of embedded electronics articles for you. Here’s a sneak peak.

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Here’s a sneak preview of January 2018 Circuit Cellar:

 

                                     IMPROVING EMBEDDED SYSTEM DESIGNS

Special Feature: Powering Commercial Drones
The amount of power a commercial drone can draw on has a direct effect on how long it can stay flying as well as on what tasks it can perform. Circuit Cellar Chief Editor Jeff Child examines solar cells, fuel cells and other technology options for powering commercial drones.

CC 330 CoverFPGA Design: A Fresh Take
Although FPGAs are well established technology, many embedded systems developers—particularly those used the microcontroller realm—have never used them before. In this article, Faiz Rahman takes a fresh look a FPGAs for those new to designing them into their embedded systems.

Product Focus: COM Express boards
COM Express boards provide a complete computing core that can be upgraded when needed, leaving the application-specific I/O on the baseboard. This brand new Product Focus section updates readers on this technology and provides a product album of representative COM Express products.

TESTING, TESTING, 1, 2, 3

LF Resonator Filter
In Ed Nisley’s November column he described how an Arduino-based tester automatically measures a resonator’s frequency response to produce data defining its electrical parameters. This time he examines the resultsand explains a tester modification to measure the resonator’s response with a variable series capacitance.

Technology Spotlight: 5G Technology and Testing
The technologies that are enabling 5G communications are creating new challenges for embedded system developers. Circuit Cellar Chief Editor Jeff Child explores the latest digital and analog ICs aimed at 5G and at the test equipment designed to work with 5G technology.

                                     MICROCONTROLLERS IN EVERYTHING

MCU-based Platform Stabilizer
Using an Inertial Measurement Unit (IMU), two 180-degree rotation servos and a Microchip PCI MCU, three Cornell students implemented a microcontroller-based platform stabilizer. Learn how they used a pre-programmed sensor fusion algorithm and I2C to get the most out of their design.

Designing a Home Cleaning Robot (Part 2)
Continuing on with this four-part article series about building a home cleaning robot, Nishant Mittal this time discusses the mechanical aspect of the design. The robot is based on Cypress Semiconductor’s PSoC microcontroller.

Massage Vest Uses PIC32 MCU
Microcontrollers are being used for all kinds of things these days. Learn how three Cornell graduates designed a low-cost massage vest that pairs seamlessly with a custom iOS app. Using the Microchip PIC32 for its brains, the massage vest has sixteen vibration motors that the user can control to create the best massage possible.

AND MORE FROM OUR EXPERT COLUMNISTS:

Five Fault Injection Attacks
Colin O’Flynn returns to the topic of fault injection security attacks. To kick off 2018, he summarizes information about five different fault injection attack stories from 2017—attacks you should be thinking about as an embedded designer.

Money Sorting Machines (Part 2)
In part 1, Jeff Bachiochi delved into the interesting world of money sort machines and their evolution. In part 2, he discusses more details about his coin sorting project. He then looks at a typical bill validator implementation used in vending systems.

Overstress Protection
Last month George Novacek reviewed the causes and results of electrical overstress (EOS). Picking up where that left off, in this article he looks at how to prevent EOS/ESD induced damage—starting with choosing properly rated components.

December Circuit Cellar: A Sneak Preview

The December issue of Circuit Cellar magazine is coming soon. Want a sneak peak? We’ve got a great selection of excellent embedded electronics articles for you.

Not a Circuit Cellar subscriber?  Don’t be left out! Sign up today:

 Here’s a sneak preview of December Circuit Cellar:

MICROCONTROLLERS IN MOTION

Special Feature: Electronics for Wearable Devices
Circuit Cellar Chief Editor Jeff Child examines how today’s microcontrollers, sensors and power electronics enable today’s wearable products.

329 Cover Screen CapSimulating a Hammond Tonewheel Organ
(Part 2)

Brian Millier continues this two-part series about simulating the Hammond tonewheel organ using a microcontrollers and DACs. This time he examines a Leslie speaker emulation.

Money Sorting Machines (Part 1)
In this new article series, Jeff Bachiochi looks the science, mechanics and electronics that are key to sorting everything from coins to paper money. This month he discusses a project that uses microcontroller technology to sort coins.

Designing a Home Cleaning Robot (Part 1)
This four-part article series about building a home cleaning robot starts with Nishant Mittal discussing his motivations behind to his design concept, some market analysis and the materials needed.

SPECIAL SECTION: GRAPHICS AND VISION

Designing High Performance GUI
It’s critical to understand the types of performance problems a typical end-user might encounter and the performance metrics relevant to user interface (UI) design. Phil Brumby of Mentor’s Embedded Systems Division examines these and other important UI design challenges.

Building a Robotic Candy Sorter
Learn how a pair of Cornell graduates designed and constructed a robotic candy sort. It includes a three degree of freedom robot arm and a vision system using a Microchip PIC32 and Raspberry Pi module.

Raster Laser Projector Uses FPGA
Two Cornell graduates describe a raster laser projector they designed that’s able to project images in 320 x 240 in monochrome red. The laser’s brightness and mirrors positions are controlled by an FPGA and analog circuitry.

ELECTRICITY UNDER CONTROL

Technology Spotlight: Power-over-Ethernet Solutions
Power-over-Ethernet (PoE) enables the delivery of electric power alongside data on twisted pair Ethernet cabling. Chief Editor Jeff Child explores the latest chips, modules and other gear for building PoE systems.

Component Overstress
When an electronic component starts to work improperly, Two likely culprits are electrical overstress (EOS) and electrostatic discharge (ESD). In his article, George Novacek breaks down the important differences between the two and how to avoid their effects.

AND MORE FROM OUR EXPERT COLUMNISTS:

Writing the Proposal
In this conclusion to his “Building an Embedded Systems Consulting Company” article series, Bob Japenga takes a detailed look at how to craft a Statement of Work (SOW) that will lead to success and provide clarity for all stakeholders.

Information Theory in a Nutshell
Claude Shannon is credited as one of the pioneers of computer science thanks to his work on Information Theory, informing how data flows in electronic systems. In this article, Robert Lacoste provides a useful exploration of Information Theory in an easily digestible way.

High Electron Mobility Transistors

gold backgroundThe TPH3002LD and the TPH3002LS are 600-V Gallium nitride (GaN)-based, low-profile power quad flat no-lead (PQFN) high electron mobility transistors (HEMTs). The HEMTs utilize Transphorm’s patented, high-performance EZ-GaNTM technology, which combines low switching and conduction losses, reducing the overall system energy dissipation up to 50%.

The TPH3002PD and TPH3002PS HEMTs are designed for use in smaller, lower-power applications (e.g., adapters and all-in-one computer power supplies). The HEMTs feature a Kelvin connection to isolate the gate circuit from the high-current output circuit to further reduce electromagnetic interference (EMI) and high-frequency switching capabilities.
Evaluation boards are available with the devices.

Contact Transphorm for pricing.

Transphorm, Inc.
www.transphormusa.com

Electromagnetic Compliance Protection (EE TIP #115)

Electromagnetic compatibility (EMC) compliance is one of the last processes before a device may be released to the public. EMC goes hand-in-hand with electromagnetic immunity (EMI), but immunity is only needed for critical devices. With EMC, it is very important to find a good EMC company to deal with. With most circuits, the weak point for EMC is any external leads. By adding a few inexpensive parts, EMC protection can be added and the EMC filtering can be adjusted by changing the values of the parts. In the figure below, the 1 µH inductors act as chokes to block any external voltage spikes above a certain frequency. The 1 nF capacitor also acts as a shock absorber to reduce any sharp voltage spikes. Effectively, this is a second-order filter and the cutoff frequency may be reduced by increasing the inductance and capacitance.

EMC ProtectionEditor’s Note: This EE Tip was written by Fergus Dixon of Sydney, Australia. Dixon, who has written two articles and an essay for Circuit Cellar, runs Electronic System Design, a website set up to promote easy to use and inexpensive development kits. Click here to read his essay “The Future of Open-Source Hardware for Medical Devices.”

Reduce EMI on a Micro (EE Tip #109)

Electromagnetic interference (EMI) on a typical microprocessor board is related to the clock. If the clock is a square wave, it contains frequencies at the clock frequency and harmonics. A perfect square wave clock would have harmonic frequencies at f, 3 × f, 5 × f, 7 × f, and so on. For a perfect square wave, or any string of pulses with a fast rise time, the strength of the harmonics declines inversely with frequency.

So, the eleventh harmonic would be one-eleventh as strong as the fundamental frequency. This corresponds to a decline in harmonic amplitude of 20 dB per decade.

Real time clocks are not perfect square waves, and pulses do not have infinitely fast rise times. As a result, the higher harmonics of any real waveform start dropping faster than 1/n at higher frequencies, generally dropping as 1/(n2), or 40 dB per decade, after the frequency is high enough.

You can see this in Figure 1. The antenna efficiency of PC board structures or cables increases 20 dB per decade as frequency increases and wavelength gets shorter and closer to the size of structures found on typical PC boards.

Figure 1—Here you can see the sources of EMI in a typical microprocessor and the resulting spectrum.

Figure 1: Here you can see the sources of EMI in a typical microprocessor and the resulting spectrum.

As a result, the beginning part of the radiated spectrum tends to be uniform, the 20 dB per decade decline in harmonic strength being balanced by the 20 dB per decade increase in antenna efficiency, until a high enough frequency is reached where the curve takes a bend and harmonics start declining at 40 dB per decade zone (see Figure 1).

Above this frequency, the radiated spectrum starts declining by 20 dB per octave. But, the amplitudes of the real harmonics of a real device are often quite irregular because of resonances that weaken some and reinforce others.

What is not usually understood is that the biggest source of EMI is not the clock directly, but a train of pulses generated on both edges of the clock when current surges into the microprocessor for a nanosecond or two when the clock transitions up or down. This pulse train has a frequency that’s double the clock frequency. It seeps out of the processor chip into the power supplies and generally infects the board with high-frequency EMI. It also gets into the output lines emanating from the processor package; therefore, it’s further spread around the board and to cables and devices connected to the board.

The current surges on both clock edges are related to the clock tree. The clock tree is a system consisting of a branching network of buffers that distribute the internal clock around the silicon die. Because these buffers drive considerable capacitance and have both polarities of the clock present, there is a surge of current on both edges of the clock. This occurs as current flows into the chip to charge up the capacitance in the part of the clock tree that is transitioning from 0 V to the power supply voltage. On-chip devices, such as flip-flops, also contain internal gates and buffers where both polarities of the clock are present and contribute to the current surge.

An additional current surge is related to the crossover current when both the N and P transistors in a CMOS buffer are momentarily conducting during a logic transition. The silicon chip tries to suck in the required current to service these fast transients through its power supply pins. However, these connections have inductance created by the bond wires and lead frame, so the voltage drops briefly on the die, creating an on-chip power supply voltage drop with an amplitude on the order of a few tenths of a volt and the duration of a nanosecond or so.

If this same on-chip power supply drives the output buffers that carry signal lines out of the chip, these lines will also be infected with the fast pulses present in the power and ground supplies. This is because the power supply noise is directly transmitted through the buffer power inputs to the output lines. The on-chip current surges create fast noise that passes out through the power supply pins to the power and ground planes on the PC board, further spreading the infection.

The amplitude of the harmonics of the periodic noise pulses, at least at lower frequencies, declines inversely with frequency (1/f). Unfortunately, the effectiveness of a short antenna, such as a PC board trace, increases directly with frequency (~f). The result is that the radiated EMI tends to be flat across the spectrum.

Fortunately, the amplitude of the harmonics starts declining more rapidly than 1/f; it’s more like 1/(f2) at some higher frequency determined by the finite rise time of the pulses in the pulse train. The balance of these countervailing effects is such that the most trouble is often found in the area of 100 to 300 MHz for lower-speed 8- and 16-bit microprocessor boards.

Decoupling capacitors and the intrinsic capacitance of the power and ground planes can be used to short circuit or filter noise on the power supply. However, this technique loses effectiveness above 100 MHz, because the decoupling capacitors have inductance of about 1 nH, giving an effective resistance of about 0.5 Ω at 200 MHz. The large currents involved will develop millivolt-level voltages across such capacitors.

REDUCTION TRICK #1

The problem of noise on the I/O lines of a processor can be addressed with two sets of power supply pins. One set is used for the processor core; the other is for the output drivers that are located in the I/O ring on the periphery of the die (see Figure 2).

Figure 2: The connection of separate power and ground pins for the core and I/O ring of a processor is shown here. A PC board filter blocks core noise from power planes. You can also see how I/O buffers spread power supply noise.

Figure 2: The connection of separate power and ground pins for the core and I/O ring
of a processor is shown here. A PC board filter blocks core noise from power planes.
You can also see how I/O buffers spread power supply noise.

If the I/O buffers are supplied with the same power that is made dirty by the fast transients in the processor core, every output pin of the processor will spread EMI. The EMI that tries to come out of the power pins for the core can be blocked by a combination of decoupling capacitors and PC board trace inductance. This keeps the PC board power planes a relatively clean source of power for the processor I/O ring. The design team figured this feature decreases EMI amplitudes by 10 dB, which is a factor of three in EMI electrical field strength measured by the prescribed calibrated antenna. This is a lot because it’s common to flunk the tests by 5 dB.

REDUCTION TRICK #2

Most microprocessors have I/O and memory devices connected to the same bus with distinct control signals for the devices. Generally, there is a lot more activity at a higher frequency for the memory devices. For instance, a Digi International Rabbit 3000 microprocessor has an option to use separate pins for memory and I/O devices, both address and data. The advantage is that the physical scope of the high-speed memory bus is limited to the memory devices. A separate address and data bus handles I/O cycles and has a much lower average operating frequency. In particular, the address lines toggle only during I/O bus cycles, greatly limiting the emissions from the I/O bus. This avoids the situation where the fast-toggling address and data lines of the memory bus have to be run all over the printed circuit board of a large system. This scheme also limits the capacitive loading on the memory bus, which does not have to extend to numerous I/O devices.

REDUCTION TRICK #3

A line spectrum is the spectrum generated by a square wave clock or by a train of short pulses. All of the energy is concentrated in a narrow spectral line at the harmonic frequencies.

When the FCC EMI measurement tests are performed, the spectrum analyzer measures the amplitude of the signal from a 120-kHz wide filter that is swept across the frequencies of interest. With a line spectrum, all of the energy in a single line passes through the filter, resulting in a strong signal. If the energy in the line could be spread out over a wider frequency, say 5 MHz, only one-fortieth the energy would pass through the 120-kHz wide filter, considerably reducing the reading (by 16 dB in amplitude for one fortieth of the energy). This is what a clock spectrum spreader does. It modulates the clock frequency by a little so as to smear out the spectral line in frequency.

The idea to do this for the purpose of reducing EMI was patented by Bell Labs in two patents during the 1960s. There are numerous ways to modulate the clock frequency. One method is to use a voltage-controlled oscillator and phase-lock loop so that the frequency sweeps back and forth at a low modulation rate (e.g., 50 kHz).

Another method is to insert random delays or dithers into the clock. These methods are all covered in the original Bell Labs patents. The Bell Labs people were probably interested in EMI because telephone switches involve a large amount of equipment in a small space. In addition, it’s conceivable that the early computerized switches suffered from EMI problems. We installed a clock spectrum spreader in the Rabbit 3000 based on a combination of digital and analog techniques. The spectrum spreader reduces FCC-style EMI readings by around 20 dB, which is a lot.

A control system makes sure that the modulated clock edge is never in error by more than 20 ns compared to where the clock edge would be if it were not modulated. This prevents disruption in serial communications or other timing functions. For example, a UART operating at 460,000 bps can tolerate about 500 ns of clock edge error before it will be near to generating errors. This is far less than our 20-ns worst error in clock edge position.—Circuit Cellar 146, Norman Rogers, “Killing the EMI Demon,” 2002.

This piece originally appeared in Circuit Cellar 146, 2002. Author: Norman Rogers, who was President of ZWorld, Inc. and Rabbit Semiconductor.