filters & ferrites

More effective EMI reduction techniques forhigh-demand consumer applications

Innovative drop-in SSC solutions effectively manage EMCpreventing costly redesign, reducing production time, and

valuable board real

NARENDAR "BUDDY"VENUGOPALPulseCore SemiconductorCampbell, CA

As technology advances becomeeven more sophisticated and applica-

tions become increasingly streamlined,traditional methods of EMI reduction arenot providing sufficient results for meet-ing regulatory standards and productionrequirements. Product engineers who de-sign a full spectrum of high-performance,high-demand consumer electronicsapplications are turning to innovativealternatives to overcome these obstacles.

REDUCING EMI WITHTRADITIONAL MEASURESWhile passive measures of reducingEMI are popular in numerous consumerproduct applications, they tend to com-promise the signal integrity of the clockand other switching signals. Such mea-sures include placing physical resistors,inductors, ferrite beads, and capacitorson the signal path to attenuate any low-pass filter signals. Other methods suchas shielding and other mechanical bandaids, add cost and weight. On the otherhand, spread spectrum clocking (SSC) isan active approach to EMI reduction thatattacks EMI at the source—a pulsatingclock that generates peaks of electro-magnetic energy. This technique involvesmodulating (increasing and decreasing)the frequency of a clock signal over a pe-riod of time thereby "spreading" the elec-

tromagnetic energy generated by theseswitching signals over a selected band ofthe frequency spectrum, a process thatsmoothes over the energy spikes of thefrequencies and harmonics. This reducespeak emissions to levels that comply withFCC or other regulatory requirements.This technique is particularly effectivewhen applied to the master or sourceclocks in systems as it provides the spreadspectrum benefit to all switching signalsderived from these master clocks.

Despite all these advantages overpassive solutions, traditional spreadspectrum measures do have several draw-backs. For instance, it is highly likely thatarchitectural changes will be needed toaccommodate these techniques if theyare applied at the source. One exampleis the need for deep FIFOs or extra pinson an ASIC to ensure that functionalityis not compromised. Maintaining clockand data synchronization is a seriouschallenge that can present itself whenthese techniques are applied directly toselected independent sub-system clocks.Also, current spread spectrum methodscannot be used in timing-sensitive ap-plications, such as USB.

SPREAD SPECTRUM CLOCKINGGOES FURTHERLimiting or confining the variations insignal frequencies to defined regions cor-responding to clock edges can eliminatethe potential misalignment of clock anddata windows. This design option solves

96 INTERFERENCE TECHNOLOGY EMC DIRECTORY S. DESIGN GUIDE 2009

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MORE EFFECTIVE EMI REDUCTION TECHNIQUES

SS Disabled Locked andSynchronized

Figure 1. Consider the waveform illustrated in two spread spectrum

devices, in which A is a standard spread spectrum device and B is a

device that employs true drop-in spread spectrum technology. When

disabled, both devices lock synchronously to the input signal.

SS Enabled

Input

Unlocked andUn-synchronizec

Figure 2. Now, consider the same two devices with spread spectrum

enabled. The output waveform of device A, the conventional spread

spectrum device, is completely out of synchronization with the input.

On the other hand, the waveform of device B, which employs a new

technology, is synchronized, in phase, and exhibits an extremely well-

controlled spread spectrum modulation.

a major drawback of traditional spread spectrum EMIreduction, and there are innovative technologies thateffectively reduce the peak amplitudes of clock signalswhile maintaining synchronization throughout thesystem and protecting data integrity—thus, eliminatingthe possibility of cycle slip, which is detailed above. Atthe same time, Federal Communications Commission(FCC) compliance can be achieved using this type oftechnology.

ELIMINATING CYCLE SLIPWhile faster data rates have made FCC compliance

98 INTERFERENCE TECHNOLOGY EMC DIRECTORY & DESIGN GUIDE 2009

VENUGOPAL filters & ferrites

a challenge, spreading the mas-ter clock at its origin is the firstchoice in meeting FCC standardsand offers an ideal approach. Thismethod keeps the data and theclock synchronized throughout thesystem. Often, the master clock getsdistributed to multiple subsystems.Several subsystems could functionwell with spread spectrum appliedto the master clock. In certain de-signs, however, subsystem clocksmay require independent spreadingof EMI. In such cases,the data and the clockcan lose synchroniza-tion and can producea cycle slip.

S t i l l , t h e r e a r eICs that can providespread spectrum clockmodulation for EMI ^^^^^^_reduction without theassociated cycle slip. These devicesemploy a unique proprietary, patent-pending technology, and they can beinserted directly into the subsystem

clock path—providing true drop-in solutions to reduce subsystemEMI on any clock path. These EMIreduction ICs are flexible and canbe added at any point in the designprocess—early or late—to achieveFCC compl iance with minimalredesign. See Figures 1 and 2 for acomparison of results from a devicethat employs conventional spreadspectrum clocking and a seconddevice that employs true drop-inspread spectrum technology.

Despite all theseover passive solutions, traditional

spread spectrum measures dohave several drawbacks.

LOWERING PEAK EMI LEVELSIN AIM LCD TV APPLICATIONLCD TVs are one example of an ap-plication that benefits from drop-in

spread spectrum EMI reductiontechnology. With high-definitionvideo increasingly in demand, LCDTV sales are on the rise. Sales areexpected to skyrocket in the UnitedStates when all broadcast televisionstations end analog signals and beginbroadcasting only in digital in June2009. LCD TV manufacturers arepreparing to take advantage of thisopportunity by offering bigger andbetter LCD TVs. Unfortunately, anincrease in size and performance

also produces moreEMI. If EMI levels donot meet FCC require-ments and a systemredesign is required atthe end of a product'sdesign cycle, produc-tion time and costs

^^^_^^__ could increase signifi-cantly.

When one leading LCD TV manu-facturer's product was nearing theend of its production cycle and didnot achieve FCC compliance, the

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interferencetechnology.com INTERFERENCE TECHNOLOGY 99

filters S ferrites MORE EFFECTIVE EMI REDUCTION TECHNIQUES

manufacturer needed an expedientsolution. Designers needed to addressthe problem of EMI originating froma 61-MHz low-voltage differentialsignaling (LVDS) clock. Previously,magnetic-shielding rings had beenused to reduce the EMI at the LVDScable. Increasing the number of ringswas considered as a possible solution,but this would have increased pro-duction time and costs—and even thesize of the cable itself. At that point inthe production cycle, this was time,money, and space the manufacturercouldn't afford to lose.

Applying the drop-in 1C at theLVDS clock dramatically loweredpeak EMI levels during subsequentcompliance testing. This new tech-nology enabled the manufacturer tomeet FCC standards without addingmore magnetic-shielding material.The manufacturer was even able toreduce the use of this material by 75percent, a change that saved valuableprinted circuit board real estate andadditional production costs.

CONCLUSIONDrop-in spread spectrum technol-ogy has dramatically changed theway product engineers can applyspread spectrum modulation tech-niques to a variety of platformsand applications. By ensuring thattiming-sensitive interfaces andIP blocks within systems are notimpacted by the altered clock, thisflexible and compact technologycan be used easily and effectivelyin systems that were once ill-suitedto SSC because of likely perfor-mance degradation and functionalfailures. This technology is wellsuited to several applications andinterfaces, such as USB, HDMI, andLVDS, among others. Designers cannow confidently push the limits ofsystem performance while comfort-ably meeting the growing demandsfor cutting-edge consumer ap-plications using this true drop-insolution.

NARENDAR "BUDDY" VENUGOPAL is a

Founder, Chief Technology Officer and Vice

President/General Manager of PulseCore

Semiconductor. Buddy has 20 years of mixed-

signal 1C design, operations, marketing, and

management experience. He holds graduate

degrees in electrical engineering and applied

physics from the Indian Institute of Science

in Bangalore, India, and the University of

California, San Diego and undergraduate

degrees in physics and electronics.

PulseCore Semiconductor is a leading

provider of standard and custom high-speed,

low-power analog and mixed-signal silicon

solutions for EMI reduction, clocking, power

management, and system monitoring. •

MORE ON OUR WEBSITE

I Spread spectrum clocking is anincreasingly popular techniquefor forestalling interference.Design concerns involving this andother techniques are discussed inpapers found in articles archivedon lnterferenceTechnology.com atwww.interferencetechnology.com/articles.htmi.

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