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How to Avoid EMC Problems in PCB Design?

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EMC Basics• EMC Defnition

 – the ability of devices and systems to operate in theirelectromagnetic environment without impairing theirfunctions

Main elements to EMC – Emissions: 

•  !he EM" emissions refer to the generation of unwantedelectromagnetic energy#

•  !hese need to be reduced below certain acceptablelimits to ensure they do not cause any disruption toother e$uipment#

 – Susceptibility & immunity: •  !he susceptibility of an item of electronics to EM" is the

way it reacts to unwanted electromagnetic energy#•  !he aim of the design of the circuit is to ensure a

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Elements of Electromagneticinterference

• A generator is an E&E devicethat produces EM"

• A receptor is an E&E devicethat receives or couples inEM"

•

A coupling path allows EM"from the generator toproduce an undesiredresponse in the receptor

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 !hree Main 'ays toMinimi(e EM"

• Reducing the noise rom the generator

 – reducing the generator circuit area – using slower digital logic

 –  reducing the circuit current

 – )ltering* and&or shielding#

• Altering the coupling path

 – moving the receptor away from the generator

 – or shielding the receptor or generator

• Improving the immunity o the receptor

 – reducing the receptor circuit area

 – using less susceptible electronic components

 –

changing the impedance of the circuit

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+uidelines to address EMC issuesduring the PCB design

•  !he following guidelines will describe how hardwareengineers can address EMC issues during the PCB design

,# +round planes

-# Component .egregation

/# Board 0ayers

1# Digital Circuits

2# Cloc3 !ermination

4# Analog Circuits

5# Decoupling Capacitors

6# Cables

7# Crosstal3

,8#.hielding

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,#+round Planes•

Ma9imi(ing the ground areaon a PCB reduces theinductance of ground in thesystem* which in turnreduces electromagnetic

emissions and crosstal3#

• A poor PCB design is onewhere components are

connected randomly toground points#

• A recommended designapproach is to have a full

ground plane because it

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-#Component .egregation

• Components on the PCB needto be grouped according totheir functionality* such asanalog* digital* power supplysections* low:speed circuits*

high:speed circuits* and so on#

•  !he trac3s for each groupshould stay in their designatedarea

• ;or a signal to

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/#Board 0ayers• "f more than two layers are

used* then one completelayer should be used as aground plane#

•  !he layer below the groundlayer should be used as apower plane#

•

"f a complete layer ofground is not possible* thenground grids should beused#

• "f a separate power plane isnot used then round

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1#Digital Circuits

•  !races connecting cloc3 andhigh speed signals should be3ept as short as possible andbe ad=acent to the groundplane to 3eep radiation and

crosstal3 under control#

•  !hese signals must also be3ept away from the powerplane since they can induce

noise on the power plane aswell#

•  !he crystal should also be3ept close to the appropriatechips#

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2#Cloc3 !ermination

•  !races carrying cloc3 signalsfrom a source to a devicemust have matchingterminations because

whenever there is an

impedance mismatch* a partof the signal gets reective termination*

including sourcetermination* endtermination* ac termination*etc

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4#Analog Circuits

•  !races carrying analog signalsshould be 3ept away fromhigh:speed or switchingsignals and must always beguarded with a ground signal#

• A low:pass )lter shouldalways be used to get rid ofhigh:fre$uency noise coupledfrom surrounding analogtraces#

•  !he ground plane of analogand digital subsystems should

not be shared#

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5#Decoupling Capacitor

• A decoupling capacitorprovides a low:impedancepath for high:fre$uencycurrent on the powerplane to ground#

•  !his path should be 3eptto a minimum possiblelevel by placing a

decoupling capacitor veryclose to the "C

• Multiple capacitors withlow E.0 e$uivalent seriesinductance@ should be

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6#Cables•

Cables carrying digitalsignals act as e%cientantenna

• sing a twisted:pair cablehelps 3eep coupling to a

low level by cancelling anyinduced magnetic )elds

• 'hen a ribbon cable isused* multiple ground

return paths must beprovided

• ;or high:fre$uency signals*shielded cable must beused* where the shielding

is connected to ground

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7#Crosstal3

• Crosstal3 is a function ofmutual inductance mutual capacitance

•

Crosstal3 caused bymutual inductance istypically larger than thecrosstal3 caused bymutual capacitance#

• Mutual inductance canbe reduced byincreasing the spacingbetween the two tracesor by reducing the

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,8#.hielding

• A mechanical approach toreducing EMC

• .hield reduces the si(e of

loop antennas byabsorbing and re

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 !han3 ou