practical shielding, emc/emi, noise reduction, earthing and circuit board layout

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Practical Shielding, EMC/EMI, Noise Reduction, Earthing and Circuit Board Layout


Coupling Paths: Sources & Victims

Source Equipment

Victim Equipment

Peripheral

Input

External mains interference

Radiated, case to mains cable

Radiated,

cable to cableRadiated, case to case

Conducted through

common earthimpedance

Conducted via common mains

impedance


Universal Interference Model

Source(Culprit)

Receptor(Victim)

Coupling Mechanism

(Interference Path)


Interference Solution

• Remove or reduce noise at source

(decouple, shield, low noise design, etc.)

• Remove or attenuate coupling path

(spacing, shielding, filter, re-route, isolate, etc.)

• Improve victim immunity

(decouple, shield,filter, high immunity design, etc.)


Sources RF Transmitters - Communication, Radar, Telemetry, Mobile Phones, X-ray

equipment etc. (Anything that generates RF)

Receivers - Local Oscillators, Computers & Peripherals

(Anything that has a clock or phase locked loop)

Motors, Switches, Power Lines, Fluorescent Lights, Engine Ignition, Switch

Mode Power Supplies etc.

(Anything that switches current)

Arc Welders, Relays, Motors etc.

(Anything that causes arcs)

Natural - Atmospheric, Lightning, Static, Galactic Noise

(Almost anything out there)


Coupling MediaSpace Separation

Shielding Materials

Absorptive Materials

Cables

Ground Inter Coupling

Filters & Circuits

Power Lines


Receptors RF, Communication, Radar, Telemetry Receivers etc.

(Anything intended for RF reception or using tuned circuits)

Digital Electronics, Software

(Anything that can be upset by shifting logic levels, timing or clock

disturbances, memory or data line toggling etc.)

Analog Electronics

(Any sensitive circuit that can amplify, rectify, saturate, shift levels etc.)

Sensitive Materials - Ammunition, Fuel

(Anything at risk of burning, exploding)

Human Beings - biological hazard

(Anything living)


Coupling Mechanisms

• Common Impedance (Conducted)

• Capacitive (Electrostatic)

• Inductive (Magnetic)

• Electromagnetic (Radiated)


Conductive Connection(Common Impedance Coupling)

System BSystem A

System B

V Connection has inductance, L

Vin

Load Input

System B input = (Vin+V)

where V~ -L.d IL /dt

System A

IL

Input

Vin

Load

IL

Problem

Solution


Magnetic Induction

Load

System A

IL

System B

Vin

Rs

Zin

Vn

Mutual Inductance, M


Electrostatic Coupling

Vin

System B

Load

System A

IL

Rs

Zin

Vn

Stray or parasiticcapacitance


Real World Coupling

Noise Source

MagneticInduction

Victim

Electrostatic(Capacitive)

Coupling

Posible Ground Loop orCommon Impedance Coupling


Mutual C & L vs Lead Spacing

0,05

0,10

0,15

0,20

0,25

0,5

0,75

1,00

1,250,25

D D

1 mm1,6 mm

1 8040201082 4 100

D (mm)

C(pF/cm)

M(nH/cm)

Mutual

Capacitance

Mutual Inductance


Coupling via the supply network

SOURCE VICTIM

50

ohms 50 µH

10

20

30Attenuation

dB/kmDistribution system

Cable only


Electromagnetic fields

V

D (m)

E-fieldI

R (m)

H-field

Propagation

V

E-field

H-field


Rayleigh/Maxwell near/far fieldsE 1/r3, H 1/r2

Electric field predominates

H 1/r3, E 1/r2

Magnetic field predominates

1 100,110

1 k

100

10 k

Zwave

Distance from source, normalised to / 2

Far field

Transition area

Plane wave

Z = 377 ohms

Near field impedance anywhere in this region

Frequency Max dimensionD (m)

Rayleigh

d = 2D2/ (m)

Maxwell

d = /2 (m)10 MHz30 MHz

0,5

2

20,5

20,5

2

100 MHz

300 MHz

1 GHz

0,80,1672,670,58,0

1,67

0,267 4,77

0,4770,4770,1590,159

0,0477

1,59

Rayleigh & Maxwell

distances for

transition to

far field


Radiated Coupling Modes

Differential mode

Antenna mode

Common mode


Radiated Emissions from PCB(Differential Mode)

Signal current

Loop of area ‘A’


Cable Radiated Emissions(Common Mode)

Ground noise voltage


Coupling Paths - Conducted Emissions

SMPS L

N

E

CIRCUIT

Signal cable

Measurement

Measurement

CS CS

ICME

0 V

IDM

VNsupply

CC


Susceptibility to Radiated Field Coupling

E

Possible standing wave in enclosure:creates susceptibility/emission peaks

Victim

Field coupling to cable induces common mode

current at input

Field coupling to PCB induces

differential mode currents in circuit


Transient Sources

• Electro Static Discharge (ESD)

• Lightning

• Switching

• Power


TransientsLightning, Switching & ESD

10%

50%

90%

< T1 >

T2

T1/T2 wave shape


Transient Frequency

Type of

areaTransients/

hour

Industrial

Commercial

Domestic

Laboratory

17,5

2,8

0,6

2,3


Electrostatic Discharge

Coupling paths likely to be:

• Stray capacitance

• Case bonding

• Track or wiring inductance due to magnetic fields generated in the discharge


Automotive Transients

Alternator load dump

Inductive switching

Alternator field decay

100 ms 200 ms 300 ms

14 V

80 V

VP

-0,2.VP

14 V

-80 V

5 ms 10 ms 15 ms

10 µs 20 µs 30 µs


Supply Voltage Phenomena


Supply voltage phenomena - important?

1 ms 3 ms 20 ms 500 ms 10 s

500%

400%

300%

200%

140%120%

70%40%

0,2 ms


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practical shielding, emc/emi, noise reduction, earthing and circuit board layout

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