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EMC Basics
concepts
Summary
1. Basic Principles
2. Specific Units
3. Fourier Transform
4. Emission Spectrum
5. Susceptibility Threshold
6. Notion of margin
7. Impedance
8. Conclusion
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EMC environment
The “EMC” way of thinking
Electrical domain
Electromagnetic domain
Voltage V (Volt)
Electric Field E (V/m)
Current I (Amp)
Magnetic field H (A/m)
Impedance Z (Ohm)
Characteristic impedance Z0 (Ohm)
Z=V/I
Z=E/H
P=I2 x R (watts)
P=H2 x 377 (watts/m2)
far field conditions
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Specific units
How do we present EMC results?
Why in frequency domain (Hz) ?
• Time domain aspect is dominated by the major frequency harmonics
• Distinguish contributions of each harmonics, even small ones
Why in logarithm scale (dB) ?
• Signals are composed of high and low amplitude harmonics
• Very large dynamic (from µV to several mV)
• Logarithm scale is requested
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Specific units
Emission and susceptibility level units
Milli
Volt
Voltage Units
Volt
dBV
Wide dynamic range of signals in EMC
→ use of dB (decibel)
100
40
1
60
10
20
0.1
40
1
0
0.01
20
0.1
-20
0.001
0
0.01
-40
0.0001
-20
0.001
-60
0.00001
-40
For example dBV, dBA :
dBV  20 logV 
dBA  20 log A
Extensive use of dBµV
 V 
  20 logV   120
VdBµV  20 log
 1µV 
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dBµV
Specific units
Emission and susceptibility level units
Power Units
The most common power unit is the “dBm” (dB milli-Watt)
PdBmW
 P 
 10 log W   10 logPW   30
 1m W 
Power
(Watt)
Power
(dBm)
1 MW
90
1 KW
60
1W
30
1 mW
0
1 µW
-30
1 nW
-60
Exercise: Specific units
1 mV = ___ dBµV
1 W = ___ dBm
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Specific units
Fourier transform: principle
Volt
dB
Time
Time domain measurement
Freq (Log)
Fourier transform
Frequency measurement
Invert Fourier transform
Spectrum analyser
Oscilloscope
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Specific units
Fourier transform
Why Frequency domain is so important ?
Time domain
Frequency domain
Only high level harmonics contribution
appears
Contribution of each harmonic appears
Low level harmonics
contribution
User’s specification
FFT
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Emission spectrum
Emission level has to be lower than customer specification
Parasitic emission
(dBµV)
EMC compatible
80
Specification for
an IC emission
70
60
Aggressor IC
50
Measured
emission
40
30
20
10
Radiated emission
0
-10
1
10
100
1000
Frequency (MHz)
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Emission spectrum
Low parasitic emission is a key commercial argument
FM
dBµV 100
80
RF
Not EMC compliant
Supplier
A
Customer's
specified limit
60
40
20
0
10
GSM
Supplier
B
EMC compliant
100
Frequency(MHz)
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1000
Susceptibility spectrum
Immunity level has to be higher than customer specification
Immunity level
(dBmA)
50
Specification for
board immunity
Current injection limit
40
30
Measured
immunity
20
10
Victim IC
0
-10
A very low energy
produces a fault
-20
-30
-40
1
10
100
1000
Frequency (MHz)
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Notion of margin
What is a margin ?
Parasitic emission (dBµV)
Nominal Level
Safety margin
•
To ensure low parasitic emission ICs
supplier has to adopt margins
Process dispersion
Measurement error/dispersion
Component/PCB/System Ageing
Environment
Design Objective
•
Margin depends on
the application domain
Domain
Lifetime
Margin
Aeronautics
30 years
40 dB
Automotive
10 years
20 dB
Consumer
1 year
0 dB
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Notion of margin
Auto-compatibility issue ?
EMC Level
(dB)
50
Susceptibility
level
40
30
20
10
0
High risk of
interference
Aggressor IC
Safe
interference
margin
Unsafe margin
-10
-20
-30
-40
1
Victim IC
Sum of
perturbations
10
100
1000
Frequency (MHz)
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Impedance
R,L,C vs. frequency
Impedance profile of:
•50 ohms resistor
•100pF capacitor
•10nH inductor
Z = constant
•a real 100 pF SMD
capacitor
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Impedance
Conductor impedance or Characteristic Impedance Z0:
• From the electromagnetic point of view:
Coaxial line
E
Z0 
H
Microstrip line
Link to conductor geometry and material properties
• From the electric point of view :
R  jL
Z0 
G  jC
lossless
conductor
L
Z0 
C
Equivalent electrical schematic
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Impedance
Impedance matching
Why impedance matching is fundamental ?
Not adapted: the line suffers
ringing, insertion losses
Adapted: the line is transparent
Voltage
Voltage
time
time
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Impedance
Characteristic Impedance Z0:
Small conductor
What is the optimum characteristic
impedance for a coaxial cable ?
Small
conductor
Power handling
X
weight
X
Low loss
x
Small capacitance
x
Small inductance
Low Impedance
Or ?
Large
conductor
Ideal values:
• Maximum power : Z0 = 32 
X
Bending
Large conductor
• Minimum loss: Z0 = 77 
Cable examples:
x
• EMC cable (compromise between
power and loss) : Z0 = 50 
x
• TV cable (minimize Loss): Z0 = 75 
x
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Impedance
50 ohm adapted systems
Spectrum analyzer
Waveform generator
Tem cell
Amplifier
Gtem
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Conclusion
• Key words of EMC for integrated circuits have been presented
• The origin of parasitic emission in Ics has been illustrated
• The trend to decrease supply voltages increase the IC
susceptibility
• Specific units used in EMC have been detailed
• The Fourier Transform is a very important tool for the
characterization of EM signals
• The notion of impedance has been introduced
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