Download Impedance - Alexandre Boyer

EMC Basics
concepts
Summary
1. Basic Principles
2. Specific Units
3. LC Resonance
4. Radiating element
5. Emission Spectrum
6. Susceptibility Spectrum
7. Notion of margin
8. Impedance
9. Conclusion
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May 17
Basic principles
CONDUCTED AND RADIATED EMI
Conducted mode
Radiated mode
The VDD supply
propagates parasits
The EM wave propagates
through the air
Power Integrity (PI)
Electromagnetic Interference (EMI)
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May 17
Specific Units
THE “EMC” WAY OF THINKING
Electrical domain
Electromagnetic domain
Voltage V (Volt)
Current I (Amp)
Impedance Z (Ohm)
Z=V/I
P=I2 x R (watts)
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May 17
Specific units
AMPLITUDE IN DB VS. FREQUENCY IN LOG
Distinguish contributions of
small harmonics
Volt
dB
Time
Freq (Log)
Cover very large bandwidth
Time domain measurement
Frequency measurement
Fourier
transform
Spectrum analyser
Oscilloscope
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May 17
Specific units
EMISSION AND SUSCEPTIBILITY LEVEL UNITS
Volt
Voltage Units
Wide dynamic range of signals in
EMC → use of dB (decibel)
For example dBV, dBA :
dBV  20  log V 
dBA  20  log  A
dBV
Milli
Volt
dBµV
100
1
10
0.1
1
0.01
0.1
0.001

  20  log V   120

0.01
0.0001
0.001
0.00001
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May 17
Extensive use of dBµV
 V
VdBµV  20  log 
 1µV
Specific units
EMISSION AND SUSCEPTIBILITY LEVEL UNITS
Power Units
1 MW
The most common power unit is the “dBm”
(dB milli-Watt)
PdBmW
Power
(Watt)
 P 
 10  log  W   10  log PW   30
 1 mW 
1 KW
1W
1 mW
Exercise: Specific units
1 µW
1 mV = ___ dBµV
1 W = ___ dBm
1 nW
IC-EMC: 0dbm in 50 
Tools > dB/Unit converter
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May 17
Power
(dBm)
LC Resonance
THE CHIP IS A LC RESONATOR

DSPIC33F DIE ALONE
f= ___
Impedance (Ω)
Tools > LC resonance
Eurodots > z11-dspic-vdd_10-vss_9.z
Impedance measurement
between Vdd and Vss
Frequency (Hz)
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May 17
Radiating Element
RADIATED EMISSION

Elementary “Hertz” current dipole.

Short wire with a length << λ , crossed
by a sinusoidal current with a constant
amplitude Io
Eθ
Z
Er
θ
h
Io
O
φ
X
R
Hφ
Y

 1
 2 I o h
j 
Er  2
cos   2 2  3 3 e jr
4
 r 
 r

 2 I o h
 1
j
j   jr
E 
sin   2 2 
 3 3 e
4
r  r 
 r
  2 Ioh
1
1  jr
H 
sin  (
 j )e
4
 ²r ²
r




E  H r  H   0
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May 17
Radiating Element
NEAR FIELD/FAR FIELD

Close to the antenna
R  1  R 


2
Rlimit


2
Far from the antenna

R  1  R 
2
Near-field
region
Far-field
region

Non radiating field (non TEM wave)

Radiating field (TEM wave)

E and H decreases rapidly in 1/r³

E and H decreases in 1/r
100 MHz : Rlimit =____
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May 17
LC Resonance
THE BOARD IS A RESONATOR

The VDD/VSS plate acts as a capacitor
Impedance (Ω)
Eurodots > z11-board-d21on.z
Frequency (Hz)
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May 17
Emission spectrum
EMISSION LEVEL VS. CUSTOMER SPECIFICATION
Parasitic emission
(dBµV)
EMC compatible
Specification
example for an IC
emission
80
70
60
50
Measured
emission
40
30
20
10
0
-10
1
10
100
1000
Frequency (MHz)
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May 17
Emission spectrum
LOW PARASITIC EMISSION IS A KEY COMMERCIAL ARGUMENT
Emission
FM
GSM
RF
dBµV 100
80
Not EMC compliant
Supplier
A
Customer's
specified
limit
60
40
20
0
10
Supplier
B
EMC compliant
100
Frequency(MHz)
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May 17
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
0
-10
A very low energy
produces a fault
-20
-30
-40
1
10
100
1000
Frequency (MHz)
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May 17
Notion of margin
WHY A MARGIN ?
Parasitic emission (dBµV)
Nominal Level
•
To ensure low parasitic emission ICs
supplier has to adopt margins
Design Objective
•
Margin depends on the
application domain
Domain
Lifetime
Aeronautics
Automotive
Consumer
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May 17
Margin
Notion of margin
INFLUENT PARAMETERS ON IC EMC

 The variability between components induce a
The temperature of a circuit has a
dispersion of emission and susceptibility level.
direct impact on the switching time of
Radiated emission in TEM cell of a 16 bit
internal devices. When temperature
microcontroller PIC18F2480. Measurement of 12
increases, the high frequency content
samples and extraction of emission level dispersion.
of the emission spectrum tends to be
reduced.
Std deviation = 1.7 dB
K. P. Slattery et al., “Modeling the radiated emissions
from microprocessors and other VLSI devices”, IEEE
Symp. on EMC, 2000.
H. Huang and A. Boyer (LAAS-CNRS)
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May 17
Notion of margin
INFLUENT PARAMETERS ON IC EMC

MOS device characteristics fluctuate by
+/- 30 %

Ageing may significantly alter EMC
performances
Ioff/Ion MOS
32-nm
PhD A. C. Ndoye, INSA, 2010
Immunity vs. ageing (LTOL)
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May 17
Impedance
R,L,C VS. FREQUENCY
Impedance profile of:
• 1 Ω resistor (z111Ohm_0603.z)
Schematic diagram:
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May 17
Impedance
R,L,C VS. FREQUENCY
Impedance profile of:
• 1 nF capacitor (z11C1nF_0603.z)
Schematic diagram:
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May 17
Impedance
R,L,C VS. FREQUENCY
Impedance profile of:
• Inductance 47 µH
(Zin_L47u.s50)
Schematic diagram:
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May 17
Characteristic Impedance
CONDUCTOR IMPEDANCE OR CHARACTERISTIC IMPEDANCE Z0:
• From the electromagnetic point of
view:
E
Z0 
H
Coaxial line
Link to conductor geometry and material properties
• From the electric point of view :
R  jL
Z0 
G  jC
Microstrip line
lossless
conductor
L
Z0 
C
Equivalent electrical schematic
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May 17
Characteristic Impedance
IMPEDANCE MATCHING
Why impedance matching is fundamental ?
IC-EMC
Not adapted:
Adapted:
impedance_mismatch.sch
Voltage
Voltage
Impedance>
time
time
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May 17
Characteristic Impedance
CHARACTERISTIC IMPEDANCE Z0: Small conductor
Large conductor
What is the optimum
characteristic impedance for a
Or ?
coaxial cable ?
Small
conductor
Large
conductor
Ideal values:
Power handling
• Maximum power : Z0 = ___
Bending
• Minimum loss: Z0 = ___ 
weight
Cable examples:
Low loss
• EMC cable (compromise between power
and loss) : Z0 = ___ 
Small capacitance
Small inductance
• TV cable : Z0 = ___ 
Low Impedance
• Base station cable : Z0 = ___ 
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May 17
Characteristic Impedance
50 OHM ADAPTED SYSTEMS
Spectrum analyzer
Tem cell
Waveform generator
Amplifier
Tools > Interconnect parameters
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May 17
Conclusion
• Specific units used in EMC have been detailed
• The current dipole is the base for radiated emission
• The Emission Spectrum has been described
• Susceptibility Threshold, margins have been discussed
• The notion of impedance has been introduced
• Characteristic impedance of cables lead to specific values
• Discrete components used in the experimental board have
been modeled up to 1 GHz
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May 17