EM Coupling Phenomena
General Remarks
•
One of the most fundamental issues in EMC is coupling. We can
distinguish four coupling mechanisms:
– Galvanic or conductive coupling.
– Capacitive near field coupling.
– Inductive near field coupling.
– Electromagnetic far field coupling.
•
A way the coupling can be quantified is by using the concept of
transfer impedance
Z
T
.
Z
T
=
V
sink
I
source
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
1
Electromagnetic Basics (1)
James Clerk Maxwell (1831 – 1879), Coulomb (1736-1806), Ampere (1775-
1836), Faraday (1791-1867), Oersted (1777-1851) Volta (1745-1827), Gauss
(1777-1855), Hertz (1857-1894), Marconi (1874-1937).
∇×
H
=
J
+
∂
D
∂
t
∇×
E
= −
∂
B
∂
t
J
=
σ
E
∇⋅
B
=
0
∇ ⋅
D
=
ρ
v
∇
2
E
s
−
γ
2
E
s
=
0
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
2
Electromagnetic Basics (2)
•
•
•
•
•
Electric charges are associated with electric fields.
Moving electric charges (currents) are associated with magnetic fields.
Time varying electric fields are associated with changing magnetic fields.
Time varying magnetic fields are associated with changing electric fields.
This inter-dependence between varying electric and magnetic fields yields
to wave propagation.
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
3
Electromagnetic Theory – Motivation
•
•
•
•
•
•
•
Emissions, in the radiated or conducted form, are usually an unwanted
side effect caused by physical and electrical characteristics of circuits.
These characteristics are often related to parasitic parameters and the
non-ideal behavior of components.
These parameters are not explicitly shown in schematics and layout
diagrams.
In the case of immunity problems, the source of the disturbance and the
coupling path are not always obvious.
To avoid EMC problems, a few fundamental electromagnetic principles
must be followed.
Knowledge of these principles will give an understanding, why certain
guidelines and rules should be applied.
It will also allow to chose the best solution for each particular case, and to
develop tailored solutions for special problems.
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
4
Principle of Cancellation (1)
z
z
2h
I
H
r r
H
⋅
dl
=
I
enclosed
Ampere:
∫
loop
H
P
P
r
r
One line current:
Two parallel line currents
r
I
H
=
2
π
r
r
H
=
I
I
−
2
π
⋅
(
r
−
h
)
2
π
⋅
(
r
+
h
)
I
(
r
+
h
)
−
(
r
−
h
)
I
2
h
=
≈
2
2
2
π
2
π
r
2
r
−
h
h
(
for :
r
>>
h
)
5
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
Principle of Cancellation (2)
Dependency on distance: single and dual conductors, influence of separation
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
6
Basic Radiators - Hertzian dipole
I
q
(
t
)
=
0
sin
ω
t
ω
+Q
l
I
0
-Q
E
r
=
E
θ
=
H
φ
=
Z
0
p
e
cos
θ
⋅
e
2
π
r
02
Z
0
p
e
sin
θ
⋅
e
4
π
r
02
p
e
sin
θ
⋅
e
4
π
r
02
−
j
−
j
r
r
0
(
i
(
t
)
=
I
0
cos
ω
t
)
−
j
r
r
0
1
j
−
(
r
/
r
)
2
(
r
/
r
)
3
0
0
j
1
j
+
−
(
r
/
r
) (
r
/
r
)
2
(
r
/
r
)
3
0
0
0
−
I
q
(
t
)
=
0
sin
ω
t
ω
r
r
0
j
1
+
(
r
/
r
) (
r
/
r
)
2
0
0
(l: dipole length)
z
θ
r
Dipole moment:
p
e
=
I
0
⋅
l
Transition point: near - far field:
r
0
=
λ
2
π
Impedance of free space:
Z
0
=
120
π
Ω ≈
377
Ω
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
7
Basic Radiators – Small current loop
H
r
=
H
θ
=
p
m
cos
θ
⋅
e
2
π
r
03
p
m
sin
θ
⋅
e
4
π
r
03
−
j
r
r
0
i
(
t
)
=
I
0
cos
ω
t
−
j
r
r
0
1
j
−
(
r
/
r
)
2
(
r
/
r
)
3
0
0
j
1
j
+
−
(
r
/
r
) (
r
/
r
)
2
(
r
/
r
)
3
0
0
0
r
r
0
A
Z p
sin
θ
⋅
e
E
φ
=
0
m
4
π
r
03
z
θ
r
−
j
j
1
(
r
/
r
)
+
(
r
/
r
)
2
0
0
Dipole moment:
Transition point: near - far field:
p
m
=
I
0
⋅
A
r
0
=
λ
2
π
(A: loop area)
Impedance of free space:
Z
0
=
120
π
Ω ≈
377
Ω
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
8
Near/Far field behaviour
~1/r
3
~1/r
3
r
=
r
0
~1/r
2
~1/r
2
r
=
r
0
E r
0
=
Z
0
H r
~1/r
E r
=
Z
0
H r
0
~1/r
E
=
Z
0
H
E
=
Z
0
H
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
9
Current Return Path (1)
The return current will usually take
the path of least
impedance!
At low frequencies, this path will
depend on the resistance
characteristic of the structure.
At high frequencies the impedance
characteristics are dominated by
inductances.
(At very high frequencies, also stray
capacitances may play a role.)
Mother Nature sets up the return path
in order to obtain the lowest possible
impedance for the pair.
c) WATRI – 2007
http://www.watri.org.au
Dr Franz Schlagenhaufer
Part II - EM Coupling Phenomena
10
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