# Engineering Specification

Document source : www.fordemc.com

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Engineering Specification
ES-3U5T-1B257-AA
EMC Design Guide for Printed Circuit Boards
Frame 33 of 78
Rev. A
10/01/2002
Printed copies are uncontrolled
Equation 3­13 gives the noise voltage, V
n
due to capacitive coupling. R
f
is the
parallel equivalence of R
1
and R
2
, which equals 2xf, where f is the frequency or
frequencies of V
g
.
V
C
R
j
V
gr
r
n
×
=
whenever
(
)
r
gr
r
C
C
j
1
R
+
<<
and
g
r
gr
gr
n
V
C
C
C
V

+
=
whenever
(
)
r
gr
r
C
C
j
1
R
+
>>
Equation 3­13. Voltage Noise due to capacitive coupling
To reduce capacitive coupling:
·
Decrease the generator frequency
·
Decrease the parallel length between the circuits
·
Increase the separation between the circuits
·
Orient the receptor circuit to the generator circuit at 90û
·
Increase C
r
·
Decrease R
r
·
Shield the generator and/or the receptor circuit
·
Place conductors over a ground plane
3.7.4. Inductive coupling
Inductive coupling results from the interaction of a time-varying magnetic field
between a generator and receptor circuit. Inductive coupling can occur at low or
high frequencies. Crosstalk from inductive coupling is more prevalent when high-
level and fast-rising currents transients are conducted in a low-impedance circuits.
Signal current creates a magnetic field that surrounds the conductor. Figure 3­11
illustrates that conductive coupling results from a mutual inductance L
gr
. The mutual
inductance provides a path for magnetic flux to couple from the generator circuit to
the receptor circuit.
Figure 3­13 shows that inductive coupling is essentially a simple magnetic
transformer. The generator circuit is the primary and the receptor circuit is the
secondary of the transformer. The figure also illustrates that when V
g
is a sine wave
then V
noise
is a sine wave as well but with a reduced amplitude. When V
g
is a square
wave then V
noise
shows noise spikes when the square wave changes.

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Summary :

## Voltage Noise due to capacitive coupling To reduce capacitive coupling: · Decrease the generator frequency · Decrease the parallel length between the circuits · Increase the separation between the circuits · Orient the receptor circuit to the generator circuit at 90û · Increase C r · Decrease R r · Shield the generator and/or the receptor circuit · Place conductors over a ground plane 3.7.4.

Tags : circuit,coupling,generator,noise,inductie,receptor,wae,magnetic,circuits,decrease,figure,between,3­13
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