# 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 36 of 78
Rev. A
10/01/2002
Printed copies are uncontrolled
3.8. Twisted Pair
A twisted pair of wires reduces inductive coupling by canceling induced magnetic field
voltages. Figure 3­15 shows magnetic field (B) coupling into a circuit. V
s
represents an
input signal to an electronic device on a vehicle. R
in
represents the input impedance of the
module. The figure shows that the device input voltage, V
in
, is the sum of V
s
and the
noise voltage V
n
, which the magnetic field induces.

Figure 3­15. Magnetic Field Coupling into Circuit

Figure 3­16. Magnetic Field Coupling into Twisted Wire Pair
Figure 3­16 shows the circuit in Figure 3­15 that uses a twisted pair. The twisting
produces four equal loop areas with equal noise voltages. By summing all the voltages
around the circuit the noise voltages cancel due to the twisting. This is why twisted pairs
work best to reduce inductive coupling into a receptor circuit.
s
n
n
s
n
n
in
V
4
V
4
V
V
4
V
4
V
V
=
+
+
+
-
-
=
Equation 3­17. Inductive Coupling in twisted-wire pair
V
n
V
s
V
in
R
in
V
in
=
V
s
+ V
n
B
V
n
/4 V
n
/4 V
n
/4 V
n
/4
V
s
R
in
V
in
B

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Summary :

## Inductive Coupling in twisted-wire pair V n V s V in R in V in = V s + V n B V n /4 V n /4 V n /4 V n /4 V s R in V in B

Tags : figure,circuit,coupling,twisted,field,magnetic,pair,oltages,into,shows,noise,3­15,input
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