Emi Source; Current Loop Radiation; Current Loop Radiation Of A Transmission Line - Intel Quark SoC X1000 Design Manual

Electromagnetic Interference—Intel
17.3

EMI Source

17.3.1

Current Loop Radiation

Current loop radiation is formed by the forward current and return current. For
example, the current loop of signals travelling on a microstrip line is shown in the
following figure. The current traveling on the trace is the forward current. The current
traveling on the ground plane is the return current. The forward and return currents
then form a current loop in between the trace and ground plane. Current loops, like
magnetic dipoles, emit signals. The emission intensity is proportional to the current
strength and the loop area. Therefore, minimizing the loop area is a key to mitigate
EMI.
Two ways to keep the microstrip-line current loop area small:
• Keep the height of the dielectric in between the trace and ground plane as small as
possible.
• Keep the length of the trace as short as possible. In other words, route the signals
short.
Figure 52.

Current Loop Radiation of a Transmission Line

A differential line is a good example of current loop radiation cancellation. A differential
line is composed of two adjacent traces. Therefore, the current loop from one trace is
very similar to the loop from the other. Since the current traveling on these traces are
in opposite direction. As a result, the emissions caused by these two loops cancel each
other. Please note that the cancellation is maximum when forward and return currents
are balanced. Unbalanced current levels induce common mode radiation introduced in
Section 17.3.3.
June 2014
Order Number: 330258-002US
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Quark™ SoC X1000
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Intel Quark™ SoC X1000
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