The Bridge - Issue 2, 2022 - 26

Feature
Mitigating Self-generated EMI for Wireless Devices
can generate the same external field distribution
as in the original problem. This concept has been
further evolved to reconstruct equivalent electric (P)
and magnetic dipole (M) moments, replacing the
radiating structure (inside the surface) according
to the known relationship between the equivalent
dipole moments and the fields on the surface. In
this case, the surface need not necessarily be closed.
Effective and accurate source reconstruction is an
ongoing research topic. Many approaches are being
investigated, such as the least squares method,
genetic algorithms, and pattern recognition based on
machine learning. The electric and/or magnetic fields
can be measured on a surface grid with a near-field
probe and an automatic near-field scanner. One
widely used near-field scanner is shown in Fig. 1. Fig.
2 shows a comparison of measured and calculated
magnetic fields (H). The calculation was performed
with the reconstructed dipole moment, M dipole in
this example.
current distribution on another, and vice versa. The
detailed derivation process is as explained previously
[5]. The coupling voltage at the victim antenna is
described as
where
and
is for each electric dipole moment
is for each magnetic dipole moment
The transfer functions
and
are associated
with the E and H fields, respectively, at the noise
source location when the victim antenna is excited.
The E and H fields can be obtained in a full wave
EM simulation or directly from measurements using
near-field probes. The transfer functions represent
how much noise is induced at the antenna port
given the source location and type. In other words,
the transfer functions are the susceptibility of the
antenna. We do not consider the coupling path and
victim antenna separately because the noise source
is located " on " the antenna. The total coupled voltage
is a linear combination of the contribution of each
noise source. Therefore, when multiple sources
exist, as is typically the case, their sum can be either
destructive or constructive depending on their phase
relationship. Of note, the interaction between the
EM radiation of the digital ICs (i.e., the source) and
the susceptibility of the RF antenna (i.e., the transfer
function) is represented as an inner product. We use
this relationship to mitigate the self-interference in
the next section.
III. Interference Mitigation
Fig. 2. H fields from measurement (top) and calculation (bottom) [4].
Second, the transfer function is needed. We
derived a new EM framework to describe the
relationship between the radiation source and the
coupled noiseby using the reciprocity theorem. The
reciprocity theorem is widely used in the circuit
domain, but the circuit reciprocity theorem is a
special case of reciprocity in electromagnetic field
theory and is derived from Maxwell's equations.
The electromagnetic reciprocity theorem describes
a relationship between the fields produced by one
THE BRIDGE
As described before, owing to the complexity of real
products, EMI/EMC troubleshooting, including that
for self-generated EMI, has been widely based on
experience or trial and error. Instead of introducing
additions late in the development stage, we
aim to " design " EMI performance at the product
development stage. New ways to improve the EMI
performance, i.e., by mitigating self-generated EMI,
are discussed. A smart speaker, shown in Fig. 3 [6],is
used as an example for demonstration.
Fig. 3. A circuit board with Wi-Fi antennas (black), a
processor (orange), and memory (red) [6]
The Bridge - Issue 2, 2022

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