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ELECTROMAGNETIC SCIENCE
Review
Regulation and Control of Electromagnetic Field in
Radio-Frequency Circuits and Systems
Kaixue Ma
Tianjin Key Laboratory of Imaging and Sensing Microelectronic Technology
,
School of Microelectronics
,
Tianjin University
,
Tianjin 300072
,
China
Corresponding author: Kaixue Ma, Email: makaixue@tju.edu.cn.
Received November 3, 2022; Accepted August 21, 2023; Published Online September 20, 2023.
Copyright © 2023 The Author(s). This is a gold open access article under a Creative Commons Attribution License (CC BY 4.0).
Abstract—Since the first demonstrations of radio-frequency (RF) circuits, the physics of the electromagnetic (EM) field and its regulation and control
with codesigned circuits, have become essential competencies of RF circuit designers. Leveraging advanced regulation or control methods, numerous
high-performance circuits have been developed at RF and millimeter-wave (mm-wave) frequencies. Three main methods of electromagnetic regulation
have been widely utilized, namely, the separation of electric and magnetic coupling paths, the manipulation of electromagnetic energy through the cou-
pling of multiple tanks or multiple resonators, and the regulation of electromagnetic fields in air cavities or meta-substrates. The separated coupling paths
of electric and magnetic fields provide guidance for designing a high-performance filter topology with a quasielliptical response through additional ze-
ros. The manipulation of the EM field through electrical and magnetic intercouplings of multitanks or multiresonators, such as are used in oscillators,
power amplifiers (PAs), etc., results in remarkable power efficiency, size reduction, and wide bandwidth. The regulation of electromagnetism through an
air cavity, patterned substrate, or metasubstrate reduces dielectric losses and size, especially when using a substrate integrated suspended line (SISL)
platform. Many excellent circuits have been reported based on SISL with low loss, high integration, and self-packaging. Here, we present state-of-the-art
cases that demonstrate the benefits of EM field regulation and control.
Keywords—Electromagnetic, Mixed coupling, Multiresonators and multitanks, Patterned substrate and metasubstrate, Radio frequency circuits,
Regulation and control, Separate electronic and magnetic coupling paths, Substrate integrated suspended line.
Citation—KaixueMa, “Regulation and Control of Electromagnetic Field in Radio-Frequency Circuits and Systems,”
Electromagnetic Science
,
vol. 1, no. 3, article no. 0030101, 2023. doi: 10.23919/emsci.2022.0010.
I. Introduction
With the advancement of wireless communication technolo-
gy and radio frequency (RF) systems, increasing demands
for miniaturization, low loss, and low cost are prompted for
integrated circuits and systems with higher performance
and compact size. Recently, various technologies for elec-
tromagnetic (EM) control in radio frequency circuits has
been extensively studied by our group and other research-
ers. Many circuits operating in RF or microwave regimes
have seen improved performance using electromagnetic
regulation and control. For example, the control technology
in [1] helped to achieve a low insertion loss and small size,
as well as controllable transmission zeros (TZs) for the fil-
ter design. Additionally, in [2], an enhanced quality-factor
(Q-factor) and wide tuning range were performed by intro-
ducing electromagnetic control theory for RF ICs. This type
of recently developed technology consists of the separation
and regulation of electric and magnetic coupling paths, the
regulation and control of electromagnetic energy, and elec-
tromagnetic control in the medium or substrate.
Commonly implemented methods of separating elec-
tric and magnetic coupling paths (SEMCP) filter topology,
as shown in Figure 1(a), include physical separation and
equivalent separation in electric, i.e., in equivalent or equa-
tion. The separation of physical electric and magnetic cou-
pling paths offers the benefits of simplifying the design, in-
tuitive analysis, and separate control of TZs. Equivalent
electronic and magnetic coupling paths can provide instruc-
tions and guidance to the filter designs according to the
equations on the required positions of TZs, return loss, and
more. Regarding the regulation and control of electromag-
netic energy in EM coupling proposed in [2], as shown in
Figure 1(b), this approach can be conducted by electrical
coupling, magnetic coupling regulation and control, and
mixed electromagnetic techniques that treat the coupled
capacitor, inductors or both as equivalent to an electric or
magnetic or electromagnetic energy storage device. This
combination offers outstanding advantages for silicon-
based circuits and systems, such as an enhanced Q-factor, a
wide tuning range, a remarkable figure-of-merit (FoM),
considerable efficiency and linearity, and a reduced core
Electromagnetic Science
vol. 1, no. 3, 0030101, September 2023
https://doi.org/10.23919/emsci.2022.0010
ELECTROMAGNETIC SCIENCE
area [2]–[11].
To regulate the electromagnetic field effect for reduc-
ing loss and minimizing size, the air cavity, embedded
medium, and metasubstrate electromagnetic regulation
techniques proposed in [12] are introduced to manipulate
the electromagnetic wave, as shown in Figure 1(c). The
concept of quasiplanar circuits with embedded air cavities,
patterned substrate, or metasubstrate is introduced. Com-
pared with traditional planar circuits where electromagnet-
ic energy is mostly confined in substrates, circuits with em-
bedded air cavities, patterned substrates, or metasubstrates,
especially those using substrate integrated suspended line
(SISL) platforms, feature lower dielectric loss. Many cir-
cuits designed based on the SISL platform of different types
of transmission lines possess the merits of low dielectric
loss, high efficiency, low cost, high integration, and self-
Waveguide filter
Microstrip filter
Coaxial filter
M
M
E
E
S
CPW filter
Substrate Metal
Substrate Metal
SIW filter
Substrate
Metal
Metallic holes
Via holes
Air cavity
Air cavity
SISL filter
L
R
n-1
R
n
R
1
R
2
C
R/N
L/N
R/N
L/N L/N
L/N
R/N
R/N
…
V
1
V
2
V
2
V
N
M
12
M
13
M
23
M
3N
M
2N
i
1
i
2
i
3
i
N
+
−
+
−
+
−
+
−
Z
in
Electric energy storage
Magnetic energy storage
M
1N
Linear circuits:
LNA, power amplifier, switch etc.
Nonlinear circuits:
VCO, ILFD, frequency multiplier etc.
Transceiver
(a)
(b)
Figure1(to be continued)
0030101-2 Electromagnetic Science, vol.1, no.3
ELECTROMAGNETIC SCIENCE
packaging [13]–[30]. To expand the advantages of SISL,
several EM regulation methods have been presented to fur-
ther reduce the loss of substrate and metal, such as double-
sided interconnected strip line (DSISL) technology [16],
patterned substrates [16], and multiple inner boards [26].
Several methods are introduced to miniaturize the circuit
designs, such as loading dielectric blocks with high dielec-
tric constant material, quasilumped elements, and metasub-
strate [30]–[34].
The remainder of this article is organized as follows.
Section II introduces the theory of separate electric and
magnetic coupling paths, combined with analyzing the ba-
sic circuits and application cases. Section III discusses the
regulation technique of electromagnetic energy in multi-
tank circuits, including principles and some circuit designs
with enhanced performance. Section IV demonstrates the
regulation of EM in air cavities, patterned substrates, load-
ed dielectric substrates, or metasurface electromagnetics,
consisting of theories, guidance for utilizing this technolo-
gy, and some specific improved methods applied to circuit
design. The conclusion summarizes three main kinds of
regulation and control technologies for EM, presenting the
advantages of applying this newly developed technology.
II. Separating Electric and Magnetic Coupling
Path Regulation Theory and Circuits
1. Overview of the SEMCP topology
In recent years, the demand for available frequency chan-
nels has continuously increased, which results in more
stringent standards for high-performance microwave band-
pass filters (BPFs) used in communication systems. Re-
search on BPFs has long been one of the key directions in
microwave fields. Direct-coupled microwave BPFs and
cross-coupled BPFs [35] were introduced in the early
1950s; since then, much effort has focused on the synthesis
and design of BPFs with advanced topologies. Recently, as
a promising method to achieve high-performance filter re-
sponses, the SEMCP topology proposed by Ma in [1] in the
filter synthesis and design process has received significant
attention.
Regarding the EM regulation theory of the separating
electric and magnetic coupling paths in circuits, two main
methods have been implemented thus far, namely, physical
path separation and equivalent electric and magnetic cou-
pling path separation. The mechanism of physical path sep-
aration is mainly the introduction of two or even more par-
allel paths in physical space, i.e., an electric coupling path
and a magnetic coupling path, and additional TZs can be
generated through the EM coupling canceling between the
paths. In general, the electric coupling path can be realized
by an equivalent capacitive element, while the magnetic
coupling path can be realized by an inductive element. Us-
ing this method can be more intuitive to reflect the main
spatial distribution of the electric and magnetic coupling
paths, so it offers the advantages of easy design, simple
analysis, and flexible control of TZs and loading effect for
size reduction. Equivalent electric and magnetic coupling
separation path theory is based on preset indicators, such as
the position of TZs, return loss, out-of-band rejection, and
other information, to determine the structure of the filter.
This approach is also divided into two methods: the first is
to optimize the circuit directly based on the coupling ma-
trix, and the second is to synthesize it in the low-pass do-
main and then convert it to the bandpass domain. That first
approach relies on the cost function, and the second relies
on rotating and scaling the transformation of the low-pass
prototype. For example, we can introduce the separating
electric and magnetic coupling path regulation theory in the
commonly used cascaded triplet (CT), cascaded quadruplet
Suspended slot line
Air cavity
Air cavity
Air cavity
Air cavity
Air cavity
Multiple inner layers
Waveguide mode
Air cavity
Air cavity
Air cavity
Air cavity
Substrate Metal
Cavities with embedded medium
Dielectric block
Dielectric block
Air cavity
Air cavity
Air cavity
Air cavity
Suspended CPW
Metasubstrate
Composite metasubstrate
Via holes
Cut out
TE
10
(c)
Figure1The concept of three methods of regulating and controlling of electromagnetic field in circuits. (a) SEMCP topology and its applications [1];
(b) The regulation and control of electromagnetic energy storage in coupling [2]; (c) The regulation of electromagnetic field in the metasubstrate or air
cavity [12], [13]. (continued)
Regulation and Control of Electromagnetic Field in Radio-Frequency Circuits and Systems 0030101-3
ELECTROMAGNETIC SCIENCE
(CQ), and other cascade topologies to generate more TZs,
thus effectively improving the filter selectivity.
2. Theory and applications in filters
As a promising method to produce flexible TZs, we have
studied the underlying synthesis theory and then implement-
ed SEMCP in various circuits. As an example, Figure 2(a)
shows a second-order microstrip bandpass filter using the
fundamental SEMCP block, which offers two independent-
ly controlled electric and magnetic coupling paths. The re-
lationship between the coupling coefficients, electrical cou-
pling, and magnetic coupling can be established using even-
odd modes analysis, and the coupling between the two
modes can be described by a separate coupling parameter
C
, as shown in equation (1).
C =
ω
2
o
−ω
2
e
ω
2
o
+ ω
2
e
= F(Y
c
L
m
−C
m
Z
c
) = M −E (1)
where
F =
4(A + Y
c
L
m
+ Z
c
C
m
)
(2Y
c
L
m
+ A)
2
+ (2Z
c
C
m
+ A)
2
(2)
A =
√
ε
re
(
l
1
+ l
2
)
c
(3)
M = FY
c
L
m
(4)
E = FZ
c
C
m
(5)
β
e
=
ω
e
√
ε
re
c
(6)
where
β
e
is the propagation constant at the even-mode reso-
nance angular frequency
ω
e
.
A
A′
I/O port
I/O port
I/O portI/O port
I/O port
CouplingResonator
E: Electrical coupling M: Magnetic coupling
l
1
l
1
l
2
l
2
C
m
L
m
E
M
1 2
(a)
(b)
Figure2Proposed second-order filter configuration and the topology.
(a) Configuration with lump coupling elements; (b) Filter topology [1].
The even-mode resonant angular frequency can be giv-
en by
ω
e
=
π
2(2L
m
Y
c
+ A)
(7)
The odd-mode resonant angular frequency can be de-
termined by
ω
o
=
π
2(2C
m
Z
c
+ A)
(8)
L
m
C
m
Z
c
= 1/Y
c
L
m
C
m
l
1
+ l
2
≈ λ
g
/4
where and are lumped elements. We can observe that
the coupling coefficients of the proposed SEMCP are devel-
oped by two separate parts, i.e., magnetic coupling
M
and
electric coupling
E
, and the two coupling paths are depen-
dent on one another and have canceling effects.
c
denotes
the speed of light in free space, and is the charac-
teristic impedance of the resonator. For a small or ,
.
Figure 2(b) shows the responses of the presented sec-
ond-order BPF using SEMCP. As shown, by selecting a dif-
ferent dominant coupling path and coupling element values,
the filter response could be totally regulated. The design of
narrow bandwidth BPFs requires a reduced coupling coeffi-
cient, which can benefit from the canceling effects in two
coupling paths. Numerous excellent works on electromag-
netic regulation of planar circuits by separating electric and
magnetic coupling paths have been presented and have
achieved favorable performance in terms of compact size,
low losses, etc. In [1], the concept of the SEMCP and the
topology (shown in Figure 2) was proposed, and the cou-
pling mechanism was analyzed based on the equivalent
electric and magnetic path separation.
L
m
C
m
C
m
= 0
C
m
Figure 3(a) illustrates the impact of the coupling ele-
ments ( and ) in the two coupling paths on the filter
when magnetic coupling is prominent. When , there
is just one TZ produced in the high stopband as a result of
the harmonic effects. When is greater than 0, a second
TZ is produced in the high stopband, and the filter exhibits
excellent roll-off in this region.
Frequency (GHz)
|S
21
| (dB)
0.5
(a)
(b)
−80
1.0
L
m
=0.3 nH, C
m
=0.05 pF
l
1
=12.5 mm
l
2
=1.5 mm
Z
c
=50 Ω
Z
L
=50 Ω
L
m
=0.15 nH, C
m
=0 pF
L
m
=0.15 nH, C
m
=0.01 pF
1.5 2.0 2.5 3.0 3.5
−60
−40
−20
0
Frequency (GHz)
|S
21
| (dB)
0.5
−80
1.0
L
m
=0.1 nH, C
m
=0.1 pF
l
1
=12.5 mm
l
2
=1.5 mm
Z
c
=50 Ω
Z
L
=50 Ω
L
m
=0 nH, C
m
=0.05 pF
L
m
=0.05 nH, C
m
=0.05 pF
1.5 2.0 2.5 3.0 3.5
−60
−40
−20
0
Figure3
S
spectrum responses of SEMCP filters. (a)
M
dominant; (b)
E
dominant [1].
L
m
C
m
Figure 3(b) shows how the coupling elements ( and
) in the two coupling paths affect filter performance
0030101-4 Electromagnetic Science, vol.1, no.3
ELECTROMAGNETIC SCIENCE
L
m
= 0
L
m
> 0
when the electrical coupling is significant. No finite TZ val-
ue may be formed in the stopband when . Two more
TZs are produced in the stopband when , and the fil-
ter exhibits a nice roll-off.
To physically implement the SEMCP, we propose
three second-order filter structures based on physical path
separation (shown in Figure 4). These designs use a modi-
fied quarter-wavelength resonator with a coupling gap or
coupled transmission line to achieve the lumped capaci-
tance and use either a via-hole or a short section of a high
characteristic impedance transmission line grounded
through a series-connected via-hole to achieve the lumped
inductance.
G
W
0
W
0
W
0
l
u
l
u
l
u
s
s
l
J
l
J
l
b
l
b
l
b
(a) (b) (c)
Via
Via
Via
Tap Tap
Tap
Figure4The SEMCP filters. (a) Structure 1; (b) Structure 2; (c) Struc-
ture 3 [1].
In such systems, the electric coupling is prominent.
The electric and magnetic coupling cancel each other out in
the operational frequency range when the external quality
factor is known. As a result, the operating frequency falls
due to the increasing load on both coupling routes, while
the filter bandwidth, which is mostly influenced by the in-
terstage coupling coefficient
C
, varies minimally. Figure
5(b) illustrates the situation where magnetic coupling domi-
nates. Because of the canceling effect of the two coupling
paths, the coupling of each path increases, resulting in a
lower operating frequency. As a result, the filter’s band-
width can remain almost unchanged even with increased
electrical and magnetic coupling.
n
Additionally, Figure 6 uses the SEMCP structure to
depict the th-order SEMCP filter topology. Its novelty is
that every pair of neighboring resonators in the filter has ac-
cess to two quasi-independent, controllable
E
and
M
cou-
pling paths. Figure 7(a) illustrates how the dominant mag-
netic coupling can be exploited to generate TZs in the up-
Frequency (GHz)
|S
21
| (dB)
1.0
(a)
−40
1.4
Z
P
Fig. 4(a)
Fig. 4(b)
Fig. 4(c)
l
J
=3.2 mm, R
via
=0.1 mm
l
J
=1.7 mm, R
via
=0.3 mm
1.8 2.6 3.4 4.62.2 3.0 3.8 4.2 5.0
−30
−20
−10
0
Frequency (GHz)
|S
21
| (dB)
1.0
(b)
−50
1.3
Z
P
Fig. 4(a)
Fig. 4(b)
Fig. 4(c)
1.9 2.2 2.81.6 2.5 3.0
−40
−30
−20
−10
0
Figure5Frequency responses of the SEMCP filters. (a)
E
dominant;
(b)
M
dominant [1].
Source
1
2
C
0,1
C
1,2
M
M
E
Load
E
n−1 n
C
2,3
C
j,j+1
C
n−1,n
C
n,n+1
Figure6Topology of the high-order SEMCP filter [1].
Frequency (GHz)
Magnitude S
21
(dB)
0
(a)
−90
0.3
E
M
i)
i)
ii)
ii)
Inverted pairs
No-inverted pairs
E
M
ZP
by M-dominant
ZP
by E-dominant
0.6 1.2 1.8 2.70.9 1.5 2.1 2.4 3.0
−70
−50
−30
−10
−20
−40
−60
−80
0
7.5 mm
8.2 mm
E
M
C
0,1
C
1,2
C
2,3
C
j,j+1
C
n,n+1
C
n−1,n
E
M
n
Load
n−1
Source
1
2
l
u
l
u
l
b
l
b
l
J
l
J
S
1
W
0
W
0
S
2
S
3
Tap
Tap
Via
Via
(b)
(c)
(d)
Figure7The fourth-order SEMCP filter. (a) Simulation; (b) Configura-
tion; (c) Topology; (d) Comparison photographs with hairpin filter [1].
Regulation and Control of Electromagnetic Field in Radio-Frequency Circuits and Systems 0030101-5
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