IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 69, NO. 11, NOVEMBER 2022
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3-D Printed mm-Wave Filter Using
Increased-Height DGS Resonator
for Spurious Suppression
Taotao Huang , Haiwen Liu , Senior Member, IEEE, Cheng Guo ,
Linping Feng , Member, IEEE, and Li Geng , Senior Member, IEEE
Abstract—A 3-D printed quasi-elliptic waveguide bandpass filter (BPF) for 5G mm-wave application is proposed in this letter.
First, by adjusting the coupling topology based on the typical 4th -order cross-coupled filter, the spurious mode below the
passband is eliminated. Second, an increased-height resonator
is proposed to decrease the electric coupling strength to avoid
extremely small features, hence effectively reducing the sensitivity
to the fabrication tolerances. Third, two types of defected ground
structures (DGSs) on the filter shell are introduced to suppress
two spurious modes above the passband. The stereolithography
apparatus (SLA) 3-D printing with the electroplating copper plating technique is utilized to fabricate this filter for demonstration.
A good agreement between simulated and experimental results
verifies this design method.
Index Terms—Bandpass filter, waveguide, defected ground
structure (DGS), 3-D printing, mm-wave, spurious suppression.
I. I NTRODUCTION
ITH the rapid development of the 5th -generation (5G)
mobile communication and its commercial applications, the higher-frequency spectrum in millimeter-wave (mmwave) bands has received considerable attention for high
data rate transmission and larger bandwidth [1]. To balance
the tradeoff between the propagation loss and the operating
bandwidth, the 28-GHz band became one of the good candidates for 5G communication [2]. Bandpass filters (BPFs) with
a sharp-rejection performance near the passband edges and
spurious-free stopbands are highly required due to their high
selection capability and the suppression of adjacent bands [3].
W
Manuscript received 8 April 2022; revised 9 June 2022; accepted 29
June 2022. Date of publication 4 July 2022; date of current version
28 October 2022. This work was supported in part by the National Key
Research and Development Project under Grant 2017YFE0128200; in part
by the National Natural Science Foundation of China under Grant U1831201
and Grant 62171363; in part by the International Cooperation Funds of Key
Research and Development Plan of Shanxi Province of China under Grant
2022KWZ-15; and in part by the Shaanxi Key Laboratory of Deep Space
Exploration Intelligent Information Technology under Grant 2021SYS-04.
This brief was recommended by Associate Editor M. Chen. (Corresponding
author: Haiwen Liu.)
Taotao Huang, Haiwen Liu, Cheng Guo, and Linping Feng are with the
Shaanxi Key Laboratory of Deep Space Exploration Intelligent Information
Technology, School of Information Communications Engineering, Xi’an
Jiaotong University, Xi’an 710049, China (e-mail: 1455690467@qq.com;
haiwen_liu@hotmail.com).
Li Geng is with the School of Electronic and Information Engineering,
Xi’an Jiaotong University, Xi’an 710049, China.
Color versions of one or more figures in this article are available at
https://doi.org/10.1109/TCSII.2022.3187970.
Digital Object Identifier 10.1109/TCSII.2022.3187970
Some BPFs based on printed circuit board (PCB) technology, such as integrated waveguide (SIW) [4], and ridge gap
waveguide (RGW) [5] are used to satisfy high-frequency applications. Although they have high selectivity by generating
transmission zeros (TZs), the dielectric loss and radiation loss
from the gap between the metallization vias are inevitable [6].
The air-filled waveguide structure is an excellent solution for
its low-loss and high-power handling capability [7], and its
major drawback, i.e., the larger size, can be neglected in the
mm-wave frequency region. In general, the metal waveguide
and cavity are manufactured through highly precise computer
numerically controlled (CNC) milling technology [8]–[9],
showing excellent performance and extremely low insertion
loss. Nevertheless, the extortionate price and heavyweight
impose additional burdens on the large-scale application of
the 5G system.
Additive manufacturing (AM), also referred to as 3-D
printing, has become increasingly attention in the microwave
field. The body volume of the plastic or metallic materials is the major acquisition cost. They are constructed
to realize microwave components layer by layer regardless of the structural complexity. To form a conductive
layer onto the surface of the printed device, the surface metallization process is subsequently implemented.
Various 3-D printing approaches including the fused deposition modeling (FDM) [10]–[11], selective laser sintering (SLS) [12], selective laser melting (SLM) [13], and
stereolithography apparatus (SLA) [14]–[19], have been utilized to fabricate microwave devices. The size of the FDM
nozzle is relatively large, resulting in a lower resolution [14]
and was merely used in mm-wave frequencies. The SLS
and SLM are commonly used to fabricate metallic materials. Although they can provide a relatively high resolution to
fabricate the mm-wave devices, the weight and costs are still
very high [11]. Considering those concerns, the SLA was chosen to fabricate the proposed filter in this letter for its high
resolution, lower cost, and better surface finish.
The quasi-elliptic response is more popular than the
Chebyshev due to its high selectivity caused by the transmission zeros (TZs) near the passband. In [8], a 4th -order
quasi-elliptic waveguide filter operating at 220 GHz using the
electric cross-coupling is presented. The CNC-milling technology can ensure high precision and fairly smoothness. Although
such a design can fulfill the quasi-elliptic response with low
loss, noticeable spurious modes located below and above the
passband limit the stopband bandwidth. Furthermore, the presence of an undersized air gap inside the filter geometry is more
sensitive to fabrication tolerance and accuracy.
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IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 69, NO. 11, NOVEMBER 2022
Fig. 2. Electric field distributions of the spurious modes in the typical BPF I.
(a) Mode A (capacitive iris resonance). (b) Mode B (TE101 ). (c) Mode C
(TE210 ).
Fig. 1. BPF I: typical fourth-order cross-coupling rectangular waveguide
filter. (a) Geometry configuration and Coupling topology. (b) Simulated results
in a wide frequency range. (c) Electric field distribution at 28 GHz. (Design
parameters: a = 8.636, b = 4.318, w1 = 6.8, w2 = 8.48, l = 6, d12 = 4,
d23 = 3.4, d34 = 4, d14 = 0.1, dS1 = d4L = 4.94, Units: mm).
In this letter, based on the typical design in [8], a quasielliptic waveguide filter with extended spurious-free stopband
and reduced sensitivity to fabrication tolerance is proposed by
SLA 3-D printing. Three approaches are applied to optimize
the filter performance. By adjusting the coupling topology,
the spurious mode below the passband induced by the capacitive iris is firstly eliminated. To reduce the sensitivity to
fabrication tolerances, an increased-height rectangular resonator is proposed, which can decrease the electric coupling
strength between two resonators while having no effect on
the filter response. Third, two types of defected ground structures (DGSs) are introduced on the filter shell to suppress two
spurious modes above the passband, which can also facilitate
the electroplating process to acquire a homogeneous conductive layer. The demonstration features a very low insertion loss
and a highly selective with a good out-of-band rejection.
II. T YPICAL C ROSS -C OUPLING F ILTER
A typical fourth-order quasi-elliptic waveguide filter (BPF I)
centered at 28GHz with 3-dB FBW of 7% and passband return
loss of 20 dB is first depicted in Fig. 1. The filter is constructed
with four dominant TE110 mode resonators and two ports using
the standard WR-34 waveguide (8.636 mm × 4.318 mm).
The resonant frequency of TE110 mode in a hollow metallic
rectangular cavity can be determined by using the following
formula [20]:
c
1
1
( )2 + ( )2
(1)
fTE110 =
2 W
L
where c is the light velocity in vacuum, W and L are the width
and length of the resonator, respectively. The frequency selectivity of the filter is improved by a cross-coupling topology
employing folded construction that can generate two transmission zeros in the vicinity of the passband. The electric
coupling M 14 is provided by a capacitive iris with a height
of d14 , while the magnetic couplings M 12 , M 23 , and M 34
are realized by inductive irises with widths of d12 , d23 , and
d34 , respectively [8]. The external coupling QeS and QeL are
achieved by inductive irises with the width of dS1 and d4L ,
respectively.
Based on the coupling matrix synthesis method [16], the
un-normalized non-zero coupling coefficients and the external quality factors are calculated to be: M 12 = M 34 = 0.062,
M 23 = 0.054, M 14 = −0.012, and QeS = QeL = 13.36.
The simulated S-parameters and electric field distribution at
28 GHz are shown in Fig. 1 (b) and (c), respectively. As can
be seen that apart from the desired passband, it also comprises
three spurious bands, i.e. modes A, B, and C, their electric
field distributions are displayed in Fig. 2. It can be found that
mode A is the resonance induced by a small capacitive iris
d14 , while mode B and mode C are caused by the degenerate
mode TE101 and the higher-order mode TE210 in resonator-2
and -3, respectively. The excitation of the spurious modes may
limit certain applications. Additionally, because of the very
small value of its electric coupling coefficient M 14 (−0.012),
the corresponding capacitive iris d14 given in Fig. 1 is only
0.1mm. Intuitively, such a small size is likely to be sensitive to
fabrication tolerances and it is difficult for the metal solution
to be uniformly plated on the surface of the resin body during
plating. Furthermore, an inadequate capacitive iris is likely to
result in the presence of a supporting structure to maintain
stability during printing [12]–[13]. Here, we have developed
an increased-height resonator filter with DGS to overcome the
drawbacks mentioned above.
III. P ROPOSED C ROSS -C OUPLING F ILTER
To enhance the out-of-band performance and address the
processing challenges posed by the inadequate capacitive iris,
three improvements are introduced based on the BPF I, which
is performed as follows.
A. Suppression of Spurious Mode A
As shown in Fig. 2 (a), mode A is generated by the capacitive iris resonance caused by the small electric coupling
coefficient M 14 (0.012). To eliminate this mode, M 23 rather
than M 14 as the negative electric coupling is preferred in our
design (BPF II), as illustrated in Fig. 3. This is since coupling coefficient M 23 (0.054) is greater than M 14 (0.012), and
accordingly, resulting in a larger capacitive iris from which
mode A can be easily eliminated. Based on the coupling matrix
theory [5], the location of the negative coupling in the coupling scheme topology has no effect on the response of the
filter. Fig. 3 (b) demonstrates that with this coupling topology, the filter still exhibits a quasi-elliptic response, and mode
A has been suppressed. In addition, with this type of solution,
the capacitive iris can be enlarged to 0.6 mm.
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HUANG et al.: 3-D PRINTED mm-WAVE FILTER USING INCREASED-HEIGHT DGS RESONATOR
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Fig. 5. (a) Coupling structure between resonator-2 and -3 under the electric
coupling. (b) Extracted electric coupling coefficients as a function of d23
under three different h values. (c) Extracted electric coupling coefficients
as a function of h under three different d23 values.
Fig. 3. BPF II: BPF I with a coupling topology adjustment. (a) Geometry configuration and Coupling topology. (b) Simulated results in a wide frequency
range. (Design parameters: a = 8.636, b = 4.318, w1 = 5.84, w2 = 8.22,
l = 6.8, d12 = 3.76, d23 = 0.6, d34 = 3.76, d14 = 2.8, dS1 = d4L = 5.34,
Units: mm).
Fig. 6.
Sensitivity analysis to capacitive iris tolerances (± 50 μm)
of (a) BPF I and (b) BPF III.
equation [21]:
k23 =
Fig. 4. BPF III: BPF II using an increased-height resonator. (Design parameters: a = 8.636, b = 4.318, w1 = 5.8, w2 = 8.2, l = 6.8, d12 = 3.76,
d23 = 1, d34 = 3.76, d14 = 2.8, dS1 = d4L = 5.34, h = 0.982, Units:
mm).
B. Increased Capacitive Iris
To further increase the size of the capacitive iris to reduce
the sensitivity to fabrication tolerances so that it is physically realizable, an increased-height resonator is proposed and
adopted. It was found that the electric coupling strength can be
adjusted by changing the height of the resonator itself, whereas
the resonant frequency of the dominant TE110 mode remains
unchanged due to its zero wavenumbers in the z-direction,
as represented in (1). Based on this property, the BPF III is
designed using the increased-height resonators-2 and -3, as
illustrated in Fig. 4. Note that the heights of resonators-1 and
-4 keep the same with the standard waveguide since there is
no effect on the magnetic coupling strength.
Based on the above-mentioned description, the relationship between the electric coupling strength and the resonator
height, as well as the size of the capacitive iris is further
investigated. The electric coupling coefficient between two
coupled resonators can be extracted by using the following
f12 − f22
f12 + f22
(2)
where f1 and f2 are the lower and higher resonance frequencies,
respectively. They can directly be calculated from the eigenmode simulation. The coupling structure of resonators-2 and
-3 under the electric coupling is shown in Fig. 5 (a), where
d23 and h are the sizes of the capacitive iris and the height
variation of the resonator, respectively. The simulated coupling
coefficient curves versus different dimensions of the d23 and
h are plotted in Fig. 5(b) and Fig. 5(c). It is clear from
Fig. 5(b) that increasing h allows the slope of the electric
coefficient k23 to be smaller. The k23 rises as h increases, as
shown in Fig. 5(c). Therefore, filters using the resonator with
increased height are expected to be less sensitive to fabrication
tolerances. Subsequently, considering the convenience of the
processing and the compactness of the overall 3-D geometry,
a 1 mm capacitive iris was used to design the filter so that the
sensitivity to fabrication tolerances can be further reduced. In
this case, the total height of the resonators-2 and -3 is 5.3 mm
(h = 0.982 mm). The sensitivity analysis to capacitive iris
tolerances values of ± 50 μm (resolution of the SLA 3-D
printer) in BPF I and BPF III are depicted in Fig. (6). As
predicted, the BPF III using an increased-height resonator is
far less sensitive to fabrication tolerance than the typical filter
using the original height resonator.
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Fig. 7.
Simulated S21 of the BPF II and BPF III.
Fig. 10.
Fig. 8.
Simulated S21 of the BPF IV with different DGSs.
BPF IV: BPF III with two types of DGSs.
Fig. 11. 3-D printed filter. (a) Photograph of the prototype before surface
metallization. (b) Photograph of prototype after electroless copper plating.
can also facilitate the electroplating process, which allows
the metal layer on the inner surface of the filter to be more
homogeneous.
Fig. 9. Surface current distributions of the resonators-2 and -3 in BPF IV.
(a) TE110 . (b) TE101 . (c) TE210 .
Moreover, to confirm that there is no effect of the resonator
height on passband performance, the simulated responses of
BPF II and BPF III are depicted in Fig. 7. It can be seen from
the results that the responses of the two filters keep consistent
with each other.
C. Suppression of Spurious Modes B and C
As illustrated in Fig. 2 (b) and (c), the spurious Modes
B and C are induced by the TE101 mode and TE210 mode,
respectively, and distributed within the resonators-2 and -3.
To suppress those two modes, two types of DGSs are etched
on the filter shell located at resonators-2 and -3, which is
named BPF IV and shown in Fig. 8. The suppression principle by adding DGS is that the surface current of the spurious
modes is disturbed and radiated while keeping the dominant
mode intact. The surface current J s on the inner wall can be
expressed using the following equation [20]:
→
J =−
e ×H
(3)
s
n
→
where −
en and H are unit vector normal to the boundary and
magnetic field, respectively. As can be seen from Fig. 9, DGS
2 intersects the currents of the TE101 on the sidewall in the
x-direction, and DGS 1 is vertical with the currents on the
upper surface of the TE210 . However, the dominant TE110 surface currents and two types of DGSs are always parallel and
the current flowing paths are not cut so that there is no significant effect on the passband. Therefore, the spurious modes
can be suppressed. Fig. 10 plots the responses of BPF IV with
different DGSs, further verifying that Modes B and C are suppressed by DGSs 2 and 3, respectively. Furthermore, DGSs
IV. FABRICATION AND M EASUREMENTS
The filter prototype is first printed monolithically using the
SLA 3-D printing process to generate a plastic body, and
then metalized utilizing electroless and electroplating copper
plating techniques. Compared with the multi-piece assembly structure, 3-D printing in one piece simplifies the design
complexity and eliminates the additional losses caused by
assembly and gaps. A ceramic-filled photosensitive resin is
used to print the filter model using an SLA printer with a
vertical resolution of 50 μm. Considering the thickness of the
metalized layer on the device surface, the overall dimension
of the internal air cavity is slightly enlarged before printing to
compensate. Subsequently, a copper layer with a thickness of
10 μm is coated onto the resin surface using the electroless
and electroplating plating process to form a metalized surface.
It is noted that the thickness of the copper layer is over twenty
times the skin depth (0.395 μm calculated at 28 GHz). A photograph of the fabricated filter before and after metallization
is shown in Fig. 11.
The proposed 3-D printed filter was measured with a vector network analyzer AV3672E under a thru-reflect-line (TRL)
waveguide calibration. Fig. 12 shows the simulated and measured results of the filter, and excellent agreement among them
can be observed. The measured passband insertion loss (IL)
is 0.16 dB and the return loss (RL) is better than 20 dB. Note
that the simulated S21 of the filter without DGSs is also given
in Fig. 12(b) for a comparison. After adding DGSs, the IL
only increased by less than 0.1 dB, which is acceptable.
Fig. 13 plots the S21 of the filter over a wideband frequency
range from 20 to 40 GHz. It can be found that the proposed
filter effectively suppresses the spurious modes by adjusting
the coupling topology and introducing DGSs, realizing an
improved out-of-band rejection performance.
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HUANG et al.: 3-D PRINTED mm-WAVE FILTER USING INCREASED-HEIGHT DGS RESONATOR
4297
By using the SLA 3-D printing technique, the prototype is fabricated. Excellent consistency is achieved between simulated
and measured results. It implies the proposed waveguide filter
can serve the 5G mm-wave applications well.
R EFERENCES
Fig. 12. Measured and simulated results of the filter. (a) Responses over the
whole band. (b) Enlarged view of S21 over the passband.
Fig. 13.
Measured and simulated results of the filter.
TABLE I
C OMPARISON W ITH THE S TATE OF THE A RTS
Finally, a comparison with state-of-the-art BPFs is listed
in Table I. It indicates that the proposed 3-D printed filter
exhibit not only a very low IL and excellent quasi-elliptic
performance, but also a good spurious suppression.
V. C ONCLUSION
In this letter, a 3-D printed waveguide filter is proposed.
Compared with the typical 4th -order cross-coupling filter, three
spurious modes below and above the passband are suppressed
by introducing the coupling topology adjustment and two types
of DGSs on the filter shell, extending the spurious-free region.
Moreover, an increased-height rectangular resonator is utilized
to reduce the sensitivity to fabrication tolerances and accuracy.
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