A Wideband Bandpass Filter with Broad Stopband and Ultra-Wide
Reflectionless Range for 5G Applications
Changkun Liu, Zhixian Deng, Xiaohui Liu, and Xun Luo
Center for Integrated Circuits, UESTC, Chengdu 611731, China
Abstract — In this paper, a wideband bandpass filter with the
broad stopband and ultra-wide reflectionless range is proposed.
Such filter consists of two parallel connected channels. The
good in-band performance and broad stopband of the filter are
achieved using the dumbbell-shaped defected ground structure
(DGS) in the main channel. Meanwhile, the auxiliary channel
with a bandstop filter (BSF) and a loaded resistor is introduced
to obtain the ultra-wide reflectionless range. Besides, to further
enhance the stopband performance of the filter, spur-lines are
embedded in the I/O port of main channel. To verify the
mechanisms mentioned above, a reflectionless filter operating at
3.1-5.3 GHz is proposed. The measured stopband is expanded to
27.4 GHz with an attenuation level higher than 30 dB. Moreover,
the reflectionless range is from 10 MHz to 26 GHz, which is the
state-of-the-art performance. With such good performance, the
proposed filter shows strong merits for the 5G applications.
Keywords — Bandpass filter, broad stopband, defected ground
structure (DGS), reflectionless, wideband.
I. INTRODUCTION
The ever-increasing demands of high data-rate and in the
wireless communication driving the development of wideband
bandpass filter with broad stopband in recent years [1]. A
large variety of wideband bandpass filter (BPF) with a wide
stopband [2]-[5] have been reported, which show merits of
the broad stopband [4] and ultra-wide passband [5]. However,
those conventional filters reflect the signals of stopband back
to the source, which cause the intermodulaiton interference
and gain fluctuations in the mixer and high gain amplifier
for practical wireless communication applications, especially
the 5G wireless communication. Thus, reflectionless filters are
paid great attention in recent years to meet the requirements of
interference suppression of 5G applications. Nevertheless, the
reported reflectionless filter implemented by lumped-element
[6] or microstrip-line [7] suffer from the fundamental limi-
tation of narrow bandwidth and poor stopband performance.
Therefore, the design of wideband bandpass filter with broad
stopband and ultra-wide reflectionless range still remains a
great challenge.
In this paper, a wideband bandpass filter with the broad
stopband and ultra-wide reflectionless range is proposed to
cover the n77 band (3.3-4.2 GHz) and n79 band (4.4-5.0
GHz) of 5G New Radio (NR) [8]. Such filter consists of
two paralleled complementary channels. Firstly, the dumbbell-
shaped defected ground structure (DGS) is introduced in
the main channel to achieve the wide passband and broad
stopband. Secondly, the auxiliary channel with a bandstop
filter (BSF) and a loaded resistor is in paralleled with the
main channel to achieve the ultra-wide reflectionless range.
Z
L
Port1
Port2
Ground
Auxiliary
channel
Spur-lines
Port1
Port2
Passband signals
Other siganls
(a) (b)
Main
channel
Auxiliary
channel
Main
channel
BSF
BPF
Vias
Fig. 1. (a) Layout and (b) signal-flow of the proposed filter.
Thirdly, the stopband performance is further enhanced based
on the spur-lines located in the I/O port of the main channel.
Based on the structures mentioned above, a reflectionless
filter is designed and fabricated. A good agreement between
measurement and simulation is achieved.
II. SCHEMATIC AND OPERATION
Fig. 1(a) shows the layout of the proposed filter with two
parallel connected channels. In the main channel, spur-lines
are located between the port 2 and a pair of T-junctions with
a dumbbell-shaped DGS. Meanwhile, the auxiliary channel
is composed of a microstrip line and a BSF loaded with a
resistor. Fig. 1(b) depicts the signal-flow of the reflectionless
filter. To investigate the mechanisms of the proposed structure,
the RT5880 substrate (i.e.,
r
= 2.2 and h = 0.508 mm), EM
simulator IE3D, and design tool ADS are used.
A. Network Model
Fig. 2(a) and (b) depict the network model and signal-flow
of the conventional bandpass filter and proposed reflectionless
filter, respectively. Compared with the conventional filter, an
auxiliary channel is introduced in parallel with the BPF in the
proposed reflectionless filter, which consists of a transmission
line (i.e., Z
0
/θ) and a BSF loaded with a resistor Z
L
. Then,
the S-parameters of proposed filter can be derived from S
11p
and S
11s
of the BPF and BSF as follow:
S
11
=
2S
11p
− (1 + S
11p
)(1 − S
11a
)/S
11a
2 + (1 + S
11p
)(1 − S
11a
)/S
11a
(1)
S
21
=
2S
21p
2 + (1 + S
11p
)(1 − S
11a
)/S
11a
(2)
978-1-7281-1309-8/19/$31.00 © 2019 IEEE 2019 IEEE/MTT-S International Microwave Symposium
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