Reconfigurable Dual-Band Bandpass Filter with Fully-Switch Operation
Using
λ/2 Folded-Resonator with Varactor-Loaded Open-Stub
Zhen Tian, Huizhen Jenny Qian, and Xun Luo
Center for Integrated Circuits, UESTC, Chengdu, 610054, China
Abstract—In this paper, a prototype of reconfigurable dual-
band bandpass filter is proposed. It features independently
frequency-controllable quasi-elliptic-type passbands with stop-
band transmission zeros that can be intrinsically switched on/off.
Tunable dual-resonance (i.e., f
1
and f
2
) is introduced in this
filter by the half-wavelength (λ/2) folded-resonator with series
varactor-loaded open-stub. To verify the operational mechanism
above, a tunable dual-band bandpass filter with added passband-
switching capability is fabricated and measured. It exhibits
frequency-tuning ranges of 38.2% and 33.1% for the dual-band,
respectively.
Index Terms—Bandpass filter, dual-band, fully-switch opera-
tion, reconfigurable, varactor-loaded.
I. INTRODUCTION
With ever-increasing demands for application flexibility to
support multiple-standard communication, wireless systems
are leaning towards multi-band with the smart tunable and
switchable operation. As one of essential components in such
system, reconfigurable bandpass filters with multi-function
capabilities are highly demanded [1]-[4], while compared to
fixed multi-band prototypes [5]-[8] with good band-selectivity
responses. Optional multi-band and accurate frequency se-
lection with a compact circuit size are focal issue in recent
filter researches. Multiple transmission zeros [3] are developed
for multi-band enhancement. Meanwhile, to increase external
quality factors of operation bands, the open/short stub [6] and
stepped-impedance resonators [7] are introduced. However,
to meet requirements of future intelligent communication,
bandpass filter with fully reconfigurable operation still remains
a great challenge.
In this paper, a compact prototype of reconfigurable band-
pass filter with individually controllable dual-resonance and
fully switchable operation is proposed. Firstly, the half-
wavelength (λ/2) folded-resonator with series varactor-loaded
open-stub are introduced to allocate the tunable dual-resonance
(i.e., f
1
and f
2
), respectively. Secondly, based on such
resonators with a 0
◦
-feed coupled-scheme, shunt varactor-
loaded circuits are embedded to the filter structure. Such
implementation can not only improve the dual-band tunable
operation, but also achieve the fully switchable responses of
dual-band. Based on the principle above, a reconfigurable dual-
band bandpass filter is fabricated. Measured tuning ranges
of 0.515–0.758 GHz and 0.977–1.304 GHz for the lower
and upper passbands, respectively, are achieved. As an added
feature, these transmission bands can also be intrinsically
switched on/off.
Fig. 1. Prototype of the proposed reconfigurable bandpass filter.
II. SCHEMATIC AND OPERATION
Fig. 1 shows the proposed prototype of the reconfigurable
dual-band bandpass filter with fully-switch operation. This
filter consists of a pair of λ/2 folded-resonators (i.e., Z
f
/θ
f
)
with loaded open-stub (i.e., Z
o
/θ
o
, Z
o
= Z
s
), which are
implemented as a 0
◦
-feed coupled scheme. Meanwhile, four
pairs of loaded-varactors are located at the end of single and
coupled loaded open-stubs, respectively. To investigate the
mechanism of the proposed structure, the EM simulator IE3D,
ADS, and RT/5880 dielectric substrate with
r
of 2.2 and a
thickness of 0.508 mm are used.
A. λ/2 Folded-Resonator with Series Varactor-Loaded Open-
Stub
Fig. 2. depicts the simple scheme of the fixed λ/2 folded-
resonator (i.e., Z
f
/θ
f
, where θ
f
= θ
f 1
+ θ
r
+ θ
b
and θ
r,b
=
θ
f 2
+ θ
cr,cb
) with a loaded open-stub (i.e., Z
o
/θ
o
, where Z
o
= Z
f
). The fundamental resonance f
1
is allocated by the λ/2
folded-resonator, which can be calculated as
f
1
=
c
(l
f1
+2l
f2
+ l
cr
+ l
cb
)
√
eff
(1)
where
l
cr,cb
=
λ arctan (Z
f
C
r,b
2πf)
2π
=
θ
cr,cb
β
(2)
l
f1
+2l
f2
is the physical length of the fixed λ/2 folded-
resonator, l
cr
and l
cb
are equivalent lengths of varactor-loaded
capacitance C
r
and C
b
, c is the velocity of light in free
space, and
eff
is the efficient dielectric constant of substrate,
respectively. Then, to further investigate the dual-resonance
mechanism, the input impedance Z
in
of the resonator is
derived as
Z
in
=
−jZ
f
[B cot θ
b
tan(θ
r
+ θ
b
)+A]
(A − B) tan(θ
r
+ θ
b
) − B cot θ
g
− cot θ
b
cot θ
r
(3)
978-1-5090-6360-4/17/$31.00 ©2017 IEEE918