Varactor-Tuned Hairpin Bandpass Filter
With An Attenuation Pole
Moon-Seok Chung, Il-Soo Kim, and Sang-Won Yun, Member, IEEE
Department of Electric Engineering, Sogang University, Sinsu-dong, Mapo-gu, Seoul, 121-742, Korea
E-mail: doorrock2@sogang.ac.kr
Abstract—In this paper, a varactor-tuned hairpin bandpass filter
using a tapped open stub is studied. In this configuration the filter’s
center frequency as well as the bandwidth can be controlled. The
tapped open stub introduces the attenuation pole which can be
located near the passband by adjusting the stub length. Varactors
loaded at the end of hairpin resonator and the tapped open stub
enable us to control the center frequency with constant bandwidth as
well as to have a large tuning bandwidth. Based on the detailed
analysis, the varactor-tuned hairpin bandpass filter is designed, in
which the tuning bandwidth reaches 15 % of the center frequency at
2 GHz.
Index Terms—Bandpass filters, microstrip filters, tunable filters,
varactors.
range [10]. In case of interdigital tunable bandpass filter, parasitic
effects of the varactor series resistance and the resonator electric
length on the overall resonator Q factor are discussed in Brown’s
paper [9].
The conventional combline and interdigital bandpass filter have
trouble in fabrication because short circuits are inevitable.
However, the proposed hairpin structure does not require short
circuits [11]. As a tapped open stub at the general hairpin
resonator is added, the filter size can be reduced as well as the
skirt characteristics become improved. The center frequency and
bandwidth can be tuned using varactors loaded at the end of
hairpin resonator and tapped open stub. Simple design equation
and the equivalent circuit are derived, and the experimental results
are compared with the theoretical ones.
I. INTRODUCTION
II. HAIRPIN TUNABLE BANDPASS DESIGN
Recent advances in modern ultrawide-band radar systems and
wireless communication applications demand high-performance
and reconfigurable RF subsystems [1], [2]. These trends have
been required on the microwave passive and active filters directed
at these application [3], [4]. Specifically, a tunable bandpass filter
which has compact size and enables us to control the center
frequency and bandwidth should be one of the main goals.
The tunable bandpass filter has been widely designed using
microstrip line resonators which offer a very compact realization
[5], [6]. These filter structures have been used variable reactance
elements to tune the center frequency and bandwidth. The use of
semiconductor varactors as variable capacitors has the most
popular choice to control the filter response [7]-[10]. Microstrip
combline and interdigital tunable filters have been described by
many authors. Hunter’s tunable bandpass filter has compact
configuration and broad tuning ranges can be achieved while
retaining minimum degradation in passband performance [5].
Also, Kim’s combline tunable bandpass filter using
step-impedance microstrip line is presented that the passband
bandwidth can be maintained nearly constant within the tuning
A. Hairpin resonator with the tapped open stub
The hairpin tunable bandpass filter is shown in Fig. 1. This
hairpin tunable bandpass filter is composed of two microstrip
quarter wavelength resonators and a tapped open stub.
W1
Z
W3
l1
0
l2
θ1
R3
W2
R1
R2
S1
Fig. 1. Schematic diagram of the hairpin bandpass filter.
0-7803-9433-X/05/$20.00 ©2005 IEEE.
APMC2005 Proceedings
That structure is the second order filter form, but keeps the first
order filter’s size. Also, attenuation poles can be changed at upper
and lower side of the passband edge by adjusting a tapped open
stub length.
The equivalent circuit of the proposed hairpin bandpass filter is
shown in Fig. 2. As shown in Fig. 2, the proposed bandpass filter
consists of front-end J inverter and open stub’s T-junction.
T-junction’s equivalent circuit is composed of LA ,
caused by discontinuity of R1 ,
the tapped open stub has
θ1
CT which
R2 and R3 in Fig. 1 [12]. Also,
LB element and electric wavelength θ 2 .
Φ/ 2 −Φ/2
−Φ/2 Φ/2
θ1
W2'
S1
W2
S1'
Fig. 4. The proposed varactor-tuned hairpin bandpass filter.
We replaced hairpin resonator and open stub’s reduced length
with varactors which have variable capacitance for compensating
reduced open microstrip lines. In detail, an open-circuited stub of
microstrip line can be equivalent to a shunt capacitor in Fig. 5
[13].
θ2
Y 'in
Yin
Fig. 2. The detailed equivalent circuit.
Finally, the equivalent circuit in Fig. 2 is converted the final
equivalent circuit in Fig. 3, which is consisted of J-K-J inverter’s
second order bandpass filter.
θ1
Φ/ 2
−Φ/ 2
−Φ/ 2
Φ/2
ZC
θ1
L<λ/4
Fig. 5. The open-circuited stub.
According to the transmission line theory, the input admittance
of an open-circuited transmission line having a characteristic
admittance YC = 1 / Z c and propagation constant β = 2π / λ is
Fig. 3. The final equivalent circuit of Fig. 2.
B. Center frequency and bandwidth tuning
For the center frequency and bandwidth tuning, general hairpin
resonator and open stub’s length must be changed in Fig. 4. The
center frequency can be controlled by adjusting hairpin
resonator’s length. Also, an attenuation pole at the upper side can
be tuned by changing the open stub length.
give by (1).
§ 2π ·
Yin = j YC tan ¨
l¸
© λ ¹
(1)
where l is the length of the stub. If l < λ / 4 , this input
admittance is capacitive. The capacitance of the open stub length
is calculated in (2), (3).
Yin = Yin'
§ 2πf ·
YC tan¨
l¸
c ¹
©
C=
2πf
(2)
(3)
In order to compensate the reduced front-end inverter values,
0
'
III. EXPERIMENTAL RESULTS
A varactor-tuned hairpin bandpass filter has been designed and
tested according to the following specifications:
Bandwidth tuning range
300 MHz
Center frequency tuning range
300 MHz
Number of filter orders
two
Type
0.01-dB Chebyshev
The varactor diode used in bandwidth tuning design is MA46601,
and the one in center frequency tuning design is 1SV277. The
substrate used is Rogers ( ε r = 3, h = 15 mil). Initially, the
characteristic impedance values of the microstrip lines of the
proposed filter are selected conveniently as Z1 = 50 Ω (width
W 1 = 0.935 mm) for the general hairpin resonator and Z 2 = 40 Ω
(width W 3 = 1.4 mm) for the open stub. The hairpin resonator and
open stub lengths are S 1 = 0.1, W 2 = 0.4, l1 = 9.065, l2 = 7 (unit:
mm) in Fig. 1.
S-parameter
-5
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2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
frequency ( GHz)
Fig. 7. (a) The experimental bandwidth tuning result (S 21 ) of the varactor-tuned
hairpin bandpass filter with an attenuation pole.
0
-5
S-parameter
'
gap ( S1 ) and width (W 1 ) are adjusted as S1 and W 1 in Fig. 4.
As a result, varactor at the end of the open stub enables us to tune
bandwidth. While the lower side cutoff frequency is fixed, the
bandwidth can be increased to upper frequency as being the
variable capacitance of the varactor in Fig. 6. Also, three varactors
at the end of the hairpin resonator and the open stub make possible
to control center frequency with the constant bandwidth in Fig. 7.
The center frequency can shift to upper or lower frequency as
three varactors controls simultaneously.
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2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
frequency ( GHz )
Fig. 7. (b) The experimental bandwidth tuning result (S 11 ) of the varactor-tuned
hairpin bandpass filter with an attenuation pole.
Fig. 6. Experimental varactor-tuned hairpin bandpass filter.
The varactor capacitance values for compensating reduced open
stub microstrip lines are considered for line lengths determination.
The experimental bandwidth tunable bandpass filter performance
is shown in Fig. 7. While the lower side cutoff frequency is fixed,
the bandwidth can be increased to upper frequency as being the
variable capacitance of the varactor. Also, the center frequency
tunable bandpass filter response is presented in Fig. 8. Three
varactors at the end of the hairpin resonator and the open stub
make possible to control center frequency with the constant
bandwidth. The center frequency can shift to upper or lower
frequency as three varactors controls simultaneously. The
1SV277 varactor diode has lower Q-factor than MA46601.
Therefore, the skirt characteristic of the bandwidth tuning filter
response is better than the one of the center frequency tuning filter
response. The measured results show that tunable bandpass filter
has 15 % tuning range of the center frequency at 2 GHz and
enables us to adjust 300 MHz bandwidth tuning. Fig. 6 shows the
photograph of the designed tunable bandpass filter.
0
S-parameter
-5
IV. CONCLUSION
We have proposed a varactor-tuned hairpin bandpass filter with
an attenuation pole. The open stub enables us to reduce the filter
size as well as improve characteristics at the skirt frequencies. The
attenuation pole can be located near the passband by adjusting the
stub length. By using varactors at the end of the hairpin resonator
and open stub for compensating reduced open microstrip lines, the
center frequencies are made tunable within the tuning bandwidth,
while the passband bandwidth remains nearly constant. Since the
proposed bandpass filter is not only compact in size but also has a
wide tuning range it can be applicable to the wireless system such
as ultrawide-band.
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ACKNOWLEDGMENT
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This research was supported by the Agency for Defense
Development, Korea, through the Radiowave Detection Research
Center at Korea Advanced Institute of Science & Technology.
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1.3
1.5
1.7
1.9
2.1
2.3
2.5
frequency ( GHz )
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[2]
Fig. 8. (a) The experimental center frequency tuning result (S 21 ) of the
[3]
varactor-tuned hairpin bandpass filter with an attenuation pole.
[4]
[5]
0
S-parameter
-5
[6]
-10
[7]
-15
[8]
-20
[9]
-25
[10]
-30
-35
1.3
[11]
1.5
1.7
1.9
2.1
2.3
2.5
frequency ( GHz )
Fig. 8. (b) The experimental center frequency tuning result (S 11 ) of the
varactor-tuned hairpin bandpass filter with an attenuation pole.
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