A Tunable 0.6 GHz 1.7 GHz Bandpass Filter With a Constant
Bandwidth Using Switchable Varactor-Tuned Resonators
Feng Lin and Mina Rais-Zadeh
Electrical Engineering and Computer Science Department, University of Michigan, Ann Arbor, MI 48109 USA
fngln@umich.edu, minar@umich.edu
Abstract — This work reports on a tunable second-order
bandpass filter with a constant bandwidth using switchable
varactor-tuned resonators. A wide center frequency tuning range
of 600 MHz to 1711 MHz is obtained using p-i-n diodes to switch
in and out λ/4 or λ/2 resonators for low-band or high-band modes,
without increasing the tuning capacitance range of the varactors.
A combination of electric and magnetic coupling is utilized to
realize a near constant absolute bandwidth across the tuning
range. The filter is fabricated on a Duroid substrate with εr =2.2
and h=0.787 mm. When the p-i-n diodes are on, the center
frequency is tuned form 1070 MHz to 600 MHz while
maintaining a 3-dB bandwidth of 114±10 MHz, insertion loss of
less than 3.3 dB, and return loss of better than 15 dB. The IIP3
and P1dB are better 18.6 dBm and 12 dBm, respectively. When
the p-i-n diodes are off, the center frequency is tuned form 1711
MHz to 1070 MHz (and potentially down to 1000 MHz) while
maintaining a 3-dB bandwidth of 88±8 MHz, insertion loss of
less than 4 dB, and return loss of better than 13 dB. The IIP3 and
P1dB are better 13.5 dBm and 10 dBm, respectively. The rejection
level at 200-MHz offset frequency from the passband center
frequency is better than 22 dB across the entire tuning range. To
our knowledge, this planar bandpass filter exhibits the widest
tuning range with a near-constant bandwidth.
Index Terms —Microstrip filters, tunable filters, UHF band,
varactor diode, wide tuning range.
CV1
Switch 1
CV1
Switch 1
Ze1, Zo1, θ1
Z4, θ4
Z4, θ4
Zea, Zoa
Zeb, Zob
θ
Port 1
Cm
Port 2
Cm
Ze2, Zo2, θ2
Switch 2
Z3, θ3
CV2
Switch 2
Z3, θ3
CV2
Fig. 1. The architecture of the tunable filter. The filter has switchable
varactor-tuned resonators operated in a low-band and a high-band
mode to achieve a wide tuning range.
I. INTRODUCTION
Modern transceiver systems are required to be
reconfigurable to address multiple frequencies and
communication standards. As a key component of RF frontends, the bandpass filter is required to be low loss, widely
tunable, low power, small size, fast, and linear [1]. In the
ultra-high frequency (UHF) band (i.e., 300 MHz to 3 GHz),
planar transmission-line filters [2]-[4] offer the widest tuning
range and a moderate unloaded quality factor (Qu) compared
to other alternative implementations such as lumped LC filters
[1] and cavity filters [5]. For the lumped LC and cavity filters,
the main tuning elements are RF microelectromechanical
systems (MEMS) switches or varactors. The challenge in
designing a widely tunable planar filter is to maintain a proper
filter response across the entire tuning range. To achieve this,
there are two main issues: 1) designing of a wide tuning range
resonator while maintaining the desired coupling coefficient
between resonators and 2) port matching across the entire
frequency range. Reducing the length of transmission lines [2]
can extend the frequency tuning range to some extent, but it
also reduces the Qu of the resonators. To further extend the
(a)
(b)
Fig. 2. Schematics of the switchable varactor-tuned resonator. (a)
Low-band mode. (b) High-band mode.
tuning range, switched-bank tunable filter configurations,
which use RF switches at input/output ports to select different
tunable filter branches, are often used. The drawback of this
method is the overall filter size and loss. In [4], an intrinsically
switched-bank tunable filter operating at 740 MHz  1644
MHz was demonstrated, where three different tunable filters
were switched on and off. Without requiring additional RF
switches, the insertion loss was improved at the cost of
increased filter size. Thus, a simple and effective design
method for wide tuning range planar filters is an ongoing
challenge.
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
To meet the requirements of having a wide tuning range and
compact size, switchable tunable resonators and switchable
feed lines are utilized in this work to provide a wide band
match and a constant bandwidth. A tunable filter is
demonstrated with an insertion loss of less than 4 dB and
tuning range of 2.85:1 from 600 MHz to 1711 MHz, covering
half of the UHF band.
Cv1, Cv2, and RF switches are realized using two 180 k
resistors and two 5 k resistors, respectively. Fig. 3 (b)-(c)
show the simulated S-parameters for different values of Cv1
and Cv2. The filter area is 17×40 mm2 not including the
input/output microstrip lines (Fig. 4).
II. TUNABLE BANDPASS FILTER DESIGN
The proposed tunable filter consists of two switchable
varactor-tuned resonators (Fig. 1). The external coupling is
realized by a switchable feed line and a matching capacitance
Cm. Fig. 2 shows the schematics of resonators at low- and
high- band modes. To design a tunable filter with a constant
bandwidth, the Y-matrix of the two-port circuits in Fig. 2
should satisfy the following resonance and coupling
coefficient conditions across the entire tuning range [3]:
Im Y11 0    0 ,
Im Y12 0  
b


g1 g 2
 k12 ,
(1)
0  Im Y11 0  
, k12 is the coupling coefficient, gi

2
(i=0,1,2) is the lumped circuit elements of the lowpass
prototype, and ∆ is the fractional bandwidth.
The second-order Butterworth low-pass prototype is chosen
to design the filter. In the low-band mode, Switch 2 is on,
while Switch 1 is off. The frequency of the quarter-wavelength
resonators is tuned by varactors Cv1. The desired coupling
coefficient k12 is controlled by the dimensions of coupled lines
(Fig. 2(a)). In the high-band mode, on the other hand, Switch 2
is off, while Switch 1 is on. Cv1 and Cv2 are used to tune the
frequency of half-wavelength resonators. The ratio of Cv1 and
Cv2 and the dimensions of stub Z3 are additional design
parameters used to realize the desired k12 (Fig. 2(b)).
Dimensions of the feed line and matching capacitances Cm are
fine tuned to obtain the needed external Q (Qext=g0g1/∆) that
satisfies the bandwidth requirement over the tuning range.
where b 
|S| (dB)
(a)
|S21|
CV1=4, 2.3, 1.5, 0.7 pF
0.5
1.0
1.5
Frequency (GHz)
2.0
(b)
|S| (dB)
III. RESULTS
For experimental demonstration, a tunable filter operating
from 600 MHz to 1.7 GHz was designed and implemented.
The filter was fabricated on a Rogers RT/Duroid 5880 with εr
=2.2 and h=0.787 mm. Device models for varactors and p-i-n
diodes were incorporated in Agilent Technologies 2012
Advanced Design System (ADS) tool to simulate the filter
response taking into account all parasitic effects. Fig. 3(a)
shows the final layout of the filter. The matching capacitor Cm
and DC blocking capacitor Cbias are realized by ATC 600S
series capacitors. The capacitors Cv1, Cv2, and RF switches are
implemented by MA46H202 GaAs diodes and Infineon
BAR65-02V p-i-n diodes, respectively. The bias circuits for
0
-10
-20
-30
-40
-50
-60
-70
-80
0
-10
-20
-30
-40
-50
-60
-70
-80
0.5
CV1=4, 2 pF;
CV1=0.7pF;
CV2=4 pF
CV2=4, 2.4, 1.6, 0.8 pF
1.0
1.5
Frequency (GHz)
2.0
(c)
Fig. 3. (a) Filter layout showing the position of varactors, RF
switches, and DC bias networks. Simulated S-parameters for
different varactor values in (b) the low-band and (c) high-band mode.
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
MH
Hz with returrn loss of moore than 13 ddB (Fig. 6).
meaasured results are summarizzed in Fig. 7.
0
-10
-20
-30
-40
-50
-60
-70
-80
|S21| (dB)
a
Figg. 4. An image oof a fabricated tunable
t
filter.
=20 V, V
=0 V
sw1
sw2
= V, V =2 V
a: V =6
cv1
cv2
b: V =23
= V, V =23 V
cv1
cv2
1.0
1.5
Frequuency (GHz)
2.0
((a)
0
-5
|S11| (dB)
When the RF
F switches in resonators arre turned on (30
( V
biaas voltage) annd the bias vooltage applied to RF switchhes on
feeed lines is less than 0 V, a lower frequenncy passband (lowbaand) is achieveed (Fig. 5). Thhe center frequuency is tunedd from
0.66 to 1.07 GHz for a Cv1, Cv2 bias voltage of 2–23 V.
V The
meeasured inserttion loss and 3-dB bandwidth are 3.3–22.5 dB
annd 114±10 MHz, respectiveely, while the return loss is better
thaan 15 dB (Fig. 5). In this mode,
m
the addittional loss is m
mainly
atttributed to the loss of RF sw
witches on the resonators (~
~0.5 
forr two switchees connected iin parallel) annd can be impproved
usiing lower resistance switchees.
b
V
0.5
A. Insertion Losss, Return Losss, and Bandwiidth
The
-10
-15
-20
-25
0.5
1.0
1.5
Frequuency (GHz)
2.0
((b)
a
b
Insertion Loss (dB)
V
=0 V,
V V
=30 V
sw1
sw2
a: V
=V
V =2 V,
cv1 cv2
b: V =V
V =23 V
cv1 cv2
0.5
1.0
Frequuency (GHz)
1.5
2
2.0
(a)
125
3.5
120
115
3.0
110
2.5
105
100
2.0
600 650 700 750 800 850 900 950 1000 1050
Frequency (MHz)
(
0
Insertion Loss (dB)
|S11| (dB)
-10
-15
-20
0.5
1.0
Frequuency (GHz)
1.5
2
2.0
(b)
Figg. 5. Measured: (a) |S21| and (b)) |S11| in the low
w-band mode.
When the RF switches in reesonators are turned
t
off (0 V bias
vooltage) and thee bias voltage for RF switchhes on feed liines is
200 V, the filter rresponse is sw
witched to higgher frequencyy band
(hiigh-band) (Figg. 6). The centter frequency iis tuned from 1.0 to
1.771 GHz for a Cv1, Cv2 bias voltage
v
of 2–23 V. The respective
meeasured loss aand 3-dB banddwidth are 4.22–3.1 dB and 88±8
8
5.0
100
4.5
95
4.0
90
3.5
85
3.0
80
2.5
75
1000 11000 1200 1300 14400 1500 1600 1700 1800
Frequency (MHz)
33-dB
dB Bandwidth (MHz)
((a)
-5
-25
0.0
130
4.0
33-dB
dB Bandwidth (MHz)
Fig.. 6. Measured: ((a) |S21| and (b) |S11| of the highh-band mode.
|S21| (dB)
0
-10
-20
-30
-40
-50
-60
-70
-80
0.0
((b)
Fig.. 7. Measured insertion loss and bandwidthh for the filterr with
swittches in (a) on and
a (b) off statees versus tuned frequency.
B. Linearity
L
and Power Handling
T
The input thirdd-order intercept point (IIP3) and the inpput 1dB gain compreession point (P
P1-dB) are meaasured at diffferent
cennter passband frequencies. The extracteed IIP3 at 1-MHz
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
offset versus different center frequencies is shown in Fig. 8 (a).
The IIP3 ranges from 13.5 to 25.4 dBm as the center frequency
varies from 600 MHz to 1711 MHz. Since the two varactors
Cv1 and Cv2 are both on in the high-band mode, the IIP3 is
lower than that at the low-band mode. Fig. 8 (b) shows the P1dB of the tunable filter. The measured input P1-dB shows that the
filter in low-band and high-band modes can handle around 12
and 10 dBm of input power, respectively, before compression
occurs. These values are limited by nonlinearities of the RF
switches and varactors and can be improved using phase
change (GeTe) switches [6], [7] or RF MEMS capacitors [1].
IIP3 (dBm)
30
25
IV. CONCLUSION
A tunable UHF bandpass filter was introduced. The design
takes advantage of switchable varactor-tuned resonators and
switchable feeding networks to realize a wide tuning range
(600 MHz1711 MHz) and matching condition without
increasing the filter size. Table I compares the performance
parameters of the tunable filter in this work with reported
filters in the UHF band. Note that the filter in [8] is made with
a higher-Q cavity resonator and is not planar. In [9], each
individual filter has 2:1 tuning range and three filters are
placed in a bank to achieve the reported 6:1 total frequency
coverage. As shown, the presented tunable planar filter of this
work offers the widest tuning range, a moderate filter size, and
competitive power handling performance.
f=1 MHz
20
V. ACKNOWLEDGMENT
15
10
600
800
1000
1200
1400
Frequency (MHz)
1600
1800
This work is supported by the Defense Advanced Research
Projects Agency (DARPA) RF-FPGA program and by the
National Science Foundation (NSF) under EARS and
CAREER programs.
REFERENCES
(a)
[1] Y. Shim, Z. Wu, and M. Rais-Zadeh, “A high-performance continuously
P1dB (dBm)
14
12
[2]
10
[3]
8
6
[4]
600
800
1000
1200
1400
Frequency (MHz)
1600
1800
[5]
(b)
Fig. 8. Measured (a) IIP3 and (b) P1dB versus tuned frequency.
[6]
TABLE I
COMPARISON OF REPORTED TUNABLE FILTER PERFORMANCE
Ref. # of
poles
[1]
3
[3]
2
[4]
3
[8]
2
[9]
2
This
work
2
Tuning
elements
MEMS
IL
Range IIP3
Size
(dB) (GHz) (dBm) (mm2)
81-135 3.0-3.6 0.6-1.01 >20 11×15
MHz
Varactor
43±3 1.9-2.9 0.9-1.3 11-15 14×25
MHz
(BW1dB)
Varactor 50 MHz 3.1-4.9 0.74- N/A 119×101
1.64
Piezoelectric 1.1% 1.56- 0.98- N/A 41.5×24.9
disc
3.57
3.48
Varactor
4.6%- 2.4-4.2 0.1-0.62 5-14 80×65
6.4%
Varactor + 80-124 2.5-4.2 0.6-1.71 13.5- 17×40
p-i-n diode MHz
25.4
[7]
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[8]
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