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726
On the Development of Tunable Microwave Devices for Frequency
Agile Applications
Jia-Sheng Hong and Young-Hoon Chun
Department of Electrical, Electronic and Computer Engineering
Heriot Watt University, Edinburgh, EH14 4AS, United Kingdom
Abstract— This paper presents a recent development of electronically tunable microwave devices for frequency agile applications such as emerging cognitive radios and ultra-wide band
(UWB) wireless systems.
1. INTRODUCTION
Since the demands for wireless communications increase and become complex, RF front-ends need
multi-band or wideband circuits to satisfy several standards of wireless systems, which makes a
tunable circuit essential [1, 2]. An adaptive controlled system which can enhance the performances
of a wireless system also requires the tunable devices and circuits such as tunable phase shifters and
tunable filters. In this paper, we review some our newly developed tunable microwave devices, which
are based on new device structures and tuning elements [3–6]. For example, we have investigated
new tunable device structures comprised of variable characteristic impedance (ZC ) transmission
line which can be used as a distributed tuning circuit. Demonstrators for this type of tunable
devices have been designed, fabricated and tested. Both simulated and experimental results are
presented.
In addition to semiconductor tuning elements used, ferroelectric materials are of great interest
for the development of electrically tuned microwave components and circuits. Rapid tuning speed,
moderate insertion loss at microwave frequencies, high power handling capacity and simple fabrication process make them more attractive. In this paper, as a practical application of ferroelectric
materials, we will present some newly developed bandstop filters based on variable capacitors on
BST thin film. A tunable bandstop or band reject filter, which is considered as an application of
BST varactors in this work, has become more essential for wideband wireless communication systems as it can reduce incoming or emitted unwanted signals effectively. The designs using defected
ground structures or electromagnetic bandgap structures will be described.
2. TUNABLE PHASE SHIFTER
Figure 1(a) is a unit cell of conventional distributed phase shifter. Overall input impedance should
be designed to vary around 50 Ω and its delay will be changed, while variable capacitance devices,
such as varactor diodes (Dp ), have different values of capacitance (Cvar1 ). In this case, the return
loss can be degraded due to varying Cvar1 while a transmission line, which is inductive, has a fixed
value of ZC . It usually limits the tuning range of phase shifters. If we are able to implement a
device in Fig. 1(b), wider tuning range of phase shifters can be achievable. Fig. 1(c) illustrates how
a tunable ZC transmission line can be implemented with conventional devices such as CPW lines
and lumped elements. Simulated results show that ZC can be electronically varied from 55 to 100 Ω
at the center frequency of 800 MHz, which has higher value of ZC for impedance of a phase shifter.
A fabricated tunable phase shifter of this type is photographed in Fig. 2(a). It consists of four
tunable ZC lines and five tuning varactor diodes, Dp , which are adjusted by dc bias, VC1 . Varactor
diode, DZ , with adjustable capacitance, Cvar2 , is controlled by co have better return losses with
consistent phase differences. All the varactors used are Infenion BB833. Its differential phase shift
is demonstrated in Fig. 2(b) which shows the differential phase shift as a function of frequency for
several bias values for the condition that the return losses are less than −12 dB up to 850 MHz. It
can be seen from this graph that, at 850 MHz, the differential phase shift is continuously variable
from 0◦ to 90◦ by adjusting the bias. When VC2 is fixed, tuning range with the same return loss
property shrinks to 85◦ . From the measured data the figure of merit (i.e., degree/dB) for the phase
shifter can be extracted. Some typical data are given as follows. For a fixed VC2 of 20 V, when
VC1 varies the figures of merit are 86 degree/dB, 104 degree/dB and 114 degree/dB for VC1 = 6.5,
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ZC_ va r
ZC
C var 1
(Dp )
727
C var1
(Dp )
C var 1
(Dp )
C var 1
(Dp )
(a)
(b)
Ground
C var2 (DZ)
ZC_ va r
C SE
CS
C SE
Ground
(c)
Figure 1: Schematic diagram of a phase shifter using novel tunable transmission line; (a) a unit cell of the
conventional distributed phase shifter, (b) a unit cell of a proposed phase shifter, and (c) a detailed block
diagram for the realization of a tunable transmission line.
8 and 10 V respectively. As can be seen the figure of merit increases against VC1 . This is because
the capacitance of the varactors used is actually decreased when the bias voltage VC1 increases,
resulting in a higher quality factor at a larger VC1 . Using high Q varactors can improve the figure
of merit. Also, a higher figure of merit can be obtained if referring to another VC2 .
Bias for ph ase
as e shift
s hift
(VC1 )
Varactors to adjust phase
phas shift
e s hift
(Dp )
Varactors to adjust
ad justZCZC
(DZ)
Bias fo r ZC
(VC2 )
Differential phase shift [Degree]
120
VC1 =6.5 V,
VC2 = 20 V
100
80
VC1 =8.0 V,
VC2 = 20 V
60
40
20
VC1 =20 V, VC2 = 10 V
0
0.2
(a)
VC1 =10 V,
VC2 = 20 V
0.4
0.6
0.8
1.0
Frequency [GHz]
(b)
Figure 2: (a) Photograph of the fabricated tunable phase shifter. (b) Differential phase shift versus frequency
for selected values of varactors bias. The phase shift is with respect to the transmitted phase at VC1 = 20 V
and VC2 = 10 V bias.
3. TUNABLE FILTERS
3.1. Tunable Bandstop Filters
A bandstop filter (BSF) is designed at the center frequency of 1.5 GHz with the bandwidth of 20%
which has a schematic diagram of Fig. 3(a). Its resonators can be easily replaced with stepped
impedance line resonators of Fig. 3(b). Their frequency responses are identical at the center frequency. In order to make the circuit tunable, we modified it further as shown in Fig. 3(c) and
(d). Whereas Fig. 3(c) can be straightforwardly realized by connecting varactor chips at the end of
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shunt stubs, the realization of Fig. 3(d) would be sophisticated. We can change either the electrical
length or the characteristic impedance, or both of them as indicated in Fig. 4. The first method is
a capacitive loaded transmission line. It is quite frequently used to implement a phase shifter. The
second method is a variable ZC transmission line which could adjust the characteristic impedance
of the transmission line with slight change of electrical length described above.
(a)
(b)
Z12 ,
Z1a ,
Z1b ,
12
Z2a,
1a
Z2b ,
1b
(c)
2a
2b
(d)
Figure 3: (a) Schematic of a bandstop filter, (b) its modified circuit with SIRs and tunable BSFs using a
tunable SIR with lumped circuit (c) and distributed circuit (d).
(b)
(a)
(c)
Figure 4: Realization methods of a tunable transmission line of (a): (b) capacitor loaded transmission line
and (c) variable ZC transmission line.
Figure 5(a) shows a physical realization of the proposed tunable BSF, where the Ferroelectric
Barium-Strontium-Titanate (BST) film varactor chips are used as the tuning elements. The measured results are plotted in Fig. 5(b), showing a tuning range that is around 10%. This type of
BSF has not only good rejection, but also has an advantage to implement a DC bias circuit. In this
case, the DC voltage is applied to an additional coupled line instead of a transmission line which
is connected with input or output, which reduces spurious response from bias circuitry.
Another newly developed BST-varactor tunable bandstop filter (BSF) is based on a defected or
slotted ground structure as shown in Fig. 6(a). This structure can be explained as a kind of electro-
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Bottom V iew
S-parameters [dB]
Top View
BST
vara ctor
chips
Va riable Z C Transmission line
DC Bias Supply
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0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
S 11S 21S 11S 21S 11S 21-
0.6
0.8
1. 0
1.2
1. 4
1.6
0V
0V
30 V
30 V
60 V
60 V
1. 8 2.0
Frequency [GHz]
(b)
(a)
Figure 5: (a) Fabricated tunable BSF. (b) Measured results.
magnetic bandgap (EBG) structure. The BST varactor chips are attached onto the fabricated BSF
as illustrated. The fabricated tunable BSF with slotted ground structure has overall dimensions
of 5.0 by 2.5 cm2 . Scattering parameter measurements are performed using Agilent 8510B network
analyzer over the frequency range from 0.5 to 2.5 GHz. Fig. 6(b) gives the simulated and measured
responses of the bandstop filter in which we can observe that it operates at the center frequency
from 1.2 to 1.4 GHz and has the bandwidth 100 MHz. Hence, the measured tuning range is 14%.
0
-5
S21 [dB]
-10
-15
S imulatio n (2pF)
S imulatio n (3pF)
Mea sured (0V)
Mea sured (35 V)
-20
BST varactor
chips
DC bias
-25
0.5
(a)
1.0
1.5
2.0
2.5
Frequency [GHz]
(b)
Figure 6: (a) Tunable microstrip BSF filter based on slotted ground structure. (b) Simulated and measured
results.
Air Br idge
0
S-Parameters [dB]
So lid (Sy mbol) lines
– Mea
Measu red results
BST chips
BS
ch
Dotte d lines
– Simulated
S
re sults
-100
35V
-200
-300
VC =0V
S 11
S 21
DC bias
ZS , θS
(Variable ZC Tx Line )
(Variab
(a)
-400
1.0
1.5
2.0
2.5
Frequency [GHz]
(b)
Figure 7: (a) BST-varactor tunable bandpass filter. (b) Simulated and measured results.
3.0
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3.2. Tunable Bandpass Filters
Figure 7(a) shows a recently developed BST-varactor tunable bandpass filter (BPF). This is a
bandwidth-tunable dual-mode ring-resonator bandpass filter, which is designed by adopting a novel
tunable circuit based on variable ZC or tunable impedance transmission line. The variable ZC
transmission line is used for the perturbation of two degenerate modes which makes the bandwidth
of filter adjusted as the value of characteristic impedance of the transmission line. A demonstrator
of thin-film BST-based tunable bandpass filter with dual mode ring resonator is fabricated on CPW
substrate and measured. Fig. 7(b) gives the simulated and measured responses of the bandpass
filter in which we can observe that it operates at the center frequency of around 1.8 GHz and has
the adjustable 3-dB bandwidth from 276 MHz to 318 MHz. Hence, the tuning ratio of maximum
to minimum bandwidth is 1.15:1 for the given DC bias range.
4. CONCLUSIONS
Several electronically tunable devices including tunable phase shifters, tunable bandstop and bandpass filters have been presented. It has been shown that the use of the variable ZC transmission
line in association with the semiconductor or ferroelectric BST varactors can make the electronic
tuning effectively.
ACKNOWLEDGMENT
The support from the U.K. Engineering and Physical Research Council (EPSRC) is acknowledged.
The authors would also like to thank their project partners at the University of Birmingham and
Cranfield University for fabricating the BST thin film devices.
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