[ieee 2014 ieee students' technology symposium (techsym) - kharagpur (2014.2.28-2014.3.2)]...
TRANSCRIPT
Novel Tunable Band Reject Filter Using RF
MEMS Technology
Buddhadev Pradhan*, Bhaskar Gupta
Department of Electronics and Tele-communication Engineering, Jadavpur University,
Kolkata, West Bengal, India.
E mails: [email protected]*, [email protected]
*corresponding author
Abstract—A novel tunable band reject filter is designed and
simulated using metamaterials and RF MEMS technology. CSRR
on CPW line are used to achieve filter characteristics, while
tunability is ensured through incorporation of a MEMS variable
capacitor, enabling compatibility with planar IC technology. The
CSRRs are etched on both the signal line and ground planes of
the CPW with different CSRR structures. Tunability of the band
reject filter is achieved by putting the MEMS bridge in either up
or down state. The designed band stop filter rejection of stop
bands are around -19.87dB for down state around the centre
frequency 29.719GHz and -17.96dB for up state around the
centre frequency 33.172GHz. The proposed device structure is
simulated using ANSOFT HFSS v13® for RF analysis.
Keywords— CSRRs Band Reject Filter; RF MEMS; device
structure; equivalent model; RF characterization.
I. INTRODUCTION
In modern wireless and mobile communication systems
microwave filter plays an important role. Tunable microwave
filters are essential in many civil, military and long-distance
(satellite or terrestrial) communications and signal processing
systems [1-2]. Metamaterial structures like Split Ring
Resonators (SRRs), Complementary Split Ring Resonators
(CSRRs) etc have been recently in detecting and controlling
the spectrum of radio frequency (RF) signals in radar and
communication systems [3-4]. Further, research on Radio
Frequency (RF) Micro-Electro-Mechanical systems (MEMS) technology has become significant in field of MEMS. This
technology provides the possibility of low loss, high linearity
filtering in volumes comparable to those of integrated circuits
[5-9].
In this paper we propose a novel type of metamaterial
structure based on the application of CSRR on conventional
Co-Planar Waveguide (CPW) with MEMS bridge. Different
CSRR structures on CPW signal line and ground planes are
shown to affect inherent resonance characteristics of the filter
to have a sharp cut off around the resonance frequency.
MEMS based variable capacitors are employed along with the
metamaterials to achieve tunability over a wide range of
frequencies depending on the different heights of MEMS
capacitors.
The tunable band-stop filter reported in this paper shows good
simulated results. The center frequency of the filter may be
varied in Ka band from 33.17GHz to 29.71GHz. The insertion loss among pass band is always less than 1 dB, the return loss
among stop band is also within 1 dB.
II. PROPOSED DEVICE STRUCTURE
The structure shown in Fig.1(b) is a three dimensional view of
the band stop filter that has been designed by embedding
CSRRs on the central line and ground planes of a CPW made
of gold housed on silicon substrate with S/W/S of
30/200/30µm. Figs.1(a) illustrate the top view of a CPW band
reject filter with embedded CSRRs along with bridge
capacitor. The gray colour represents the silicon substrate, while yellow colour is used for the CPW with MEMS beam.
The CSRR resonators are patterned on the CPW central line
and both ground plane. For the silicon substrate relative
permittivity is 11.9, thickness is 275µm and width & length
are 1460µm and 1000µm respectively. An 1µm thick SiO2
layer acts as buffer.
The CPW transmission line and the metallic switch
membranes are made of a thin film of Au (1µm thick) having
high conductivity and good mechanical properties. Gap
between the CPW signal line and metallic beams is 2µm.
Dimensions of the metallic beams length, width and thickness
are given by l, w and t respectively. The length and width of the complementary split ring resonators for central line are
given by ‘h’ and ‘a’ respectively. ‘c’, ‘e’ and ‘f’, ‘d’ are the
width & spacing of the split rings respectively, while ‘g’ and
‘t’ are the gaps between the split rings. The ground plane
resonators length and width are ‘L’ and ‘A’ respectively. The
width & spacing of the split rings are ‘r’, ‘n’ and ‘k’, ‘q’
respectively. The gap between the split rings are ‘m’ and ‘v’
(Please see Fig.1(a)).
Proceeding of the 2014 IEEE Students' Technology Symposium
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Fig.1(a): Top view of the RF MEMS bridge with CSRRs loaded CPW
transmission line.
Fig.1(b): Three dimensional view of the RF MEMS bridge with CSRRs
loaded CPW transmission line.
III. EQUIVALENT CIRCUIT DEVELOPMENT AND
PARAMETER EXTRACTION
1. Equivalent circuit:-
We propose an equivalent circuit model as shown in Fig.2 to
provide a better understanding of the system. Lt, Ct are the CPW transmission-line inductance and capacitance, where as
Ls, Cs and Lr, Cr are CSRR loaded CPW signal line and
ground plane inductance and capacitance respectively. Cb is
the bridge capacitance of the MEMS beam. The equivalent
circuit developed from CSRR filter device structure, where we
embedded CSRR resonator with CPW transmission-line and
MEMS beam capacitors are put on the signal line in shunt.
The analytical expressions of the different elements of the
proposed equivalent model are given below. The total
capacitance presented by the CPW loaded transmission- line is
given by [10-11].
where
Here h is the height of the substrate.
Further,
which leads to
where c = velocity of light in free space. The proposed device equivalent circuit is simulated using Agilent ADS [12]
followed by RF device level simulation using HFSS [13].
Fig.2: CSRR filter CPW loaded transmission-line
Equivalent circuit network
The simulated up and down state results are shown in Fig.3
with superimposed HFSS and ADS generated data. In ADS
simulation results, the up and down state rejection of stop
bands are around -50.15 dB and -53.79 dB whereas the
corresponding values as obtained by HFSS are -18.10dB and -
Proceeding of the 2014 IEEE Students' Technology Symposium
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20.05dB respectively. ADS & HFSS combined plots show
close agreement validating our proposed circuit model. The
parametric study of proposed device structure is then
simulated using standard FEM tools for its RF characteristics.
The results are discussed in the following section.
Fig.3: Tunable CSRR filter HFSS & ADS simulation
S-Parameter results for up-state & down-state of RFMEMS bridge.
2. Parameters extraction:-
2.(a) Parametric study of the CSRR on the
central line of CPW:-
We started parameter extraction process through central line
CSRR simulation for different values of resonator length ‘a’
by keeping fixed the optimal width of the rings ‘c’, the
spacing between the rings ‘e’ and split between the rings ‘t’.
The variations of up and down state simulated results are
shown in Fig.4(a). As seen in Fig.4(a), as the length of the
central line of CSRR is decreased, the resonance frequency
increases. It is because the capacitance due to the CSRR is thus decreased.
Fig.4(a):Parametric study of the length ‘a’ of CSRR
on central line of CPW in HFSS simulator.
Next, simulations are run for same for different values of
CSRR ring width ‘c’, keeping the optimal length of the CSRR
‘a’, the spacing between the fixed rings ‘e’ and split between
the rings ‘t’ unchanged. FEM simulations clearly depict that,
as the width of the ring increases the inductive & capacitive
effects of the CSRR decreases, causing an increase in the
resonance frequency. The variations of up and down state
simulated results are shown in Fig.4(b).
Fig.4(b): Parametric study of the width of the rings ‘c’ of CSRR on central
line of CPW in HFSS simulator.
After parametric studies on the CSRR length & width, we simulated the CSRR for different values of the spacing
between the rings ‘e’, keeping the optimal CSRR length ‘a’,
width of the rings ‘c’ and split between the rings ‘t’. As seen
in the Fig.4(c), spacing between the rings increases, the
inductive effects of the CSRR decreases, leading to an
increase in resonance frequency.
Fig.4(c):Parametric study of the spacing between the rings ‘e’ of CSRR on
central line of CPW in HFSS simulator.
Finally the central line of CPW the CSRR is simulated for
different values of the split between the rings ‘t’, keeping the
fixed optimal length ‘a’, width of the rings ‘c’ and spacing
between the rings ‘e’ unchanged. As seen the variation in the
split in the rings do not cause much shift in resonance
frequency. The variations of up and down state simulated
results are shown in Fig.4(d).
Proceeding of the 2014 IEEE Students' Technology Symposium
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Fig.4(d):Parametric study of the split between the rings ‘t’ of CSRR on central
line of CPW in HFSS simulator.
2.(b) Parametric study of the CSRR on the
ground planes of CPW:-
The ground planes of the CSRRs are simulated for different
values of resonator length ‘L’, width of the rings ‘r’, the spacing between the rings ‘n’ and split between the rings ‘v’,
by keeping fixed the optimal dimensions of the other
parameters . The variations of up and down state simulated
results are shown in Figs.5(a, b, c, d) respectively.
Fig.5(a):Parametric study of the length ‘L’ of CSRRs on ground planes of
CPW in HFSS simulator.
Fig.5(b):Parametric study of the width of the rings ‘r’ of CSRRs on ground
planes of CPW in HFSS simulator.
Fig.5(c):Parametric study of the spacing between the rings ‘n’ of CSRRs on
ground planes of CPW in HFSS simulator.
Fig.5(d):Parametric study of the split between the rings ‘v’ of CSRRs on
ground planes of CPW in HFSS simulator.
From the parametric studies performed, list of parameters are
chosen to optimize design the CSRR based band reject filter for up & down state required frequencies of 33.17 GHz and
29.71 GHz. The list of the optimized chosen parameters of the
band reject filter for CSRR central and ground planes are
tabulated below [Table 1, 2].
TABLE I: LIST OF THE OPTIMIZED PARAMETERS OF THE
CSRR FILTER ON CENTRAL LINE OF CPW.
Parameters Values (µm)
CSRR length (a) 600µm
CSRR rings width (c) 25µm
spacing between rings
(e)
25µm
Split on the rings (t) 20 µm
Proceeding of the 2014 IEEE Students' Technology Symposium
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TABLE II: LIST OF THE OPTIMIZED PARAMETERS OF THE
CSRR FILTER ON GROUND PLANE OF CPW.
IV. PERFORMANCE OF TUNABLE FILTER & MEMS
SWITCH
1. Performance of tunable band-stop filter:
The dimensions for the CSRRs on central line and ground
plane are very crucial for obtaining the tunable band stop filter
performance [14]. The tunability of CSRR band reject filter is
achieved when applied voltage is varied between the MEMS
bridge and the CPW ground plane. Due to change of
capacitance value of the bridge in its up & down states, we
observe tunability of CSRR filter rejection frequency band.
Fig.6 shows result for simulated S-parameters.
Fig.6: CSRR filter optimized HFSS-simulation ‘S-Parameters’ result for
both up & down state of RFMEMS bridge.
2. MEMS switch:
When a voltage is applied between a fixed-fixed beam and the
ground plane, an electrostatic force is induced on the beam
[15]. This electrostatic force causes the deflection of the beam
gap height. An actuation voltage is required for transition
between up & down state configurations. The down state is
considered to be at a height of 2g/3 above the CPW ground
planes [15].
V. CONCLUSIONS
The concept of CSRR based CPW transmission-line filter with
MEMS capacitive loading is proposed. It used Micro-Electromechanical System (MEMS) fixed-fixed beam switch
and coplanar waveguide (CPW) structures to tune the
resonance frequency by DC actuation. The results of RF
simulation are satisfactory. An equivalent circuit is also
proposed. Good agreement between extracted circuit and
simulation results are observed.
ACKNOWLEDGMENT
The authors would like to acknowledge the National
Programme on Micro and Smart Systems (NPMASS) for
providing the necessary support.
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Parameters Values (µm)
CSRR length (L) 600µm
CSRR rings width
(r)
50µm
spacing between
rings (n)
50µm
Split on the rings (v) 50 µm
Proceeding of the 2014 IEEE Students' Technology Symposium
TS14P02 091 978-1-4799-2608-4/14/$31.00 ©2014 IEEE 339