RF MEMS Tunable Band Reject Filter using Metamaterials Buddhadev Pradhan , Bhaskar Gupta
Department of Electronics andTele-communication Engineering, Jadavpur University,Kolkata: 700032, West Bengal, India.
[email protected]* gupta
[email protected] *corresponding author _
Abstract:-.
Tuneable band reject filter is a very critical component of electronic warfare. The design is of such a filter using MEMS technology presented on CPW transmission line on silicon substrate using Complementary Split Ring Resonators (CSRRs) and RF MEMS variable capacitor, enabling compatibility with planar IC technology. The CSRRs are etched on both the signal line and ground planes of the CPW. Tunability of the band reject filter is achieved by putting the MEMS bridge in either up or down state. Through electrostatic actuation of the RF-MEMS switched capacitor, the electrical characteristics of the filter is modified, so that its resonance frequency can be tuned. The rejection of stop bands are around -20.19dB for down state around the centre frequency 35.32GHz and -18.29dB for up state around the centre frequency 38.80GHz. Keywords:-
Filter, characterization
CSRR
RF
MEMS,
metamaterial,
appropriate as substrate for fabrication of RF MEMS filters since it would introduce unacceptably high substrate losses. Due to this reason RF MEMS filters have so far been fabricated on high resistivity substrates like as glass, quartz, alumina [5-7] etc. However, a silicon substrate is necessary from the viewpoint of system integration. Keeping this in mind, we used high resistivity silicon as substrate for design and fabrication of RF MEMS tuneable CSRR filter. II. PROPOSED DEVICE DESIGN The length of the CSRR based metamaterial band reject filter i.e. "L" for ground plane and 'a' for signal plane are very crucial for obtaining proper filtering action. Figs.l(a) and l(b) illustrate the top view and cross section of a CPW band reject filter with embedded CSRRs along with bridge capacitor.
RF -+ ,
I. INTRODUCTION The study and realization of planer filter at microwave and millimetre wave frequencies using metamaterial structures is already reported [1]. Microwave filters are critical components for electronic warfare, radar and communication systems. But typical tunable filters are bulky, expensive and consume large amounts of power [2]. To overcome these limitations, planer filters utilizing metamaterial structures like Split Ring Resonators (SRRs), Complementary Split Ring Resonators (CSRRs) etc have been developed [3]. The inherent resonance characteristics present in such structures (CSRRs) enables the filter to have a sharp cut off around the resonance frequency. This technology provides the possibility of low loss, high linearity filtering in volumes comparable to those of integrated circuits. As CSRRs can be implemented by planer IC technology [4], an added advantage of CMOS compatibility may also be achieved. Standard normal resistivity silicon is not
-+
L
k
p
I M
'1
Fig.l(a): Top view of the RF MEMS bridge with CSRRs loaded CPW transmission line.
RF MEMS bri d :e
• D
Silicon substrate (275 ).1m)
•
Gold (Au) (i) RF MEMS beam (111m) (ii) CPW
Ct Eeft = -------(11) Cair
Silicon dioxide (111m)
Fig.l(b): Cross sectional view of the RF MEMS bridge
which leads to
Zo =
1
CCair .JEett -----
(12)
where c velocity of light in free space. Based on the theoretical model presented, parametric study of proposed device structure is performed using standard FEM tools for its RF characteristics. The results are discussed in the following section. =
with CSRRs loaded CPW transmission line.
It is seen that the length of CSRRs; 'L' for ground plane and 'a' for signal plane are dependent on length of the unit cell (y) as given in equation (1) [8].
Y=Ag/4 ------(1) where Ag is the guided wavelength and is given by
Ag = AO/ JEett ----- (2) Here, Celf is the effective permittivity and is approximately given by
Eett = (Er +1)/2 - - - - - (3) where, Cr is the relative permittivity of the substrate which is 11.9 for silicon. Metamaterials are used to denote isotropic, effective homogeneous media with negative permittivity and/or permeability. The total capacitance presented by the CPW loaded transmission- line is given by [9]. where
Ct = Csubs +Cair ----- (4) K(ko) Cair = 4Eo K(k' 0 ) - - - - - -(5) - - - -(6) Csub = 2E0 (Er - 1) -K(k'l) K(kl)
sinh[��lrrS/ 4h) kl = - - - - - (7) sinh[�H1T(S+2W)]/4h} Here h is the height of the substrate.
J
K'l = 1 - ki -- - - - - -(8) ko = S
J
S
+2W
------(9)
k'0 = 1 - k5 ------(10) Further,
III. RESULTS AND DISCUSSIONS 1. RF Characterization :(a) Parametric study of the CSRR on the central line of CPW:The CSRR is simulated 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.2(a). As seen in Fig.2(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.
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"
�
-10
�
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- a-400Upstate - a-500Upstate - a-600Upstate - a-400Downstate - a-500Downstate - a-600Downstate
-15
-20 10
20
30 Freq
IGHzj
"0
60
Fig.2(a):Parametric study of the length 'a' of CSRR on central line of CPW in HFSS simulator.
Next we simulate the same for different values of CSRR width of the rings 'c', keeping the optimal length of the CSRR 'a', the spacing between the fixed rings 'e' and split between the rings 't' . 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.2(b).
In ground planes 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.3(a, b, c, d) respectively.
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I
�
�
en
-I.
- 10
- c-15-upstate - c-20-upstat e - c-25-upstate - c-15-Downstate - c-20- 0 own state - c-25-Downstate
-2. -lO W
H
•
�
�
�
Freq [GHz[
Fig.2(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.2(c), spacing between the rings increases, the inductive effects of the CSRR decreases, leading to an increase in resonance frequency.
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-I.
�
;;
E
�
l-500Upstate -L-600Upstale
-1'
-L-700Upstale
en
-L-500DOlMlstate -l-600Downstate
-2.
-L-70000WTlState
I.
2.
3.
40
••
6()
Freq [GHz[
Fig.3(a):Parametric study of the length 'L' of CSRRs on ground planes of CPW in HFSS simulator.
-. -.
- r-40Upstate - r-SOUpstate - r-QOUpstate - r-40Downstate
e-1S ,Upstate
-2
-e-20,Upstale
.
- r-50Downstate - r-QODownstate
-e-25,Upstale -e-15,Downstate
. . ----.l 6. '. ----� 3.---� - " ,+.----2�. ---�
-e-20,OOwnstate - e-2S Downstate
-2.
Freq IGHz]
Fig.3(b):Parametric study of the width of the rings 'r' of 2.
I.
30
;$0
�
6.
CSRRs on ground planes of CPW in HFSS simulator.
Freq [GHz)
Fig.2(c):Parametric study of the spacing between the rings 'e' of CSRR on central line of CPW in HFSS simulator.
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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.2(d).
-I.
;;
E �
n-40,Upstate
&;
-n-SO,Upstate
N
'"
-n-40,Downslate - n-SO,Downstate
;;
I.
�
2.
3.
4.
50
60
Freq IGHz)
Fig.3(c):Parametric study of the spacing between the rings 'n' of CSRRs on ground planes of CPW in HFSS simulator.
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:8�
.1.
E
�
- n-60 Downstate
-2.
-. iii :8�
- n-60,Upstate
-10
-
�
t- 15 . Upstate
�
-t-20.Upstate - t-25.Upstate -t-15.downstate - t-20.downstate - t-25.downstate
-15
en _2.
'"
W
-v-40,Upstate -v-50,Upstate
-IS
- v-60,Upstate - v-40.Downstate -v·50,DCNtInstate -v-60,DCNtInstate
-2.
-25
10
-10
30
�
�
®
Freq [GHz)
Fig.2(d):Parametric study of the split between the rings 't' of CSRR on central line of CPW in HFSS simulator.
(b) Parametric study o f the CSRR CPW:-
on
the ground planes o f
I.
2.
3.
4.
••
Freq [GHz]
Fig.3(d):Parametric study of the split between the rings
60 '
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 38.80 GHz and 35.32 GHz.
(c)
Optimal filter performance :-
The choice of the dimensions for the CSRRs on central line and ground plane are very crucial for obtaining the optimized performance. The list of the chosen parameters of the band reject filter for CSRR central and ground planes are tabulated below [Table 1.1 & 1.2]. Optimization is done with the help of FEM simulation using ANSOFT- HFSS v13®. 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.
IV. CONCLUSIONS :In this paper, the characteristics and behaviour of a microwave signal passing through a CSRR based CPW transmission-line filter are studied with MEMS capacitive loading. We use ANSOFT-HFSSv13® to find out the RF characteristics of the CSRR band reject filter. With its high tuning range, the proposed compact filter is extremely promising for applications in microwave communications and radars in satellite communications. Fabrication process for the filter is presently going on. V. ACKNOWLEDGEMENT :The authors would like to acknowledge the National Programme on Micro and Smart Systems (NPMASS) for providing the necessary support. REFERENCES :-
Table 1.1: List of the optimized parameters of the CSRR filter on central line of CPW
Parameters
Values (p.tm)
CPW configuration(WIS/W) CSRR length (a) CSRR rings width (c)
30/200/30 500llm 251lm
spacing between rings (e) Split on the rings (t)
251lm 20 11m
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supporting
2-D
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Falcone,
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M.A.G, Marques. R, Medina. F and Sorolla, "Super compact split ring resonators CPW bandpass filters". IEEE-MTT Int. Microwave Symp. Dig., Fort Worth, TX, USA, June 2004, pp. 1483-1486
Table 1.2: List of the optimized parameters of the CSRR filter on ground plane of CPW
Parameters
Values (p.tm)
CPW configuration(WIS/W) CSRR length (L) CSRR rings width (r)
30/200/30 600llm 50llm
spacing between rings (n) Split on the rings (v)
50llm 50 11m
[3].
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1.,
1.
Garcia-Garcia,
Bonache,
J.
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Marques, "Varactor-Ioaded split rings resonators for tunable notch filters at microwave frequencies", Electron. Letter 2004, 40, (21), pp. 1347-1348. [4].
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The simulated up and down state results are shown in FigA with superimposed HFSS generated data.
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·s iij '2-
i
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-IS
S2t-up state - Stt-down state - S2t-down state
& InstTuments,2009.
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[8].
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1.
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40
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Fig.3(a)Tunable CSRR filter S-Parameter result for up & down-
performance shows clear tunability with acceptable level of losses, which may be improved further through miniaturezation.
Rings
Optical Technology Letters , Vol. 46, No. 3, August 5 2005,
Freq [GHz)
RF
Yong-hua
Based on MEMS and CPW", International Conference on
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�
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