[ieee 2014 ieee students' technology symposium (techsym) - kharagpur (2014.2.28-2014.3.2)]...

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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 AbstractA 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. KeywordsCSRRs 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 SiO 2 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 TS14P02 091 978-1-4799-2608-4/14/$31.00 ©2014 IEEE 335

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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

TS14P02 091 978-1-4799-2608-4/14/$31.00 ©2014 IEEE 335

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

TS14P02 091 978-1-4799-2608-4/14/$31.00 ©2014 IEEE 336

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

TS14P02 091 978-1-4799-2608-4/14/$31.00 ©2014 IEEE 337

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

TS14P02 091 978-1-4799-2608-4/14/$31.00 ©2014 IEEE 338

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.

REFERENCES

[1]. C.-K. Liao and C.-Y. Chang, IEEE Trans. Microwave Theory

Tech, vol.53, (2005), p. 2302.

[2]. K. Entesari and G. M. Rebeiz, IEEE Trans. Microwave Theory

Tech., vol. 53, (2005), p. 1103.

[3]. Iyer.A.K. and Eleftheriades.G.V., “Negative refractive index

metamaterials supporting 2-D waves”. IEEE-MTT Int. Microwave

Symp., Seattle, WA, USA, June 2002, Vol. 2, pp. 412– 415.

[4]. Bonache, Martin, Falcone, Garcia, Gil. I, Lopetegi, Laso, 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.

[5]. J. Brank, J. Yao, A. Malczewski, K. Varian, and C. L. Goldsmith,

Int. J. RF Microwave Computer-Aided Eng., vol. 11, (2001), p.

276.

[6]. Gabriel M. Rebeiz, “RF MEMS: Theory, Design, and

Technology”, John Wiley & Sons Ltd, Chapters 1, 9 and 10, pp.1-

20, 259-324, 2003.

[7]. Jae-Hyoung Park, Hong-Teuk Kim, Youngwoo Kwon and Yong-

Kweon Kim “Tunable millimeter-wave filters using a coplanar

waveguide and micromachined variable capacitors”, Journal of

Micromechanics and Microengineering, 12 October 2001.

[8]. Gil, I., Garcia-Garcia, J. Bonache, J. Martin, F. Sorolla, Marques,

“Varactor-loaded split rings resonators for tunable notch filters at

microwave frequencies”, Electron. Letter 2004, 40, (21), pp. 1347–

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[9]. I. Gil, M. Morata, R. Fernández , X. Rottenberg , W. De Raedt,

“Characterization and modelling of switchable stop-band filters

based on RF-MEMS and complementary split ring

resonators”,Microelectronic Engineering 88 (2011), pp. 1–5

[10]. Coplanar Waveguide Circuits, Components, and Systems. Rainee

N. Simons. A John Wiley & Sons, Inc., Publication New York · P-

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[11]. Chao Wang1 Xing-long Guo1 Wei-xia Ou-Yang1 Yong-hua

Zhang1 Zong-sheng Lai, “A Novel Tunable Low-pass Filter Based

on MEMS and CPW”, International Conference on Electronic

Measurement & Instruments, 2009.

[12]. http://www.ADS2009.com/manuals.htm

[13]. ANSOFT HFSS™ V-13, available at

http://www.ansoft.com/products/hf/hfss

[14]. Michael A. Mariani, Langis Roy, and R. Niall Tait, “Switchable

Patterned Centreconductor CPW Filter Using RF MEMS”,

Microwave and Optical Technology Letters, Vol. 48, No. 5, May

2006.

[15]. Brown, E.R.: “RF-MEMS switches for reconfigurable

integrated Circuits”, IEEE Trans. Microw. Theory Tech., 1998, 46,

pp. 1868–1880.

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

Proceeding of the 2014 IEEE Students' Technology Symposium

TS14P02 091 978-1-4799-2608-4/14/$31.00 ©2014 IEEE 340