Modeling of Rf Mems Switchable Band Pass Filter

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  • Topic: Band-pass filter, Capacitor, RLC circuit
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Modeling of RF MEMS Switchable Band Pass Filter
M. Jeya Rani, M. Anitha, M. Arul Jothi and S. Kanthamani*
MEMS Design Center, Department of ECE,
Thiagarajar College of Engineering, Madurai, Tamil Nadu, INDIA. This paper presents the design and modeling of Radio Frequency (RF) Micro Electro Mechanical System (MEMS) based Switchable Band Pass Filter. The switchable filter is used in space applications for frequency switchability where electromagnetic spectrum can be effectively utilized. Switchable Band pass filter is designed at a frequency of 20 GHz using Distributed MEMS Transmission Line (DMTL) structure . The DMTL structure has periodically loaded variable MEMS capacitors. This paper focuses on the development of Capacitor – Inductor – Resistor (CLR) model of the periodically loaded variable MEMS capacitors. The RF performance for filter and MEMS variable capacitors are carried out using Advanced Design System (ADS) for both lumped model and distributed model. The simulation results in a bandwidth of 2 GHz with an insertion loss of -0.074dB and a return loss of -17.72 dB. Keywords – CPW, RF MEMS, Band Pass Filter, DMTL, and ADS.

Introduction
The
rapid
development
in
wireless
communication Systems has led to an explosion of
interest in improved design approaches for the RF and
microwave areas. The recurring demand for flexible,
compact and low power wireless systems has
generated the need for technological solutions that
can dramatically reduce manufacturing cost, size,
weight, improved performance, battery life and
provides the possibility of low loss , near zero power
consumption, high linearity filtering in comparable to
those of integrated circuits1.The use of MEMS
technology for RF and Microwave application
promises to solve several technological limitations
that have plagued the high frequency electronics field
for decades2.
MEMS capacitive devices have recently been
investigated for low loss and high performance
microwave control circuit applications3.Tunable band
pass filter are a crucial component in a variety of
radar and communication systems.
Existing tunable filters are fabricated using YIG
resonators, active inductors
and semiconductor
4
variable diodes , Improvements in MEMS technology
have made it possible to use MEMS variable
capacitors instead5. Recently, DMTL technique has
*Author for correspondence
Email: skmece@tce.edu

been used to implement very low loss millimeter
wave true time delay phase shifters6. These structures
provide electronically variable phase and group
velocity through voltage control of the MEMS
loading capacitance. This technique can also be used
to design tunable resonators, using a capacitively
coupled DMTL3.
This paper proposes and focuses on the modeling
of periodically loaded MEMS variable capacitor
using equivalent CLR model. The switchable band
pass filter is simulated at a center frequency of
20 GHz and RF performance is obtained using ADS.
MEMS Tunable Filter
The development of MEMS tunable filter with
variable capacitors have been reported . The DMTL is
comprised of a high - impedance transmission line
(>50Ω) periodically loaded with MEMS variable
capacitors (Fig. 1).

Fig. 1 - Layout of nine section tunable filter in coplanar
waveguide form

The periodicity of the structure is given by the Bragg
frequency fBragg (1) as

f Bragg 
L

1
 LT (CT  CMEMS )

….. (1)

: Length of transmission line per section

sect
Where LT and CT are the inductance and capacitance
CT : Transmission line capacitance per section
per unit length of the unloaded line and CMEMS is
LT : Transmission line capacitance per section
continuously variable MEMS Capacitor.

Circuit Model of periodically loaded variable
capacitors
The equivalent circuit model available in
[Muldavin J.B and Rebeiz G M 2000], for RF MEMS
switch realized on CPW platform is used as a basis
for obtaining the equivalent circuit model for...
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