Gross R., Sidorenko A., Tagirov L. Nanoscale Devices

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Unpublished

MEMS Tunable Dielectric Resonator

G. Panaitov

1,2

, R. Ott

, N. Klein

ISG-2, Research Centre Jülich GmbH, Germany

Abstract: We present a novel approach for tuning of dielectric resonators by

microelectromechanical switches (MEMS). The concept is based on

intermodal coupling between the TE

01δ

mode of a cylindrical

dielectric resonator and a planar slotline resonator. The resonance

frequency of the resonator can be changed by MEMS switch leading

to variation of the coupling between the planar resonator mode and

the TE

01δ

mode. As a consequence, the resonance frequency of the

01δ

mode changes.

Keywords: MEMS, microwave resonator, slotline

Introduction

The rapidly growing market of microwave communication systems requires

new approaches which may allow to more effectively utilize the available

frequency bandwidth. Increasing of cannel capacity can be provided by

improvement of the quality of frequency selective elements, like filters and

resonators. Moreover, a confident operation of mobile systems under strong

interferences requires fast switchable filters and resonators. Due to higher

data rate in new 3-generation UMTS (universal mobile telecommunication

system) the requirements to the tuning time even faster than for

conventional cellular mobile systems.

MEMS (microelectromechanical systems) technology has proved to be

an appropriate method to attain switching and tuning capability with losses

being superior to any other techniques such as varactors or field effect

transistors [1]. However, due to the small dimensions of MEMS of the

111

R. Gross et al. (eds.), Nanoscale Devices - Fundamentals and Applications, 111–121.

MD-2028 Kishinev, Moldova

Institute of Electronic Engineering and Industrial Technologies ASM,

112 G. Panaitov, R. Ott, N. Klein

order of several hundred micrometers, their use is restricted to planar

microwave circuits. In order to achieve tuning of a bulky dielectric

resonator, coupled modes of planar and dielectric resonator will be

investigated here.

Tuning Concept

We propose a tuning mechanism, which is based on three main

components: a basic dielectric resonator, which determines the main

frequency of the resonator; a slotline, as an additional planar microwave

resonator; and a MEMS switch, which is used to control the intermodal

coupling between dielectric and planar resonator modes.

Dielectric resonator

We apply dielectric resonators (DRs), which conventionally play an

important role for microwave communication systems. Due to the high

quality factors, high mechanical stability, and small temperature coefficient

of permittivity of advanced microwave ceramics, dielectric resonators are

of common use in high-Q filters for base stations and satellite

communication, as local oscillators with low phase noise, and integrated

satellite receivers [2]. We use a common cylindrical dielectric resonator

which operates at TE

01δ

mode. At high permittivity constant of the dielectric

material (

~30) the 95% of the stored electric energy of the TE

01δ

mode, as

well as a great part of the stored magnetic energy (~ 60%) are located

within the cylinder [3]. The resonance frequency of the resonator is strongly

dependent on the distribution of the rf electromagnetic fields in the

resonator. Any variation of the fields in the resonator volume ∆V leads to

change of the resonance frequency

, according to perturbation principle

[3]:

()

HEdV

µε

ωω

µε

∆

−

≅

∫∫∫

(1)

where

, E

, H

- frequency and field parameters of non-disturbed

resonator. In our experiments we change the frequency of DR resonator by

coupling the magnetic field of TE

01δ

mode to the field of planar slotline

resonator.

MEMS Tunable Dielectric Resonator 113

Slotline

A slotline is a planar transmission line structure, which can be fabricated by

etching of a narrow slot in the metallization on one side of the dielectric

substrate. If short circuited at one end, it may be considered as a resonator,

which frequency depends on the length and the load at the open end of the

slotline.

Fig. 1. Slotline as the transmission line circuit loaded at the ends.

impedances Z

and Z

. Resonance parameters of the slotline can be

calculated using a transmission line formulation. The impute impedance for

the load Z

at the length l is given by

()

tan

ZjZ

(2)

where Z

is the characteristic impedance; 2

πλ

is the wave

propagation constant. At the left end of the slotline Z

(- l ) = Z

. If this end

is short-circuited, i.e. Z

= 0, then the equation (2) gives:

tan

=l .

(3)

Next, we consider two cases, when the right end of the slotline is open

and closed. For the open end case, i.e. when Z

= ∞, the ratio between the

slotline length and wavelength can be found as following:

tan

ββ

=∞ ⇒ = ⇒ =ll l,

(4)

for short slotlines,

l <

. As the length of open end slotline is a quarter of

the wavelength this slotline resonator can be considered as a quarter-wave

resonator. Similarly, one can show that in the case of shorted end (Z

=0)

holds:

Figure 1 presents a slotline as a transmission line loaded at two ends with

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