JOURNAL OF INFORMATION AND COMMUNICATION TECHNOLOGIES, VOLUME 2, ISSUE 8, SEPTEMBER 2012 5

DESIGNING OF LOW LOSS DIELECTRIC

RESONATOR FILTER Engr. Muhammad Yameen, Engr. Sharjeel Afridi and Engr. Jamil Ahmed

Abstract—Emerging wireless, space and satellite communication links require light weight, low loss, temperature stable and inexpensive

communication devices. Microwave filters are the key element of any communication system or microwave link. Microwave filters are usually implemented

by using waveguide, coaxial, microstrip or lumped element resonators. Only dielectric resonator filters provide best combination of volume versus insertion

loss of filter. Dielectric resonator filters offer very high Q factors to provide lowest pass band insertion loss and higher selectivity with compactness and

minimal power requirements [1]. And satisfy vital demand of emerging space communication systems and cellular industry by offering very high-quality

factor due to their inherent low loss material [2]. This paper describes basic steps to design dielectric resonator filter with the help of a 3D finite-element

method (FEM) simulation tool, Ansoft HFSS (High Frequency Structure Simulator). Dielectric disks are used to tune the filter response within a limited

range. Index Terms— Dielectric resonator, Filter, Insertion loss method, Puck, Spur free response.

—————————— u ——————————

1 INTRODUCTION

Dielectric resonator filter was introduced by Cohn in 1968

using a dielectric material of permittivity 100 and loss tangent

0.0001 [3]. Then up to 1980’s DR filters were not preferred

due to their temperature instability. There are several shapes of

DR commercially available but commonly used structure is

cylindrical DR. The fundamental TE mode of cylindrical DR is

TE01δ [4]. A detailed mathematical explanation of resonant

frequencies, filed distribution and magnetic coupling is

discussed for dielectric resonator band pass filters in [3].

In this paper, a single mode 3rd order dielectric resonator filter

is realized using insertion loss filter design method. The HFSS

simulation tool is used to compute the s-parameters, radiation

patterns, filed strengths and resonant frequencies of the

dielectric resonant structures.

2 DIELECTRIC RESONATORS

A small piece of high dielectric constant material having a

cylindrical, cubic or other shape can be used as a dielectric

resonator. The high dielectric constant material used as a

resonator is named as ‘puck’. A dielectric resonator sustains

resonance due to the difference of permittivity of dielectric and

surrounding air region. Puck is held with a material of low

dielectric constant within a conducting enclosure, usually

Teflon with dielectric constant of 2.1 and loss tangent of

0.0003 [5]. The purpose of enclosure is to stop radiations of

energy from puck to outside Resonant frequency depends upon

dimensions and dielectric constant of the puck. Important

properties of a dielectric resonator are its Q-Factor, field

patterns, resonant frequencies and spur free bandwidth [6]. The

spur-free response of the dielectric resonator can be enhanced

by introducing a hole inside the puck to make it as a ring.

Resonators can be tuned using an adjustable metal plate above

the resonator [4]. Dielectric resonators have low loss tangent

and good temperature stability [7]. Resonator modes are very

sensitive to the dielectric diameter [2]. Dielectric resonator

filters exhibit high Q-factor to the volume ratio [8]. A typical

dielectric resonator structure shown below in the figure.

————————————————

• Muhammad Yameen is with the University of Leeds, United kingdom. On

leave from Sukkur Institute of Business Administration, Sukkur, Pakistan.

Sharjeel Afridi is with the University of Leeds, United Kingdom. On leave

from Sukkur Institute of Business Administration, Sukkur, Pakistan.

Jamil ahmed is a MSc student at university of Leeds

6

Fig.1. A typical dielectric resoantor structure

2.1 PERMITIVITY OF DIELECTRIC

Permitivity of a dielectric material determines the capability of

the material to store electric and magnetic energy at its

resoanant frequency. The higher the permitivity of a material

the lower the speed of microwave signal passing through it [9].

The wavelength passing through a dielectric material is

decreased by a factor of √€r . [10]

(1)

 

2.2 QUALITY FACTOR

Quality factor is defined as

Q= [2π * Mximum energy stored per cycle / average energy

dissipated per cycle ]

The unloaded Q factor can be calculated as

(2)

Where Qd, Qc and Qr represent dielectric , conduction and

radiation losses respectively. When conduction, radiation and

external losses are considered negligible then unloaded Q can

be approximated as

(3)  

Where tan δ is the loss tangent of the resonator [4].

2.3 RESONANT FREQUENCY

Following expression gives the resonant frequency of a

dielectric resonator with 2 % error.

(4)

Where Dmm is the diameter of the puck in milli meters and the

H represents height of the dielectric puck.

2.4 TUNING

Operating frequency of a dielectric resonator depends upon

permitivity of the material, surrounding permitivities and its

shape. By using dielectric tuning disk a change of upto 15%

can be achieved [1]. Tuning disk can easily be mounted in the

cavity by tuning screws.

2.5 COUPLING

Coupling depends upon the mode of the resonator to be

excited, and amount of coupling required [11]. For

fundamental TE01δ mode of cylindrical dielectric resonator’s

magnetic coupling is optimum solution as there is enough

magnetic field coming out of the resonator radially.. Dielectric

resonator size and distance between resonators define the

internal coupling for resonators [11].

3 BAND PASS FILTER DESIGN USING INSERTION

LOSS METHOD

Following is the steps to design a microwave filter using

insertion loss method.

 At first specifications of a filter such as resonant frequency,

roll off rate, pass band bandwidth, stop band bandwidth,

maximum pass band insertion loss, minimum stop band

attenuation, order of filter and filter type are determined. A

normalized lumped element low pass prototype filter is

designed. Then low pass prototype is transformed to band pass

by using frequency and impedance scaling [12]. Finally, a

physical realization is selected to implement the filter.

Following figure shows 2GHz, 3-pole circuit based on low

pass prototype response obtained from AWR microwave

office.

Fig.2. Low pass prototype response of 3rd order chebyshev filter

7

3.1 SCALING AND FILTER TRANSFORMATION

If W1 and W2 represent the pass band lower and upper edges

respectively, and W0 is the centre frequency of the pass band,

then band pass filter response can be obtained replacing ‘W’ by

(5)

Where ∆= (W2-W1)/ W0 is the fractional bandwidth of the

pass band, and W0 is the geometric mean of lower and upper

pass band edge frequencies [13]. Thus low-pass filter elements

are now converted into series resonant circuits having low

impedance at resonance in series arms and parallel resonant

circuits with high impedances in parallel arms. The series

element of low-pass filter are transformed in a series resonant

circuit according to relation given below

(6)

And

(7)

Similarly parallel branch elements can be impedance, and

frequency transformed as

(8)

And

(9)

[5]. Thus using above relations the filter response of fig.1.2 is

transformed into a band pass 3-pole filter with a centre

frequency of 2GHz and new element values as shown in

fig.3 Fig.3. Circuit based 3rd order chebyshev band pass filter Following figure shows the response of a 2GHz band pass

filter having a fractional bandwidth of 50MHz produced with

the AWR microwave office.

Fig.4. Circuit based 3rd order chebyshev band pass filter response.

4 IMPLEMENTATION

The above described lumped element design is proficient at

low frequencies, but at microwave frequencies wavelength

becomes comparable to the physical dimensions of the lumped

circuit elements so these become too small to be realized [12].

Also capacitors and inductors are manufactured for specific

range of values so they can be only approximated with

distributed circuit values. Distributed circuits can be realized

using transmission lines, rectangular or circular waveguide

resonators or dielectric resonators. Following figure shows a

relative comparison between insertion loss and size of a

general filter types.

Fig.5. A relative comparison between volume and Q-Factor of different

microwave resonators at f=1GHz

Dielectric resonator offer better quality factor ranging up to

50000, more temperature stability, low pass band insertion loss

and more selectivity [1]. A Dielectric puck (Calcium

titanateneodymium) with permittivity of 45and having been

following specification is used to design DR filter.

8

Table.1. Dielectric Puck dimensions

Dimension Measurement (mm)

Puck height 10.8mm

Puck outer diameter 25.4mm

Puck inner diameter 10.3mm

Support outer diameter 22mm

Support inner diameter 18mm

Support height 10mm

Tuning disk diameter 24.8mm

Tuning disk height 4mm

Metallic enclousure dimension 170 * 50 * 50.

Figure.6 shows a photograph of a dielectric resonator with

above mentioned dimensions

(a)

(b)

Fig.6. 2GHz DR section photograph with tuning disk (a) top view (b) side

view.

When simulated in HFSS, the following Eigen mode

resonances and Q-factor data is obtained for dielectric

resonator of figure.7

Fig.7. Resonant modes and Q factor of DR

This Eigen mode data represents that first resonant mode, i.e.

TE01∂ occurs at 2GHz and the second resonant mode occurs at

2.68GHz which is much far away from the fundamental mode

and can easily be eliminated by using a secondary low-pass

filter. Figure.8 and figure.9 show electric and magnetic-field

patterns of the above structure computed by HFSS.

Fig.8. E-Field distribution in the DR

Fig.9. H-field distribution in the DR

Due to the high dielectric constant most of the electric and

magnetic energy is stored within the dielectric puck [6]. It is

observed that by allowing a tuning range of 15mm a frequency

tuning range of 140 MHz can be achieved.

A 3rd order dielectric resonator filter structure and its response

simulated in HFSS are shown in figure below. Input and output

coupling is achieved by using standard 50 ohm coaxial probes

while inter resonator coupling is achieved using magnetic field

as it forms vertical loops around the dielectric puck. DR

section’s proximity determines the amount of energy coupled

from one resonator section to the other. In case of over

coupling sharp peaks occur at resonances of both resonators

with deep ripples. It is observed that when spacing between

resonators is 33 mm, and input output probes are 3.3 mm away

from the puck the critical coupling is achieved.

9

Fig.10. Three pole dielectric resonator filter

Fig.11. 2GHz three pole DR filter S21 response

Fig.12. 2GHz three pole DR filter S11 response

5 CONCLUSION AND FUTURE WORK

A 3rd order 2GHz single mode dielectric resonator band pass

filter with 2 % fractional bandwidth is designed. A tuning disk

is put above the dielectric puck to fine-tune the centre

frequency of DR. This paper provides the basic methodology

and steps to produce single mode DR filter. Further research is

required to apply the concept of multimode DR filters in order

to achieve higher Q to volume ratio to meet the emerging

industry requirements.

6 ACKNOWLEDGMENTS

This is extended version of our own paper presented and

published as conference proceedings in “2nd International

conference on communication and signal processing” (MIC-

CSP2012) held on 4-6 April 2012 at Barcelona Spain.

7 REFRENCES [1]. Raafat R. Mansour “High Q tunable Dielectric-Resonator Filters”, IEEE

microwave magazine, Oct 2009

[2]. Chi Wang, Kawthar A. Zaki,, Ali E..Atia, and Tim G. Dolan, “Dielectric

Combline Resonators and Filters”, IEEE transactions on microwave theory

and techniques, vol. 46, NO. 12, December 1998

[3]. Seymour B. Cohn, “Microwave Band pass Filters Containing High-Q

Dielectric Resonators”, IEEE transactions on microwave theory and

techniques, vol. MTT-16, NO. 4, April 1968

[4] Ian C. Hunter, J. David Rhodes, and Vanessa Dassonville, “Dual-Mode

Filters with Conductor-Loaded Dielectric Resonators”, IEEE transactions on

microwave theory and techniques, vol. 47, NO. 12, December 1999

[5] Mohammad Memarian, and Raafat R. Mansour, “Quad-Mode and Dual-

Mode Dielectric Resonator Filters’’, IEEE transactions on microwave theory

and techniques, vol.57, No.12, December 2009.

[6] Ian Hunter, “Theory and design of microwave filters”, IEE electromagnetic

wave series, volume 48, The institution of electrical engineers, London

,UK,2001

[7] Darko Kajfez and Pierre Guillon, “Dielectric resonators,” Artech house,

INC, 1986

[8] Rafaut R. Mansur, “High Q tunable dielectric resonator filters’’, IEEE

microwave magazine, October 2009.

[9] Perambur S.Neelakanta, “Handbook of electromagnetic materials

Monolithic and composite versions and their applications”, CRC press New

York, January 1995

[10] Mailadil T.Sebastian, “Dielectric materials for wireless communication”,

first edition, Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP, UK, 2008

[11]. “Tuning and Exciting Dielectric Resonator Modes”, Trans-Tech ceramics

advanced materials, Application note No. 851: March 9, 2007

[12]. Ian C. Hunter, Laurent Billonet, Bernard Jarry, and Pierre Guillon

“Microwave Filters—Applications and Technology”, IEEE transactions on

microwave theory and techniques, vol. 50, NO. 3, March 2002

[13] John B. Ness, “A Unified Approach to the Design, Measurement, and

Tuning of Coupled-Resonator Filters’’, IEEE transactions on microwave

theory and techniques, vol 46, NO. 4, April 1998.

10

Muhammad Y Sandhu: M.Y Sandhu received

his B.E degree in Telecommunication Engineering

from Mehran Univerity of Engineering and

Technology, Jamshoro, Pakistan in 2009. He

completed his MSc in Communication Engineering

from University of Leeds, Uk in 2011.

From 2009 to 2011 he has worked as Lecturer in

department of Electrical Engineering at Sukkur

Institute of Business Administration, Sukkur Pakistan.

His Research interests include Microwave filters,

antenna and multiplexer design.

Sharjeel Afridi: Sharjeel Afridi received his B.E

degree in Telecommunication Engineering from

Mehran Univerity of Engineering and Technology,

Jamshoro, Pakistan in 2007. He just completed his

MSc in Communication Engineering from University

of Leeds, Uk in 2012.

From 2007 to 2011 he has worked as Lecturer in

department of Electrical Engineering at Sukkur

Institute of Business Administration, Sukkur Pakistan.

His Research interests include Microwave and

Wireless Communication, Communication Networks

and Protocols and algorithms.

Jamil Ahmed: Jamil received his BSc Degree in

Electronics Engineering from Islamia University

Bahawalpur Pakistan in 2010. He has accomplished

Post graduation in Electrical & Electronics

Engineering from University of Leeds UK in 2012.

From 2010-2011, he has served as Service Engineer

in Pakistan Microbiological Associates. His potential

field of interest include but not limited to

Communication Network, Microwave Theory and

Wireless Communication.