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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.