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Digital Object Identifier 10.1109/MMM.2009.933591
High-performance radio frequency (RF)
tunable fi lters are needed in reconfi gu-
rable systems to facilitate effi cient utiliza-
tion of the available frequency spectrum.
They are in demand in front-end receivers
for suppression of interfering signals and for relaxation of os-
cillator phase noise and dynamic range requirements. Tunable
fi lters are also used to replace large fi lter banks in advanced
systems concepts that self-adapt to environmental require-
ments. Tunable fi lters have been proposed as well for high
power applications. The advantages in this case are sup-
pression of harmonics generated from the power am-
plifi ers. In majority of these applications, the insertion
loss of the tunable fi lter is a key design parameter. It
directly impacts the noise fi gure for the front-end
receiver applications while it directly impacts the
transmitted power for high power applications.
The current generation of wireless and satellite sys-
tems is designed with a specific function under restricted
operating conditions such as a specific frequency band,
channel bandwidth, interference and traffic patterns. These
systems lack the agility and adaptability to vary their operat-
ing conditions, which, in turn, limits their performance. While
cell phones currently come with multiband capabilities, signifi-
cant research effort is being directed toward implementing a simi-
lar functionality for future generations of wireless and satellite
communication systems. These communication systems, however, require the use
of microwave filters [1] with very high Q values, requiring the development of novel
tunable filter configurations.
Raafat R. Mansour is with the University of Waterloo, Ontario, Canada.
Raafat R. Mansour
1527-3342/09/$26.00©2009 IEEE
Raafat R. Mansour is with the University of Waterloo, Ontario, Canada.
FOCUSED
ISSUE FEATU
RE
84 October 2009
October 2009 85
The availability of high-Q tunable filters may also
have a significant impact on production cost and deliv-
ery schedule in some communication systems. Such
systems use multiple filters that are usually identical
with the exception of center frequency and bandwidth.
The production cost can be significantly reduced by
building standard filter units that can be easily recon-
figured during production phase to fit the required
frequency plan. In wireless and satellite applications,
the delivery schedule has become a major key factor in
winning or losing a contract. Tunable devices can be
built ahead of time and used to offer a competitive
delivery schedule.
The ideal tunable filter must exhibit the following
features: high loaded-Q value, wide tuning range,
high tuning speed, good linearity, high power han-
dling capability, small in size and mass, and high reli-
ability. The loaded-Q value of the tunable filter is
determined by the Q value of the filter structure itself
and by the inherent loss of the tuning element used.
The type of the tuning element used also impacts the
tuning speed, tuning range and dc power consump-
tion. There are several different technologies avail-
able to realize the tuning elements. The list includes
semiconductors [2] – [4] , ferroelectric materials [5] –
[6] , ferromagnetics [Yttrium iron garnet (YIG) and
ferrite] [7] – [9] , and mechanical systems [miniature
motors, piezoelectric, microelectromechanical sys-
tems (MEMS)] [10] – [14] . Several excellent publica-
tions in this issue of IEEE Microwave Magazine provide
a detailed review of the various technologies avail-
able for tuning elements.
While some of the desirable features listed above
can be met by existing tunable filter designs, no tun-
able filter has been reported that can meet the stringent
Q requirements of wireless and satellite systems. In
such systems, the filters have a very narrow bandwidth
and the emphasis is on the tunable filters loaded-Q
value, linearity performance
and size. A tuning range of
only 5%–15% is usually
enough for such applications.
These features can be poten-
tially met by integrating
mechanical tuning (motors,
piezoelectric materials and
MEMS) with three-dimen-
sional (3-D) filters (metal
cavity filters or dielectric res-
onator filters). In this paper,
we focus on tunable dielectric
resonator filters, reviewing
the results presented over the
past two decades and high-
lighting the potential of this
technology in realizing high
Q tunable filters.
Why Dielectric Resonator Filters Figure 1 shows the relative insertion loss and size of
typical microwave resonators. The estimated range of
unloaded Q values for each resonator category at 5
GHz is also shown in the same figure. There is a wide
range of resonator configurations under each resona-
tor category. The Q value can therefore vary widely
for each resonator category. For example a patch
microstrip resonator would have a higher Q value
than a standard l/2 resonator, and a full height TE 101
waveguide cavity would have a higher Q than a
reduced height TE 101 waveguide cavity resonating at
the same frequency. It can be seen from Figure 1 that
regardless of the tuning element used, tunable filters
that employ planar or lumped-element resonator
configurations exhibit a very low Q value. Once
loaded with the tuning elements, the overall loaded
Q will be further reduced. The resonator structures
that can provide Q-values in the range of 4,000–10,000
at 5 GHz are 3-D cavities or dielectric resonators.
At the present time, dielectric resonator filters are
emerging as the baseline designs for the majority of RF
filters used in wireless and satellite applications. They
offer high Q values with a relatively high Q/volume ratio
in comparison with any other known filter technology. If
reconfigurable RF filters are ever employed in wireless
base stations and satellite systems, tunable dielectric
resonator filters stand to be the optimum solution.
Lumped
Element
Microstrip
Coaxial
Dielectric
Resonator
WaveguideSuperconductor
Size
Insert
ion L
oss
Q = 10–50
Q = 50–200
Q = 200–3,000
Q = 1,000–10,000
Q = 1,000–12,000
Figure 1. Relative insertion loss and size for various RF resonators (from [1]).
The ideal tunable filter must exhibit the following features: high loaded-Q value, wide tuning range, high tuning speed, good linearity, high power handling capability, small in size and mass, and high reliability.
86 October 2009
High Q dielectric materials with dielectric constants
ranging from 20 to 90, are now commercially available
from various manufacturers. Dielectric resonators with
er5 30 are commercially available with a Q 3 f prod-
uct values of 100,000, i.e., an unloaded Q value of about
50,000 can be achieved at 2.0 GHz. As the dielectric
constant increases, the achievable unloaded Q typically
decreases. For materials with a dielectric constant of 45,
the Q 3 f value reduces to 44,000. Dielectic resona-
tors can operate at various modes giving the designers
of tunable filters the flexibility to select the tuning ele-
ment that can easily interact with field distribution of
that particular mode. Dielectric resonators can also be
easily machined to have various shapes using low-cost
water-jet machining allowing ease of realization of
highly novel resonator configurations [14] – [15] .
Over the past two decades, various dielectric resona-
tor configurations have been reported in the literature
[16] – [18] . A typical dielectric resonator filter consists of a
number of dielectric resonators mounted inside cavities
operating below cutoff, as shown in Figure 2 . The cavities
are separated by irises to provide the necessary coupling
between resonators. Since the field is mainly concen-
trated in the dielectric, the use of such conventional cylin-
drical dielectric resonators in tunable filter applications
limits the tuning range. The use of resonators of various
shapes can help to enhance tunability and add flexibility
of integration of several tuning techniques.
Figure 3 shows a configuration for dielectric reso-
nator reported recently employing high dielectric fil-
ters in the form of high-K ceramic substrates [14] and
[19] . The filter structure offers a high Q (6,000–8,000 at
5 GHz) with a profile that resembles microstrip planar
filters. It makes it possible to integrate MEMS devices
or any other tuning elements (semiconductor, ferro-
electric) directly on the resonator.
In the following section, we review the published
work reported highlighting several new ideas that
can be used to implement tunable dielectric resona-
tor filters.
Filters Employing Mechanical or Piezoelectric Actuators The first report on mechanically tuned dielectric res-
onator filters was by Wakino in 1987 [20] . Figure 4
illustrates two tunable dielectric resonator configu-
rations disclosed in Wakino’s patent [20] . In
Figure 4 (a), a piezoelectric actuator is inserted on the
top of a dielectric resonator, and it acts as a tuning
plate whose distance from the resonator is controlled
by a dc applied voltage. Wakino reported a tuning
range of 8% as the gap between the resonator is varied
Figure 2. A typical dielectric resonator filter (BL Microwave, used with permission).
Strip Line
Figure 3. Novel dielectric resonator filters made on a high-K substrate (from [14]).
Figure 4. Tunable dielectric resonators disclosed in a 1987 patent by Wakino. (a) The tuning element is a piezoelectric actuator and (b) the tuning element is a piezoelectric attached to a tuning dielectric disk (after [20]).
Piezoelectric Actuator
Piezoelectric Actuator
(a)
(b)
Tuning
Dielectric
Disk
Dielectric
Resonator
Dielectric
Resonator
+
+
Dielectric
Resonator
+
Dielectric
Resonato
+
October 2009 87
from 1 to 5 mm. In Figure 4 (b), a second dielectric
disk attached to the piezoelectric actuator is brought
in proximity of the dielectric resonator for tuning.
Wakino [20] reported a 12% tuning range with that
approach over a 4 mm change in the gap between the
dielectric resonator and the tuning dielectric disk. It
should be mentioned that the same tuning range can
be obtained with a smaller change in gap, by placing
the actuator closer to resonator. This, however, would
have an impact on the resonator Q. With the avail-
ability of electromagnetic (EM) simulation tools such
as Ansoft High Frequency Structure Simulator (HFSS),
tunable filter designers can easily optimize the reso-
nator/actuator position for optimal tuning range and
Q value.
Over the past years, several publications have been
reported addressing the tuning range of dielectric res-
onator filters. The concept of using a double-resonator
to improve the tuning range was implemented in [21]
and [22] . Since the field is mainly concentrated inside
the dielectric resonator, the use of dielectric materials
of the same dielectric constant or higher is an effective
way to perturb the field in the original resonator and
hence change its resonant frequency. In [21] , a dielectric
resonator is brought in proximity to the main dielectric
resonator for tuning, whereas, in [22] , a dielectric reso-
nator of a smaller diameter is inserted in the main
dielectric resonator as shown in Figure 5 . The concepts
in [21] and [22] were demonstrated using manual
tuning with no integration of mechanical tuning ele-
ments, presenting experimental results for dielectric
resonator filters with a 4–5% tuning range.
More recently, a cone-shaped dielectric resonator
[23] was used to realize a tunable dielectric resonator
filter that tunes in both center frequency and band-
width. An experimental filter was demonstrated where
the bandwidth was tuned from 5 to 20 MHz and the
center frequency was tuned from 1,930 to 1,960 MHz
with an achievable Q value of 16,000. A 200 MHz
tuning range (1,965–2,165 MHz) was also demonstrated
using this concept [24] . Figure 6 shows a seven-pole
filter version employing the concept [24] . Dielectric
plugs (basically, dielectric resonators of smaller dimen-
sions) are used to tune the resonant frequency of the
cone-shaped hollow resonator. The dielectric plugs are
brought in and out of the cone-shaped resonator by
screws. Figure 6( a) and (b) shows the filter with the
dielectric plugs out and in respectively. As the dielec-
tric plug is moved inside the cone-shaped resonator, it
shifts its frequency down. The coupling between the
resonators is varied through the use of tuning screws
located between the cone-shaped resonators. The
reason for using a cone shaped resonator rather than a
cylindrical-shaped resonator [23] is to improve the fil-
ter’s spurious performance. Figure 7 illustrates the
experimental results obtained from the filter shown in
Figure 6 . Excellent experimental results were obtained
using this concept. The results show a filter having a
20 MHz bandwidth tuned by 50 MHz in center fre-
quency. The filter is retuned to have a bandwidth of
5 MHz and is then tuned in center frequency by 50 MHz.
The structure allows manual tuning of both bandwidth
and center frequency. The filter is useful in reducing
manufacturing cost, by having one design that can be
configured to any desired frequency around 2 GHz or
any bandwidth when needed by manual tuning. The
design of [23] represents an important step toward the
goal of remotely controlled filters in wireless base
Tuning Dielectric Resonator
Dielectric
Resonator
Support
Figure 5. The use of a second smaller dielectric resonator to improve the tuning range (from [22]).
(a)
(b)
TuningDisks AreIn
TuningDisks AreOut
Figure 6. A cone-shaped dielectric resonator filter tuned manually using dielectric plugs (other dielectric resonators). (a) The dielectric plug is outside the shaped cone resonator, and (b) the dielectric plug is inside the cone-shaped resonator (from [24]).
88 October 2009
stations through the use of
screws controlled by motors or
any other means.
Mechanically tunable di-
electric resonator filters using
piezoelectric actuators were
also reported in [25]– [28] . Fig-
ure 8 illustrates the four-pole
filter structure reported in
[27] . The filter consists of two
dielectric resonators operating
in dual mode coupled by an
iris. Tuning of the resonator is
achieved by having a metallic
disk suspended above the res-
onator through a metal rod
connected to a piezoelectric
actuator located outside the
cavity. A two-pole version of
the filter was demonstrated in
[26] using a single dielectric
resonator operating in a dual
mode as shown in Figure 9 (a).
The experimental results ob-
tained are shown in Figure
9 (b). The filter tunes over 2.25
to 2.45 GHz, demonstrating
nearly 8% tuning range.
A piezoelectric tunable
dielectric resonator filter op-
erating in a TM mode is dem-
onstrated in [29] . This TM
mode is also referred to as
TME mode [1] . The resonator
is quite small in size and is
configured to enhance tun-
ability. Figure 10 illustrates
the dielectric resonator con-
figuration [29] . The resonator
is mounted directly on the
bottom cavity wall, forcing
the TME mode to be the dom-
inant mode. As a result, the
resonant frequency shifts
down significantly while
maintaining a relatively ac-
ceptable high Q value (greater
than 2,000), even when the
gap between the dielectric
resonator and the cavity top
cover is very small. Higher Q
values can be achieved with
the use of larger cavities.
Figure 11(a) and (b) shows
the HFSS simulated results
for the tuning range and the
Q values of the resonator as
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120Frequency (MHz)
1,890 1,910 1,930 1,950 1,970 1,990 2,010 2,030
Attenuation (
dB
)
, , , , , , , ,
5 MHz
50 MHz
Figure 7. The experimental results of the cone-shaped resonator shown in Figure 5 (from [24]).
Piezoelectric ActuatorPiezoelectricActuator
Top Cap(ActuatorLocker)
Top Cap (ActuatorLocker)
Metal Rod Metal Rod
Tuning Disk
Tuning Disk
Metal Chamber(Bottom Cover)
Metal Chamber(Bottom Cover)
Substrate Substrate
Air Gap BetweenDielectricResonatorand Tunable Disk
Cavities SeparationWall
CouplingIris
(a)
(b)
AdjustableScrew
Adjustable Screw
Adjustable Screw
OutputDielectricResonator
OutputDielectricResonator
OutputMicrostripLine
InputMicrostripLine
Output MW Connector
Input MW Connector
Tuning Screw(Control CouplingBetween Resonators)
d
α
α
Figure 8. A dual-mode tunable dielectric-resonator filter, tuned by piezoelectric actuators (from [27], used with permission). (a) Side view and (b) top view. The screw at the angle a is used to couple the dual modes.
October 2009 89
the gap between the top cover and the resonator is
varied from 0.1 to 2 mm. It can be seen that more than
a 70% theoretical tuning range can be achieved with
this type of resonator [29] .
Tuning of the dielectric resonator of [29] is achieved
using a piezoelectric concept, where a thin metallic
copper sheet is used to cover the top of the cavity that
houses the TME dielectric resonator. The piezo-
electric actuator is attached to the filter housing and
is placed in contact with the copper sheet, as shown
in Figure 12 . With application of dc voltage to the
piezoelectric actuators, the copper film, which covers
the top of the cavity, is deflected by the force applied
on its surface. Figure 12( a) illustrates a side view of
the deflected cover of the cavity. The piezoelectric
used only exerts a small force on the thin metallic
copper sheet, causing it to deflect by 100–200 µm.
Figure 11 shows that such a small gap change can
cause a relatively large change in frequency when the
starting gap between the top cover and the dielectric
resonator is small, however,
the Q drops sharply hen
operating at that range.
A four-pole tunable dielec-
tric resonator filter was built
using this concept [29] . It con-
sists of four coupled dielectric
resonators; each integrated
with a thin film piezoelectric
transducer. The resonator is
cut from a low loss high
K- dielectric substrate of thick-
ness 2.54 mm, er5 45, and a
loss tangent of 10 25 . Figure 13
shows the HFSS simulation
Perturber
+
+ −
−
ConductorFlim
Cavity
PiezoelectricTransducer
ConductorWall
dcSource
DR
(a)
(b)
Figure 12. Piezoelectric tuning mechanisms (from [29]).
PiezoelectricBimorph
Top Cap (ActuatorLocker)
Top Cover
DielectricResonator
Pusher
TunableDisk
BottomCover
Air GapBetween DRand TuningDisk
d
lectricph
Locker
Top Co
DielecReso
r
e
Air GaaBetweeand TuDisk
d
S21 (dB
)
–5
–10–15
–20
–25
–30
–35
–40
–45
–50
–55
–602.10 2.15 2.20 2.25 2.30
(b)
(a)
2.35 2.40 2.45f (GHz)
0 V 300 V
Substrate
Figure 9. A two-pole dual-mode dielectric resonator filter implemented using piezoelectric actuators (from [26]).
Figure 10. A dielectric resonator operating in the TM mode (from [29]).
9
8
7
6
5
40 0.5 1 1.5 2 0
3,500
3,000
2,500
2,0000.5 1 1.5 2
Gap (mm) Gap (mm)
(a) (b)
Unloaded Q of
TME Mode Versus Gap
Fre
quency (
GH
z)
Q F
acto
r
TME Mode Versus Gap
Figure 11. The tuning range and the unloaded Q versus gap for the dielectric resonator shown in Figure 9 (from [29]).
90 October 2009
re sults of the four-pole tunable filter when the gap
between the top cover and the resonator is changed by
only 20 µm. With no tuning and an initial gap of 160 µm,
the filter has a center frequency of 5 GHz and
a bandwidth of 50 MHz. It
exhibits an insertion loss of
1.14 dB and a return loss of 17.5
dB. When the gap is reduced to
140 µm, the filter’s theoretical
insertion loss has increased to
1.75 dB with. The center fre-
quency has shifted to 4.83
GHz, a 170-MHz shift in fre-
quency. It should be noted that
the filter is capable of achiev-
ing a wider tuning range, how-
ever, since no tuning was used
for the coupling between reso-
nators the return loss degrades
over a wider tuning range.
Therefore the tuning range quoted in [29] is the range
that can provide reasonable return loss.
Figure 14 shows a picture of the fabricated filter. The
filter is first tested with a regular top cover to evaluate
its RF performance. Figure 15 shows the experimental
results of the filter using a solid top cover (i.e. with
no piezoelectric tuning). The filter exhibits an inser-
tion loss of 2.7 dB. The tuning elements are then assem-
bled and the filter was tested under various dc
actuation voltages. The mea-
sured results of the tunable
dielectric resonator filter are
shown in Figure 16 . While the
theoretical Q is close to 2,000,
the experimental Q obtained
is 550. The high insertion loss
obtained in this prototype
unit is attributed to many fac-
tors including the use of high
loss adhesive materials to
support the dielectric resona-
tor in the cavity and the oxi-
dation of the copper cavity
and the input/output probes
as well as the use of unplated
tuning screws.
Mechanically tunable two-
pole evanescent-mode metal
cavity filters have been re-
ported in [30] – [32] . These ev-
anescent-mode filters are also
known as ridge waveguide
filters or post filters. MEMS
actuators are used for tuning
in [30] , piezoelectric actua-
tors are used in [31], and
varactors are used in [32] . In
order to compare the performance of tunable dielec-
tric resonator filters operating in the TM mode versus
ridge waveguide filters having the same dimensions,
Figure 17 illustrates a comparison between the Q
0
–20
–40
–604.6 4.8 5 5.2
Frequency (GHz)
Gap = 160 um Gap = 150 um
Gap = 140 um
Figure 13. Filter schematic and HFSS simulation results (from [29]).
(a) (b)
Figure 14. Picture of the fabricated filter (from [29]).
CH1 S11 LOG 5 dB/REF 0 dB 1:–24.934 dBCH2 S21 LOG 10 dB/REF 0 dB 1:–2.7630 dB 5.293 750 0000 GHz
Cor
Cor
START 4.600 000 000 GHz STOP 5.600 000 000 GHz
Figure 15. The measured dieletric resonator filter shown in Figure 14 with no tuning (from [29]).
October 2009 91
factor of the two resonators as the gap between the
top of the resonator and the cover is varied. It can be
seen that for a small gap the TME dielectric resonator
offers a much high Q than that achieved by the eva-
nescent mode cavity resonator.
MEMS-Based Tunable Dielectric Resonator Filters A MEMS-based tunable dielectric resonator filter was
reported in [33] . The basic building block consists of a
dielectric resonator operating in a TE01d mode, a tuning
disk, and multiple MEMS. The MEMS actuators along
with the disk are employed to replace the tuning screws
typically used in conventional dielectric resonator fil-
ters. All the components are contained in a metallic
cavity as shown in Figure 18 . The tunability is achieved
by moving the tuning disk along the z axis with the use
of the MEMS actuator. With no dc bias applied to the
MEMS actuator, the tuning disk is at the closest position
to the dielectric resonator, i.e., the value of tuning gap h
is at a minimum. This state corresponds to the highest
resonant frequency. When a dc voltage is applied to the
MEMS actuator, the tuning disk is pulled away from the
dielectric resonator, and the resonant frequency begins
to decrease. Note that the dielectric resonator here
behaves differently from the TME dielectric resonator
shown in Figure 10 .
There are several constraints in developing such
MEMS tuning elements. EM (HFSS) and MEMS (Coven-
torWare) simulation tools were employed in [33] to
determine the optimum tuning disk size for maximum
tuning range and maximum Q, while taking into account
the structure’s mechanical characteristics. The most
important design parameters of this structure are the
tuning gap h and the diameter of the tuning disk d, since
they have the most effect on the tuning range and Q of
the dielectric resonator. Two
sets of simulations were per-
formed to study the effects of
these two parameters [33] . Fig-
ures 19 and 20 illustrate the
EM simulated variation in the
frequency shift and Q as a
function of the tuning gap, and
disk diameter respectively.
While a larger diameter disk
will provide a wider tuning
range, large tuning disks are
subject to other design issues
such as mechanical stability
and warpage that will con-
strain the actuator design.
Figure 21 shows the HFSS sim-
ulation results of a three-pole
tunable dielectric resonator
filter. Circular tuning disks
leaving a 3-mm diameter are
employed in the simulation model. With a deflection of
0.7 mm provided by the MEMS tuning elements, the
filter can be continuously tuned from 15.65 GHz to as
high as 16.45 GHz, i.e., a tuning range of 800 MHz.
In order to achieve a wide tuning range, it is essen-
tial to design a MEMS actuator that can generate
enough force to lift a relatively large tuning disk as
well as to generate a large deflection. In addition, a low
actuation voltage and fast tuning speed are desirable.
The concept of thermal plastic deformation assembly
was adopted in [34] to design the MEMS tuning ele-
ment with a vertical displacement for this filter. It is
Metallic ResonatorTME Mode of DR
H H
DC
DC
L L
DielectricResonator
ConductorWall
ConductorWall
MetallicPost
Q Value Versus Gap4,000
3,000
2,000Q F
acto
r
1,0000 0.5
Gap (mm)
1 1.5 2
Figure 17. Q of a dielectric resonator operating in TME versus Q of an evanescent-mode resonator (metallic resonator) having the same dimensions (from [29]).
0
Tuning No Tuning
–20
–404.7 4.9
Frequency (GHz)
5.1
Insert
ion L
oss (
dB
)
5.3 5.5
Figure 16. The measured tunable dielectric-resonator filter shown in Figure 14 (from [29]).
If reconfigurable RF filters are ever employed in wireless base stations and satellite systems, tunable dielectric resonator filters stand to be the optimum solution.
92 October 2009
demonstrated in [34] that the thermal actuator can lift
a disk of size 3,000 µm 3 3,000 µm 3 2 µm plate more
than 1 mm in the z direction. To elimi nate warpage
a hexagonal plate rather than a circular plate was used
[33] as shown in Figure 22 .
Figure 23 illustrates the tunable dielectric resonator
filter. It is manufactured into two detached parts: the
top cover in Figure 23 (a) and
the body in Figure 23 (b) [34] .
These two parts are gold
plated to reduce the conduc-
tive losses from the metal
cavity. Dielectric resonators
are assembled with the body
and the MEMS tuning ele-
ments are integrated on the
top cover. Each MEMS tuning
element is connected to two
dc feed-through pins for
applying the control voltage.
This configuration allows
each tuning element to be
independently controlled. A
few mechanical tuning screws are also included in the
tunable filter design to compensate for the machining
tolerance and variation of material properties.
The measured results for the insertion loss and
the return loss response of the three-pole tunable
0.7
0.6
0.5
0.4
Tuning Gap (mm)
Quality
Facto
r
Fre
quency S
hift (G
Hz)
0.3
0.2
0.10.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
1,800
2,200
2,600
3,000
Frequency ShiftQuality Factor
Figure 19. Frequency shift and Q as a function of the tuning gap, when the tuning disk diameter is fixed (from [33]).
2.5
2
1.5
1
0.5
01 1.5 2 2.5 3 3.5 4
Tuning Disk Size (mm)
1,000
1,500
2,000
2,500
3,000
3,500
Fre
quency S
hift (G
Hz)
Quality
Facto
r
f0, h = 0.1 mmf0, h = 0.2 mm
Q, h = 0.1 mm
Q, h = 0.2 mm
Figure 20. Frequency shift and Q as a function of the tuning disk diameter when the tuning gap is 0.1 and 0.2 mm (from [33]).
0
No Tuning
(h = 1 mm)
Tuning
(h = 0.3 mm)
–20
–40
–60
–80
–100
Insert
ion L
oss (
dB
)
15.0 15.2 15.4 15.6 15.8 16.0 16.2 16.4 16.616.8 17.0
Frequency (GHz)
Figure 21. HFSS simulation results of a three-pole dielectric resonator filter tuned to different frequencies (without loss) (from [33]).
TuningDisk
DielectricResonator
Trans-Tech, Er = 31
SupportCavity
MEMSActuators
d h
Z
X
Figure 18. Schematic of the proposed tuning structure (from [33]).
(a) (b)
MEMS
Tuning Elements
Figure 22. MEMS tuning elements: (a) a solid circular tuning disk with warpage and (b) a hexagonal tuning disk without warpage (from [33]).
October 2009 93
dielectric resonator bandpass filter are presented in
Figure 24 . The filter’s center frequency is synchronously
tuned from 15.6 GHz to 16.0 GHz. When the filter is
tuned from 15.6 GHz to 16.0 GHz, the mid-band inser-
tion loss of the filter increases from 1.5 dB to 4.5 dB.
This high insertion loss is attributed to the assembly
procedure and lossy materials (epoxy, stainless tuning
screws, etc.) used to construct this prototype filter.
Obviously, much better insertion loss performance can
be achieved with further design optimization.
Another tunable dielectric resonator concept was
reported in [28] and [35] . The concept was demon-
strated using piezoelectric material but can be imple-
mented as well using MEMS technology. The tuning
mechanism proposed in [35] consists of three main
components: a dielectric resonator operating in the
TE01d mode, a metal disk comprising a certain number
of radial-arranged quarter-wave slotline resonators,
and a switch for each slot-line resonator then later is
used to control the coupling between the TE01d mode
and the slot-line mode. The concept is illustrated in
Figure 25 . A substrate with radial slot-lines is sym-
metrically arranged above a cylindrical dielectric
resonator. Eight switches could be placed on the edge
of the slots. With the switches in the off state, the
eight slot-lines represent capacitively loaded quarter-
wave slot line resonators that possess a strong
magnetic-field component in the radial direction,
providing strong coupling to the resonant fields of
the mode. Due to the interaction between the dielec-
tric-resonator mode and the slotline resonator mode,
the stored electromagnetic energy is distributed
between the dielectric resonator and the slot-lines.
This leads to an increase in the effective size of the
resonator and, as a consequence, to a decrease of the
resonant frequency.
Upon closing the switches, the corresponding quar-
ter-wave resonance changes into a half-wave resonance,
as shown in Figure 26 . Due to the sign change of mag-
netic field in the middle of the slot line, the coupling
between slot-line mode and the dielectric-resonator
mode is much weaker. In this case, there is almost no
intermodal coupling and the resonance frequency of the
mode remains almost unaffected by the presence of slot
lines. The tuning range of the resonator is determined
by the coupling strength between the dielectric-resona-
tor mode and the quarter-wave slot line mode, which
can be controlled by a variation in the distance between
the dielectric resonator and the slot line disk. The major
advantage of this concept [35] is that in the MEMS
closed state there is almost no coupling between the
resonator and the slot line resonator. Therefore, one can
expect only small additional losses induced by the
switch contact resistance.
The concept was demonstrated experimentally
using the cavity shown in Figure 25( b). The slot lines
were switched ON and OFF using bimorph piezoelec-
tric actuators. A tuning range of 5 MHz was achieved
with a Q value of 12,000 at 2 GHz.
This concept was combined with the planar dielec-
tric resonator filters [19] to construct a tunable dielectric
(a)
(b) (c)
Figure 23. Proposed three-pole tunable dielectric resonator filter: (a) the top cover with MEMS tuning elements, (b) the filter body, and (c) the assembled filter (from [33]).
One key challenge in the design of tunable filters is to maintain a constant filter bandwidth and a reasonable return loss performance over the tuning range.
0
–20
–40
–60
–80
–100
Insert
ion L
oss (
dB
)R
etu
rn L
oss (
dB
)
14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0Frequency (GHz)
14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0Frequency (GHz)
(a)
(b)
0
–5
–10
–15
–20
Figure 24. Comparison of the measured results of the proposed tunable dielectric resonator filter for different tuning states (from [33]).
94 October 2009
resonator filter. Figure 27 sh ows
a two-pole filter designed, fab-
ricated and tested [36] . The
filter consists of two dielectric
resonators formed from a
high-K substrate. Two metal
disks, attached to the top cover
each with four slots, are placed
in proximity to the resonators.
In order to study the effects of
the disk on the resonator Q,
Table 1 compares the theoreti-
cal resonant frequency and Q
of the resonator without the
disk and with the disk with
switches in the ON and OFF
state. The switches here are
simulated in HFSS using a
metallic wire. The presence of
the disk in proximity of the
resonator reduces its Q by
around 40%. The Q is further
reduced when the switches
are in the ON state. The fabri-
cated unit is shown in Figure
27 . The HFSS simulation re-
sults, as the edges of the slots
are switched ON and OFF, are
shown in Figure 28 . The ex-
perimental results of this
two-pole filter are shown in
Figure 29 , where a 60 MHz
tuning range has been ac-
hieved. A larger tuning range can be achieved by
bringing the disk closer to the resonator but this will
lower the Q value.
Other Means for Tuning Dielectric Resonators
Magnetically Tuned Dielectric Resonators Magnetically tuned dielectric resonators have been
reported in [37] – [38] . The principle is to control the mag-
netic field pattern in the vicinity of the dielectric resona-
tor using ferrite material, which in turn results in a
change of the resonant frequency. A tunable dielectric
resonator was realized in [37] by placing a low-loss fer-
rite disk directly in proximity of a high-Q dielectric reso-
nator. An applied magnetic field was used to control the
magnetic properties of the ferrite, and hence the field
distribution in the vicinity of and within the dielectric
reson ator. Such variations of the magnetic field strength
caused a noticeable shift in resonant frequency.
Figure 30 illustrates two configurations reported in
[38] using axially and circumferentially magnetized
ferrite. Pictures of cavities built using this concept are
shown in Figures 31 and 32 . The experimental results
Substratewith Slotlines
Substrate with
Slotlines
(a) (b)
Cavity
Bimorph Actuators Actuation Supply
HF
CouplingDR
DR
h
Figure 25. A tunable dielectric resonator using a disk integrated with switches (from [35]).
(a) (b)
Figure 26. The magnetic field distribution with (a) the switches OFF and (b) the switches ON (from [28]).
Figure 27. A two-pole filter dieletric resonator filter with two disks integrated in the cover and with integrated switches (from [36]).
TABLE 1. Comparison between the resonance frequency and Q of a resonator loaded with a disk.
No Disk
With Disk Switch OFF
With Disk Switch ON
Freq. 4.008 GHz 4.167 GHz 4.227 GHz
Q 6,000 3,730 2,760
October 2009 95
achieved for the ferrite-tuned dielectric resonator
with the circumferentially magnetized ferrite are
shown in Figure 33 [38] . A 30 MHz tuning range
was achieved at 2.2 GHz with a Q of 4,000. A tunable
two-pole magnetically tuned dielectric resonator
filter was demonstrated in [38] . Hysteresis effects
were however, reported in the return loss as the mag-
netizing bias current changed directions.
Varactor Tuned Dielectric Resonators Dielectric resonators employ-
ing varactors for tuning were
reported in [37], [39] and [40] .
A substrate having a metal
ring loaded with two varactors
placed on the top of the reso-
nator [37] was used to demon-
strate tuning as shown in
Figure 34 , whereas in [39]
the metal ring was placed in
proximity of the dielectric
resonator on a substrate, as
shown in Figure 35 . The prin-
ciple of both approaches is to
(a)
(b)
Figure 31. Picture of a two-pole ferrite tunable dielectric resonator filter, axially magnetized.
(a) (b)
Figure 32. Picture of a tunable dielectric resonator filter with a circumferentially magnetized ferrite(from [28]).
0
–10
–20
–30
–40
Insert
ion L
oss (
dB
)
4 4.1 4.2 4.3 4.4Frequency (GHz)
Simulation Results
Switch OffSwitch On
Figure 28. The HFSS simulation results of the filter shown in Figure 27 (from [36]).
0
–10
–20
–30
–40
Insert
ion L
oss (
dB
)
4 4.1 4.2 4.3 4.4Frequency (GHz)
Measured Results
Switch OffSwitch On
Figure 29. The measured results of the filter shown in Figure 27 (from [36]).
Ferrite Rod
Ferrite Disks
DielectricResonator
Teflon Supports
External
Magnetic Circuit
TuningWinding
TuningWinding
Dielectric Resonator
(a) (b)
Figure 30. A schematic of a tunable dielectric resonator filter with (a) axially magnetized ferrite and (b) circumgenntically magnetized ferrite (from [38]).
96 October 2009
control the field distribution
in the vicinity of the dielectric
resonator, by varying the
boundary conditions on the
metal ring through the use of
varactors. This in turn will
result in a change in the di-
electric resonator center fre-
quency. In the latter approach
[39] , a tuning range of 1.6%
and a Q close to 8,000 at
3.5 GHz was demonstrated.
The concept was also used to
realize a tunable bandstop
varactor-tuned dielectric reso-
nator filter. In [40] varactor
diodes were embedded in a
slot machined in a dielectric resonator. A relatively
wide tuning range of 7.1% centered around 4.47 GHz
was achieved. However, a large degradation in the
resonator Q was reported using this approach.
Optically Tuned Dielectric Resonators A dielectric resonator integrated with a photosensitive
material was reported in [41] to demonstrate the
possibility of tuning the dielectric resonator by optical
means. A GaAs sample was placed on the top of the
dielectric resonator and was illuminated by a light
source. The idea here is that as the conductivity of the
sample changes with light, the electromagnetic bound-
ary conditions will change causing a shift in the resonant
frequency. The use of a strong white light with a total
output of 100 mW/cm 2 caused a 15 MHz shift n resonant
frequency for a dielectric resonator operating at 10 GHz.
Impact of Spurious Performance and Temperature Drift on Tuning Range Dielectric resonators are known to have a relatively
narrow spurious-free window in comparison with
other micro wave resonator structures. At 4 GHz the
spurious free window for a conventional dielectric res-
onator of cylindrical shape operating in the TE01d mode
is around (700–800 MHz). One approach to improve
the spurious performance is to reshape dielectric reso-
nator structures by removing dielectric materials in
areas where the field of the spurious mode has high
concentration. The concept has been applied success-
fully in [14] , [15], and [42] – [44] in improving the spuri-
ous performance of dielectric resonators operating in
single modes, dual modes and quad-modes. A 50% in-
crease in the spurious free window can be potentially
achieved with such techniques.
To maintain a reasonable spurious free window over
the tuning range, one needs to use tuning elements that
can shift the resonant frequency of both the operating
mode and the spurious mode in the same direction.
Depending on the field distribution of these two modes,
2,295
2,290
2,285
2,280
2,275
2,270
2,265
2,2600 1 2 3 4 5 6 7 8
I (A)0 1 2 3 4 5 6 7 8
I (A)
(a) (b)
f (M
Hz)
Q0
5,000
4,000
3,000
2,000
1,000
0
Figure 33. Resonant frequency and Q of the ferrite tunable dielectric resonator filter shown in Figure 32 (from [28]).
Substrate
Dielectric
Resonator
Varactor
Metal Ring
Figure 34. A varactor-tuned dielectric resonator. The varactors are integrated on a substrate placed on top of the dielectric resonator.
Substrate
Dielectric
Resonator
Varactor
Metal Ring
lectric
sonator
V
Figure 35. A varactor-tuned dielectric resonator filter. The varactors are integrated on a substrate placed in proximity of the dielectric resonator.
October 2009 97
some tuning elements, if placed improperly inside the
cavity, can cause a different impact on the resonant fre-
quencies of the two modes reducing the spurious free
window. However, in high-Q wireless and satellite
applications, filters need only to operate over a 5–15%
tuning range. Thus, the limited spurious performance
of dielectric resonators should not limit the use of tun-
able dielectric resonator filters in such applications.
The overall temperature drift of the dielectric reso-
nator filter is determined by the temperature coeffi-
cient of the dielectric resonator, the support structure
and the thermal expansion coefficient of the enclo-
sure. Dielectric resonators are offered with a wide
range of temperature coefficients (26 ppm/°C to 16
ppm/°C) to allow designers to compensate for the
combined temperature drift that results from the
above factors. Such combined temperature drift could
be positive or negative depending on the type of mate-
rial used. With the proper choice of the temperature
coefficient of the dielectric resonator materials the
overall temperature drift of the dielectric resonator
filter can be reduced to 1 ppm/°C. (i.e., over a nT of
50 °C the frequency drift at 5 GHz frequencies would
be only 250 kHz). This has been consistently demon-
strated for conventional dielectric resonator filters.
However, for tunable dielectric resonator filters that
are tuned mechanically (piezoelectric, MEMS) one
needs to take into consideration not only the impact of
the thermal expansion coefficient of the tuning ele-
ment but also the impact of varying the gap between
the tuning element and the resonator over the tuning
range. The temperature drift problem exists in all
other known tunable filters technologies. The advan-
tage, however, of tunable dielectric resonator filters is
that designers have the flexibility to select dielectric
resonator materials with negative or positive temper-
ature drifts that can do proper temperature compen-
sation. With a careful thermal design of the dielectric
resonator cavity we believe an overall temperature
drift of less than 1 ppm/°C can be potentially achieved
over a tuning range of 5–15%.
Constant Bandwidth and Return Loss Performance over the Tuning Range One key challenge in the design of tunable filters is to
maintain a constant filter bandwidth and a reasonable
return loss performance over the tuning range. This is
an important requirement for wireless and satellite
applications and is also an important requirement in
the majority of other system applications that require
the use of tunable filters. Yet it is interesting to note
that, with the exception of few publications, most of
the papers published on tunable filters do not demon-
strate a constant bandwidth and often show a
degraded return loss over the tuning range. In many
other cases the achievable return loss over the tuning
range is not reported.
This problem is attributed to the fact that synchro-
nous tuning of the resonators has traditionally been
the preferred method for tunable filters. In synchro-
nous tuning, all resonators are shifted by the same nf
during tuning. In addition, no mechanism is usually
used to tune the coupling between resonators as the
filter center frequency is changed. In most filter struc-
tures, the variations of inter-resonator coupling and
input/output coupling do not follow the variation in
resonant frequency. This in turn causes the filter band-
width to change and the return loss to degrade over
the tuning range. The problem can be circumvented by
having tuning elements that can tune the resonator
center frequencies, the inter-resonator coupling and
the input/output coupling. This however may increase
the filter insertion loss and complicates the tunable
filter design because of the large number of tuning
elements needed.
In tunable filter applications that require a relatively
small tuning range (less than 15%), the problem can be
potentially circumvented with the use of non-synchro-
nous tuning of the resonators. The idea is to control the
resonant frequencies such that the shift nf in the reso-
nant frequency of the filter resonators is not uniform to
compensate for the variations in coupling values. The
exact shift nf for each resonator to maintain a reason-
able return loss performance over the tuning range can
be determined from the filter coupling matrix model
[1] by applying optimization [45] .
Conclusions Tunable dielectric resonator filters can potentially
address wireless and satellite applications that require
very high Q values (4,000 and up) with a limited
tuning range (less than 15%). Such high Q require-
ments cannot be met by any other known non-super-
conductor tunable filter technology at the present
time. The intent of this paper is to provide newcomers
and end users with the current status and prospective
of using dielectric resonators for tunable filters. It is an
enabling technology for high-Q tunable filter applica-
tions. A key challenge, however, is to increase the
tuning range without degrading the Q value. While
several techniques have been reported to demonstrate
the feasibility of tuning dielectric resonators, the tun-
able dielectric resonator filter technology is still in its
infancy. Very limited research effort has been dedi-
cated to explore the potential for improving the tuning
range. Most of the work reported thus far has focused
Tunable dielectric resonator filters can potentially address wireless and satellite applications that require very high Q values with a limited tuning range.
98 October 2009
on the use of TE01d modes and standard shape resona-
tors demonstrating a narrow tuning range. We believe
that the tuning range can be increased while maintain-
ing reasonably high Q values by exploring the use of
other modes and by the use of non-standard-shape
dielectric resonators.
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