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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