a)

b) c)

Tunable filters based on metal-dielectric resonators

Borys Pratsiuk1, Yuriy Prokopenko1, Yuriy Poplavko2 1 Physical & Biomedical Electronics Dept., 2 Microelectronics Dept.

National Technical University of Ukraine “KPI” Kyiv, Ukraine

b.pratsiuk@gmail.com, prok@phbme.ntu-kpi.kiev.ua, poplavko@ieee.org

Abstract— A method of metal-dielectric resonator’s frequency tuning is proposed. The method is suitable for electromechanical control using piezoelectric or MEMS actuator. The two-resonator and three-resonator tunable filters based on metal-dielectric structures are developed.

Keywords-tunable; filter; dielectric; resonator;

I. INTRODUCTION Tunable filters are very attractive in modern wireless

communication systems. They are widely used in front-end receivers for suppression of interfering signals and for relaxation of oscillator phase noise and dynamic range requirements. While cellular phones and wireless networks come with multiband capabilities, significant research effort is being directed toward implementation of new microwave components design.

At the present time, dielectric resonator filters are emerging as the baseline design for the majority of radio frequency filters used in wireless and satellite applications [1]. They offer high Q values with relatively high Q/volume ratio in comparison with other known filter technologies.

In order to decrease the filter size, compact filter designs should be used [2]. The dielectric resonator placed between the metallic walls may operate on different TE, TM or HE modes. It’s important to emphasize that TE modes will significantly be shifted in the frequency range. HE100 mode has lowest frequency value and the mode is chosen as the main working mode. In paper [3] design of the filter based on dielectric resonators with TM mode excitation was presented. The same principle is applied to compact duplexer design [4]. All these devices present an important class of devices with static characteristics. Improvement of their functionality by development of the filters with tunable characteristics is very important purpose and should be investigated.

Recent achievements in MEMS and piezoelectric technologies open an opportunity for elaboration of electromechanically controlled devices. The main advantage of such devices is low loss and high quality factor. To achieve valuable control a small displacement of components shall noticeably perturb electromagnetic field. This requirement could be satisfied if electrical field of non-perturbed device is perpendicular to dielectric discontinuity. This idea was implemented in tunable dielectric resonators [5] operated in a TE01δ mode. In this paper this principle is extended to metal-dielectric resonators.

II. TUNABLE METAL-DIELECTRIC RESONATOR The metal-dielectric resonator is a monolithic design

consisting of dielectric with metalized end walls (Fig.1a). The metal disk could be used as top metalized face of the dielectric disk. Peculiarity of TMmn0 and hybrid HEmn0 modes of metal-dielectric resonator is distribution of electrical field parallel to cylinder axis and perpendicular to metal walls. That is why to achieve noticeable frequency shift a discontinuity shall be created perpendicularly to dielectric cylinder axis.

One of the solutions is to tear a metal perturber from a dielectric disk face (Fig.1b). Another way is to cut the dielectric into sections in the plane perpendicular to electrical field of the resonator (Fig.1c). For both solutions electrical field is perpendicular to air gap and ensure significant electrical field perturbation.

As a result resonant frequency of metal-dielectric resonator significantly depends on the distance d between perturber and dielectric disk. Fig. 2 demonstrates the dependence of normalized wave-number for the main resonant mode versus normalized distance between metal perturber and dielectric disk. The normalized wave-number is kRε , where ε is the dielectric permittivity of a disk,

Figure 1. Metal-dielectric resonator: (a) non-tunable structure, (b) tunable by metal perturber structure (c) tunable split resonator

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Figure 2. Normalized resonant wave number versus normalized distance between metal perturber and dielectric disk.

02 fkcπ= is the resonant wave-number, f0 is the resonant

frequency of the main HE100 mode, c is the light velocity in vacuum, R is the radius of a disk, h is the disk height. As it is seen in Fig. 2 the range of the resonant frequency tuning for metal-dielectric resonator reaches several tens percents while distance between dielectric and perturber is only a few percents from dimension of the resonators. Required absolute displacement of perturber is only tens micrometers in centimetre wavelength bands and a few micrometers for millimetre waves. Such small displacement could be easily achieved by piezoelectric or MEMS actuators. That is why this method of metal-dielectric resonator’s tuning is very promising for electromechanical control. Absolute displacement necessary for essential tuning depends on dielectric permittivity and sizes of the resonator and reduces with increasing of the permittivity and applied frequency.

Figure 3. Tunable two resonator filter based on metal-disk dielectric resonators

Figure 4. Tunable three resonator filter based on metal-disk dielectric resonators

Required displacement also decreases with decreasing of dielectric thickness. For very thin resonators frequency tuning is over 100%. However, the coupling of thin resonators with transmission line should be well considered.

III. TUNABLE FILTERS BASED ON HE100 METAL-DIELECTRIC RESONATORS

Multiresonator filters have slope of amplitude versus frequency dependence steeper than for singleresonator filters, but presence of mutual coupling between resonators and transmission line results in unbalance filter characteristics while tuning. The difficulty could be overcome by excitation of confluent resonant modes [6] in a single resonator. In paper [7] the method of mutual coupling compensation by non-synchronous resonator tuning is proposed. Using those concepts two different types of tunable dielectric resonators filters were designed [8].

Another solution consists in the fact that mutual coupling between resonators are minimal in cutoff waveguide.

In paper [9] tunable dielectric resonator filter operating in a TE01δ mode was presented. The tunability is achieved by moving of the perturber disk. Alteration of filter central frequency is about 15% while metal disk displacement is around one millimeter. This tuning method is not efficient because the metal disk moves parallel to electric field lines and electromagnetic field disturbance is negligible. Metal-dielectric resonators operating in a HE100 mode is preferable for electromechanical tuning because dielectric discontinuity is placed perpendicular to electrical field and it significantly disturbs electromagnetic field and shifts resonant frequency up to 100%. Similar concept was applied for TME mode resonator in [10].

An example of two-resonator filter is given in Fig.3.

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Figure 5. Insertion loss of a two metal dielectric resonator filter tuned at different frequencies

The bottom face of the dielectric disk is not metalized Non-perturbed resonators operate on HE100 modes and are placed in cutoff section of rectangular waveguide. So, metal wall of a waveguide plays a role of the perturber. Tuning of resonant frequencies of resonators is provided by alteration of a distance between dielectric disk and waveguide wall. Both of resonators shall be tuned synchronously. Filter central frequency tuning versus the metal perturber displacement is shown in the Fig.5. The tuning range of the resonant frequency is about hundreds megahertz while displacement of perturber is about units of micrometers. This concept of filter design can be applied to multiresonator systems as well. As an example, the three-resonator filter is shown in the Fig. 4. It should be noted, that with increase in number of resonators, the number of mutual coupling coefficients increases too.

Figure 6. Insertion and return losses of a three metal dielectric resonator filter tuned at different frequencies

As a result, compensation of unbalanced filter’s characteristic becomes more complicated. For instance, in order to adjust filter frequency band the size of the central resonator was slightly changed. Filter central frequency tuning ensured by synchronized tuning of all resonance frequencies of the resonators is shown in the Fig.6.

IV. CONCLUSION Alteration of HE mn0 or TMmn0 mode’s resonant frequency

in a metal-dielectric resonator can be achieved by creation of air discontinuity in cross-section perpendicular to axis of dielectric cylinder. This discontinuity appreciably perturbs electromagnetic field of a resonator and as a result it leads to shift of its resonant frequency. The range of frequency deviation is several tens percents and can reach 100% or even more depending on dielectric disk thickness while thickness of air gap is only tens or even several micrometers depending on dielectric permittivity of a disk and frequency band. It opens an opportunity to realize electromechanical control of resonant frequency using piezoelectric or MEMS actuators.

This principle can be applied for design of tunable multiresonator filters. A simplest way to realize such kind of filters is placing the resonators in cutoff waveguide to minimize mutual coupling between resonators. For another designs of tunable multiresonator filters the influence of frequency on coupling between resonators and transmission line should be taken into account.

REFERENCES [1] R. Mansour “High-Q tunable dielectric resonator filters,” IEEE

Microwave Magazine, vol. 10, pp. 84–98, October 2009. [2] J. Hattori, H. Wakamatsu, H. Kubo, and Y. Ishikawa, “Low profile

dielectric band elimination filter using thin film layered electrode for 2GHz band cellular base station,” IEEE MTT-S Digest, pp. 1025–1028, 1999.

[3] Y. Kobayashi, and M. Minigishi, “A low-loss bandpass filter using electrically coupled High-Q TM01δ dielectric rod resonators,” IEEE Trans. Microwave Theory Tech., vol. 36, pp. 1727–1732, December 1988.

[4] Y. Ishikawa, J. Hattori, M. Andoh, and T. Nishikawa, “800MHz high power duplexer using TM dual mode dielectric resonators,” Proc. IEEE MTT-Symposium, 1992, pp. 1617–1620.

[5] Yu. M. Poplavko, Yu. V. Prokopenko, V. I. Molchanov, and A. Dogan, “Frequency-tunable microwave dielectric resonator,” IEEE Trans. Microwave Theory Tech., vol. 48, pp. 1020–1027, June 2001.

[6] B. Pratsiuk, Y. Prokopenko, and Y. Poplavko, “Tunable sphere and cubic dielectric resonators,” Microwave, Radar and Wireless Communications, 2008. MIKON 2008, vol. 2, pp. 549–552.

[7] B. B. Pratsiuk, Y. V. Prokopenko, and Y. M Poplavko, “Method of disparity compensation in tunable filter based on dielectric resonators,” (in Russian) Electronics and Communications, Ukraine, Kiev 2009. vol. 4-5, part 2, pp. 19–22.

[8] B. B. Pratsiuk, Y. V. Prokopenko, and Y. M. Poplavko, “Tunable microwave filters based on uncoupled dielectric resonators,” Microwave & Telecommunication Technology 2009, CriMiCo 2009, vol. 2, pp. 523–524.

[9] W. D. Yan, and R. R. Mansour, “Tunable dielectric resonator bandpass filter with embedded MEMS tuning elements,” IEEE Trans. Microwave Theory Tech., vol. 55, pp. 154–160, January 2007.

[10] F. Huang, and R. R. Mansour, “Tunable compact dielectric resonator filters,” presented at 2009 European Microwave Conf., Rome, Italy Sept. 2009.

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