Tunable dielectric resonator bandpass filter

5 17 TUNABLE DIELECTRIC RESONATOR BANDPASS FILTER Jerzy Krupka*, Adam AbramowiczA,KrzysztofDerzakowski" Abstract - A simple but efective technique has been applied to magnetically tune the quasi-TEoll mode dielectric resonators. The tuning was achieved by means of thefkrrite rod situated centrally in the dielectric resonator and extending over the shield The fkrrite parameters were changed by DC magneticfield. The method assures the high qualityfactor of the resonator, over 3.4% tuning range and suflcientb good spurious response. INTRODUCTION In modem microwave systems there is an increasing demand for the inclusion of electronic controls in components. In a tunable high-Q filters the function to be controlled is the resonant kquency of one or more elements within the filter. Several Merent methods of tuning dielectric resonator filters have been presented. [1,2,3]. The electronic tuning methods can be divided into three groups: various voltage controlled vamctor circuits with up to 1-2% tuning m g e , optically controlled light sensitive layers deposited on a resonator surface with up to 0.15% tuning range and various techniques applying femtes resulting in 1-2% tuning range. The most popular tuning method with femtes is using femte disks located directly above the dielectric resonator. However this method due to sigmficant degradation of the resonator quality factor and a large size, power consuming electromagnet, which is necessary to provide the variable magnetic field, is rarely used in practice. It is possible to avoid some of these disadvantages by using a thin ferrite rod located in an axial hole of the dielectric resonator as presented in Fig.l [4]. The magnetic filed controls the permeability of the femte and hence the resonant frequency of the resonant system. This paper presents the analysis and realisation of the tunable dielectric resonator filter based on the geometry shown in Fig.1. The same structure can be used to construct electronically controlled dielectric resonator oscillators. THEORY The resonant frequencies of the resonant system shown in Fig.1 can be found as the solution to the following eigenvalue problem [4]: The permeability tensor of the femte medium is characterisedby the following expression: -jK p - where: p = p' jp" ,K = K' - jK'' 0 01 ,and b = CL: - jb". The relative permittivities of the femte medium and of the dielectric resonator are assumed to be scalars, denoted by Ef = E{ js;l and q = jG", respectively. - - Rigorous solutions to the problem given by (1) have been obtained by means of the Rayleigh-Ritz method using empty cavity functions as a basis [5]. The resonant frequencies have been found for the lossless system. The quality factors can be computed using perturbation theory as shown in [4]. The results of computation show that for the quasi-TEolI mode the resonant frequency shift is predominantly * J e w Krupka, Institute of Microelectronics and Optoelectronics, "Adam Abramowicz, Institute of Electronic Systems, "Krzysztof Derzakowski, Institute of Radioelectronics, Warsaw University of Technology, Koszykowa 75,00462 Warsaw 518 c o ~ e ~ t ewith d the change in the parallel component h'of the permeability tensor. This is clear since for the mode resonators the H, component of the magnetic field has its maximum value at the dielectric resonator axis, while the remaining filed components have minima there. The femte should be selected in such a way that the parallel component k' exhibits significant dependence on applied DC magnetic field. In [5] the computed and measured characteristics of tuned structure have been presented. Over 4% frequency shift have been demonstrated. But in a practical realisation of a tunable resonator the are other factors having hlfluence on the frequency characteristicsshould be taken into account. These are mainly the other modes that can interfere with the quasi-"E,,l~mode. In the structure fiom Fig. 1 the hybrid HEllI orthogonal modes, which for the demagnetized state have lower fr-equency than the quasi-TE&,tl mode, along with magnetization split into two resonances. One of which decreasing its frequency with magnetic field and the other increasing its frequency. The latter can interfere with the quasi-T&ll mode. To avoid such a situation a proper distance between the upper metal plate and the dielectric resonator should be selected. On the base of computed results and h o w dielectric resonator permittivity the following structure parameters have been found: E, = 32, D = 9.1 mm, L = 7.05 mm, d =4 mm, h = 7.3 nun, S = 15.5 mm, ~r=15.5,k=O.75 For given above parameters the computed maximum tuning range (i.e. for a change of the parallel component from k = 0.75 to k = 1.0) was 4.1 %or 284 MHz. FILTER REALISATION AND MEASURED RESULTS The filter has been realised in a copper housing. The size and shape of the input and output structures have been found experimentally. The dielectric resonator is excited trough the open ended probes made of silverplated wires. The S M A connectors are fixed directly to the housing. As a result the dimensions of the filter are very small. But it is the electromagnet that decides on the overall structure dimensions (see Fig.2.). The dielectric resonator has been fixed between upper and lower metal plates by means of the thin polyethylene ring. The filter has been measured using vector network analyzer. The measured resonant frequenciesversus curent in the coil are shown in Fig.3. The value of the current was limited to 500 mA because of the significant changes of the filter characteristic for higher currents. The resonant fkquency of the quasi-TI%ll mode changes between 6.924 GHz and 6.689 GHz, which is slightly lower range than computed one what means that the DC magnetic field strength was still too small to magnetize the femte. It must be added that the Anyway the relative frequency change is 3.4 %. The measured filter Characteristics are shown in Fig.4. The filter has the nearly constant 3 dB bandwidth of approximately 103 MHz. The insertion loss changes between 0.32 dE3 and 0.65 dB.The retum loss at the center frequency is always better than 30 dB.The effective spurious free band for the worst case of the lowest resonant frequency is 625 M H z at a level of -10 dB. From the m d characteristics the unloaded quality factor of the tunable resonator can be computed. At the highest frequency (demagnetized state of the ferrite) the unloaded quality factor is 1710. The observed changes in the unloaded quality factor are related to the changes of the ferrite parameters and the influence of the HE,I mode on the filter characteristics. CONCLUSIONS A simple but effective technique has been used to magnetically tune the quasi-mll mode dielectric resonators. The tuning has been achieved by means of the ferrite rod situated centrally in the dielectric resonator and extending over the shield to the electromagnet system. The ferrite parameters were changed by DC magnetic field produced by the DC current. The method assures the high quality factor of the resonator, 3.4% tuning range and quite good spurious resppnse. Measured filter characteristics show low insertion loss 0.32 + 0.65 dB and excellent retum loss better than 30 dB at center frequency in the full tuning range. REFERENCES [l] S. Watanabe et al.: Very high* dielectric resonator volege controlled oscillators, Roc. 8th European Microwave Conf.,pp. 267-278, Paris 1978 [2] P.R Hercifeld et al.: Optically tuned and FM modulated X band dielectric resonator oscillator, hoc. 14th European microwave Conf.,Liege 1984 [3] A.N.Faar et al.: Novel techniques for electronic tuning of dielectric resonators, Roc. 13th European microwave Conf.,pp. 791-796, Numberg 1983 (41 J. Krupka: Magnetic tuning of cylindrical -mode dielectric resonators, IEEE Trans. On Microwave Theory and Tech., vo1.37, N0.4,pp. 743-747, April 1989 [ 5 ] J. Krupka: Resonant modes in shielded cylindrical ferrite and shgle-crystal dielectric resonators, IEEE Trans.On Microwave Theoxy and Tech., vo1.37, No.4, pp. 691-697, April 1989 5 19 I metal housing d c-, D Fig.1 Configuration of a tunable dielectric resonator. Coil /RF ferrite core 1 mode magnetically tuned resonant system with DC magnetic field circuit which Fig.2. Shielded quasi allows a very efficient magnetic tuning. 520 7.0 6.8 6.6 6.4 6.2 + 6.0 5.8 HE 111 5.6 5.4 200 100 0 300 400 500 DCcurrent /mA Fig.3. Resonant frequencies of the tunable resonator versus DC current 0 -5 -10 -15 -20 -25 -30 -35 -40 6.3 6.4 6.5 6.6 6.7 6.8 frequency /GHz Fig.4. Measured characteristics of the filter. View publication stats 6.9 7.0 7.1