диафрагмированные волноводные фильтры / 40e641f2-9bf9-44f5-a68d-735b8ff64ebb

I. INTRODUCTION

Cellular-radio base stations demand low-loss, high- power-handling selective filters with small physical size and low cost. These demands have led to advances in coaxial resonators. Combline cavity resonators are special coaxial resonators, which are a popular type of resonator because they are comparatively inexpensive to manufacture and their physical size is small. They are a kind of wide spurious-free resonator. A problem of its inner conductor is high loss, which lowers its Q-factor. Much research has been done in order to improve the Q-factor of the conventional combline resonator. Although dielectric resonators can achieve higher Q-factor with small size [1], they are higher in cost and have a poor spurious characteristic. Analysing the corrugated conductor structure, we find that these solutions for the corrugated conducting surface under the condition that period p are the same as those of the dielectric coated conducting plane [2]. Filters loaded with periodic helix resonator structures are miniature filters [3]. There-

Correspondence to: D. Budimir; email: d.budimir@ westminster.ac.uk.

DOI 10.1002/mmce.20123

Published online 21 October 2005 in Wiley InterScience (www. interscience.wiley.com).

fore, using periodic structures to replace the dielectric materials is a method to improve the performance of the resonator. In this article, we investigate filter applications using corrugated-structure-loaded combline cavity resonators, which combine the advantages of the conventional combline resonator and high Q-performance of the dielectric resonator. Using these novel combline resonators, we can either reduce the size (compared to conventional combline resonators), or obtain a higher Q-factor than a conventional combline resonator within the comparable volume. Full-wave analysis for this kind of resonator using the mode-matching method is depended on the numerical method [4]. In this article, we express the eigenvalue equation with the condition of a single-team approximation in the gap region using the mode-matching method. The bandwidth of the filters is found to be easily adjusted by different couplings. External couplings are discussed in this article.

II. ANALYSIS OF THE COMBLINE CAVITY RESONATORS

The general type of combline cavity resonator, as shown in Figure 1(a), is a circular cylindrical-cavity resonator with a small gap at the central part of the

where T denotes the mode angular wave number with respect to the r, plan. Ez is the electric-field component for the TM modes (E modes) with respect to the z direction, expressed in region s II or I as follows [4]:

Transverse field expressions in regions II and I can be expressed for the TEz mode as follows:

For the combline resonator, DmpIE 0 and DIHmp 0 in region I. Using the boundary condition Et 0 at r R, in region II, we obtain

where BIImpE, BIImpH are constants.

For the circumferential uniform TM modes, m 0. If the width of gap d is much smaller than the wavelength, that is, d , we may neglect the high-order teams in region I. The field-matching equations are then reduced. Using mode orthogonality, the fieldmatching equations at the interface of r R0 become

BIIE0p Y0 TpIIER J0 TIIEp R0 J0 TIIEp R Y0 TIIEp R0 eIIE0p

p

hIIE0p CIE00J0 kR0 eIE00 hIIE0p , (13)

p

B0IIEp T IIEp Y0 T pIIER J1 T IIEp R0

p

J0 T IIEp R Y1 T IIEp R0

hIIE0p eIE00 kC00IE J1 kR0 hIE00 eIE00. (14)

Due to the independent mode mechanism for an ideal assumption [8], when ignoring the modes’ intervention, the expression of eq. (13) over eq. (14) becomes

J0 kR0 eIE00 hIIE0p

kJ1 kR0 hIE00 eIE00 . (15)

The inner products are as defined in the literature [9]. Substituting the inner products of eq. (16) into eq. (15), we obtain

we obtain the eigenvalue equation as follows:

where TIIEp k2 2p k2 (( p )/L)2, , TIE0 k, p 0, 1, 2 … , k 2 f/c, c 3 108

m/s. p 1 when p 0 and p 2 when p 0. Eq. (17) is the eigenvalue equation, which can also

be obtained by applying the average matching condi-

tion [4]. When L 19.5 mm, frequencies from 1 to 6 GHz and p 1, (TIIp E)2 are less than zero. Jm(x) and

Ym(x) in the equation should be replaced by Im(x) and

K0 T pIIER I1 T IIEp R0 I0 T IIEp R K1 T pIIER0K0 T IIEp R I0 T IIEp R0 I0 T IIEp R K0 T IIEp R0

sin c p d/L 2 J1 kR0

J0 kR0

d Y0 kR J1 kR0 J0 kR Y1 kR0

L Y0 kR J0 kR0 J0 kR Y0 kR0 . (18)

Eq. (18) is convergence at the resonant frequency f0 2.340 GHz when modes p 10 in region II using the

174 Shen and Budimir

Figure 2. Resonant frequency convergence of eq. (18).

parameters H 18 mm, R0 4.44 mm, L 19.5 mm, R 16 mm, as shown in Figure 2.

III. SIMULATION AND EXPERIMENT RESULTS

A. Resonators

The configuration of the novel combline resonators is the conventional combline resonator with N disks periodically mounted onto the inner conductor of the resonator. The specific design for the conventional combline resonator is readily available in the literature [5, 6]. The periodically loaded N disks symmetrically positioned onto its inner conductor have a radius of 5.12 mm and a height of 1.8 mm, as shown in Figure 1(b). Table I lists a comparison of the f0 values obtained via the methods of measurement, simulation, and calculation. A combline resonator with five periodically-loaded disks displays higher Q-factor than the conventional combline resonator (without N disks). Figure 3 shows a comparison of the measured S21 parameter of the proposed combline resonator (Qu 449) with that of the conventional combline resonator (Qu 359). The experiment also shows that external couplings have a sensitive influence for both the Q-factor and the free spurious range. The responses with varying external couplings are shown in Figure 4. Obviously, another mechanism of external couplings for the resonator structure is impedance matching. Usually, deep feeding and loading are necessary for filter applications. Figure 5 shows a comparison of the measured and simulated responses of the novel combline cavity resonator in a wide frequency range. The second resonant frequency is found to be 4.133 times the frequency of the centre of the first resonant frequency. As we can see, all the properties of the conventional combline resonator are

TABLE I. Comparison of Measured, Simulated, and Calculated Resonator Frequency f0

retained. To obtain a wide-frequency tuning range, we can adjust the following two factors: (i) the equivalent gap capacitor between the wall of the enclosure and standing inner conductor and (ii) the radius of disks. The equivalent capacitor of the resonator increases while the radius of the disks is increased. The influences of these two factors are shown in Figures 6 and 7.

B. Filters

Probe stimulation is a method to set the TEM mode for filters using combline cavity resonators. Usually the first section of the combline filter acts provides the

Figure 3. Comparison of the measured responses. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 4. S21 Response vs. distance d of the feeding and loading of the novel combline cavity resonator. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

impedance-matching function. For the specification of the centre frequency at 2.155 GHz and the bandwidth at 80 MHz, the arc-plate probe approach is found to match the structure well. Figure 8(a) shows the structure of the arc-plate-probe-stimulated two-pole filter. The H-field diagram in Figure 8(b) displays the efficiency of this approach. Figure 8(c) shows the corresponding stimulated and measured S21 responses of the two-pole novel combline cavity resonator filters loaded using the arc-plate approach. Figure 9(a) shows the structure of the bend-bar-probe-stimulated two-pole filter. Figure 9(b) displays the H-field distribution of the combline filter loaded using the bend-bar probe. Figure 9(c) shows the corresponding simulated S21 responses of the two-pole novel filter loaded using the bend-bar probe. Matching in a much narrower area than that of the arc-plate probe approach is achieved.

Figure 6. Resonator frequency vs. variant R (simulated). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Band-tuneable filters using combline cavity resonators are also investigated. Figure 10 shows the measured filters’ responses using slot coupling or space coupling between two resonators. It is easy to see that the larger the slot width, the wider the filter bandwidth, which is demonstrated in Figure 10(a). For a wider bandwidth, we take out the plate between the resonators and space coupling occurs.

For the filter specification of the centre frequency at 2.166 GHz and a 3-dB bandwidth of 120 MHz, the measured response of the two-pole Chebyshev bandpass filter using space coupling between resonators has the following characteristics: 3-dB bandwidth equals 129.30 MHz, 1-dB bandwidth equals 96.8 MHz, the centre frequency is at 2.167 GHz, the return loss is larger than 17 dB at the centre frequency, and the insertion loss is less than 0.7 dB at the centre frequency. For the filter specification of the centre frequency at 2.155 GHz and the bandwidth at 83 MHz, the measured response of a two-pole Chebyshev bandpass filter using slot coupling between res-

Figure 11. Photograph of the fabricated prototype combline filter with capacitor slot coupling.

onators has the following characteristics: 3-dB bandwidth equals 82.9 MHz, 1-dB bandwidth equals 58.19 MHz, the centre frequency is at 2.152 GHz, the return loss is larger than 22.5 dB at the centre frequency, and the insertion loss is less than 0.9 dB at the centre frequency. These responses are shown in Figure 10(b). Figure 11 shows a photograph of the fabricated prototype combline filter with capacitor slot coupling between the two resonators.

IV. CONCLUSION

By periodically mounting the disks onto the inner conductor of the combline resonator, a higher Q- factor can be achieved in the comparable volume of a conventional combline resonator. Filters using novel combline resonators have been presented in this article. By using arc-plate stimulating and loading in this filter structure, efficient optimization of the filter design was realized. Slot coupling and space coupling both have narrowband and wideband filter applica-

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Guohua Shen received B.Eng. and M.Eng. degrees from Shanghai University, Shanghai, China and his Ph.D. degree from the University of Westminster, London, all in electronic engineering. He was with the East China University of Science and Technology from 1991 to 2001, where he was responsible for microprocessor system design and CAD system design for telecommunication.

He has been with the Wireless Communication Group, University of Westminster since 2001. He is involved with the research and design of new RF and microwave components and EM simulations such as resonator structures and filter circuits.

Djuradj Budimir was born in Serbian Krajina (formerly Yugoslavia). He received Dipl. Ing. and MSc. degrees, both in electronic engineering, from the University of Belgrade, Belgrade, Serbia, and his Ph.D. degree in electronic and electrical engineering from the University of Leeds, Leeds, U.K., in the area of waveguide filters for microwave and millimetre-wave applica-

tions. In March l994, he joined the Department of Electronic and Electrical Engineering at Kings College London, University of London as a postdoctoral research fellow supported by two EPSRC contracts. Since January 1997, he has been with the Department of Electronic Systems, University of Westminster, London, U.K. His research interests include analysis and design of hybrid and monolithic microwave integrated circuits and subsystems such as filters, amplifiers, and linearizers for modern wireless-communication sys-

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tems, the design of EBG waveguide filters and multiplexing networks for microwave and millimetre-wave applications, the application of numerical methods to the electromagnetic-field analysis of 2D and 3D structures for RF, microwave, and millimetre-wave circuits, and RF and microwave design of wireless systems. He is the owner of DBFILTER software. He is a Senior Member of IEEE, a Member IEE, and a Chartered Engineer. He is also a regular referee for IEE Electronic Letters, IEE Proceedings Microwaves, Antennas, and Propagation, IEEE Microwave and Wireless Components Letters, IEEE Transactions on Microwave Theory and

Techniques, and IEEE Transactions on Circuits and Systems and the author of the books “Generalized Filter Design by Computer Optimization” (Artech House, 1998) and “Software and Users Manual EPFIL: Waveguide E-plane Filter Design” (Artech House, 2000). He has published papers in more than110 refereed journals, including IEEE Trans on Microwave Theory and Techniques, IEE Proc Microwaves, Antennas and Propagation, IEEE Microwave and Wireless Components Letters, IEE Electronics Letters and Microwave Technology Optical Letters, and conference proceeding papers in the field of RF, microwave, and millimetre-wave CAD.