диафрагмированные волноводные фильтры / 81f891dd-b969-4f91-9f4e-02a6642b4a71

Abstract—This paper presents design techniques for improving the spurious performance of an integrated ceramic waveguide filter. It is argued that these filters offer superior performance in terms of spurious free bandwidth when compared to an integrated ceramic waveguide filter. Results of simulations for an improved spurious ceramic TEM waveguide filter and an integrated stepped impedance resonator filter are presented in the current paper.

Keywords— ceramic, Waveguide Filter, TEM filter, Spurious performance, Stepped Impedance Resonator

I.INTRODUCTION

Microwave filters are an essential component of a cellular base station, used to cancel system noise and to provide isolation between transmit and receive channels .Cellular base stations widely use low cost, high Q coaxial TEM filters[1]. Despite having good electrical performance, they suffer from large physical volume which is needs to be reduced in future cellular systems. The dielectric loaded filters and waveguide filters offer high Q but have poor out of band rejection due to their crowded mode spectrum[2]. A high permittivity integrated ceramic waveguide filter offers 50% size reduction in comparison with conventional coaxial TEM filters[3]. The spurious performance of the integrated ceramic waveguide filter needs to be further enhanced to make it a suitable candidate for cellular base stations. Different methods have been presented in the literature for improving the stop band performance of dielectric filter. The TEM resonator can be added to one or both ends of the dielectric filter[4], alternatively a dielectric filter can be designed with different transversal dimensions whereby the fundamental frequency is same but the out of band frequencies are different[7,9].

In this paper, therefore, two different improved spurious integrated ceramic waveguide filters are designed using monolithic TEM resonators and integrated stepped impedance resonators (SIR).

II. CERAMIC TEM RESONATOR

An integrated ceramic TEM resonator (see Fig. 1) consists of a high permittivity rectangular ceramic block, with a metalcoated blind hole, placed at the centre of its broad wall.

Fig. 1 TEM Ceramic Resonator

The exterior surface of the resonator is coated with a metal ink. The resonant frequency and Q-factor of TEM ceramic resonator can be computed from [5]. The metal coated blind holes act as inner conductors of the TEM resonator. It offers better spurious performance and further size reduction compared to integrated ceramic waveguide resonator.

III.CHEBYSHEV INTEGRATED CERAMIC WAVEGUIDE FILTER

WITH CERAMIC TEM RESONATORS

A six pole waveguide provides inter-resonator couplings and the design technique described in [3] is used. The input /output coupling is achieved through the use of coaxial probes, where the radius and position of the probe inside the resonator determines the bandwidth, centre frequency and amount of coupling. The ceramic TEM resonators improve the overall spurious response of the filter up to 1.70* fo without affecting the selectivity of the filter. Fig.2 shows the final layout of a ceramic waveguide filter with first and last ceramic TEM resonators. The HFSS simulated results of the designed filter are compared with an integrated ceramic waveguide filter designed in [3], (see Fig.3). The results show that the ceramic TEM resonators offer a significant improvement in the spurious performance of a ceramic waveguide filter. The spurious rejection can be further enhanced by making the filter with all ceramic TEM resonators at the expense of increased pass band insertion loss.

Chebyshev ceramic waveguide filter is designed with two integrated ceramic TEM resonators having following specifications;

The ceramic waveguide resonators employed have the unloaded Q-factor of 2268, while ceramic TEM resonators have the unloaded Q-factor of 1826. The Q-factor of the first and last resonator in a filter has no effect on selectivity of the filter but, only on the overall pass band insertion loss [6]. Therefore, the low Q ceramic TEM resonators are used as first and last resonator of the filter.

Another approach to improve the spurious performance of a uniform impedance ceramic waveguide filter is to use the ceramic stepped impedance resonator as shown in Fig. 4. The Uniform Impedance resonator (UIR) controlled its resonant frequencies by changing the length of resonator, which is the only freedom of structure. Here the fundamental frequency Fo, can be controlled only by the length of resonator. In Stepped impedance resonator (SIR), more degree of freedom is available to control the fundamental frequency and higher order modes by changing the length and ratio between two impedance of resonator. In [7,8] Morelli et al introduces the use of uniform impedance resonator (UIR) and stepped impedance resonator (SIR) for a wide spurious performance of wave guide filters. He achieved the more than an octave wide spurious performance in waveguide filter by employing UIR and SIR resonators. As UIR have same impedance across the length of resonator, the SIR impedance ratio can be determined by the following equation [8].

Here Z1 and Z2 are the parameters used to control the fundamental and spurious frequencies of resonator. The stepped impedance can be achieved by introducing the steps in E-plane or H-plane of the waveguide resonator. The fundamental and harmonic frequencies of the stepped impedance resonator can be controlled by the physical length of sections and the ratio of two impedances. The E-plane stepped impedance resonator is realized to achieve the broader range of impedance ratios because the H-plane stepped impedance resonators have a limited ratio between the broader dimensions of resonator [7,8]. The ceramic stepped impedance resonator offers significant spurious free bandwidth compared to uniform impedance ceramic waveguide resonator at an expense of degraded q-factor.

IV. CHEBYSHEV INTEGRATED CERAMIC STEPPED

IMPEDANCE RESONATOR FILTER

A Six pole Ceramic waveguide filter with stepped impedance resonators was designed with following specifications;

The ceramic waveguide filter with stepped impedance resonators is designed to push the spurious beyond the cut-off frequency of TE30 mode.

The first and last resonators are selected to be Uniform Impedance resonators to facilitate the input and output couplings through coaxial probe. The Spurious free window of up to 2.02*fo is achieved with the significant reduction in the physical length of the filter in comparison with ceramic uniform impedance resonator filter. The simulated unloaded Q-factor of the ceramic stepped impedance resonator is 1342.

Both the merits of superior and better spurious performance and miniaturization of the resonators are achieved at an expense of lower unloaded Q-factor. Fig 5 shows the final layout of the integrated ceramic waveguide stepped impedance resonator filter, wherein first and last resonator are simple uniform impedance resonator having input and output coupling probe while rest of the resonators are stepped impedance resonators.

Fig. 2 Ceramic Waveguide Filter with TEM Resonator (i) Top View (ii) Side View

Fig. 3 Spurious Performance Comparison of Ceramic Waveguide Filter with Ceramic TEM Resonator Filter

Fig.6 shows the comparison of spurious performance of a ceramic uniform impedance waveguide resonator, a ceramic TEM resonator and a ceramic stepped impedance resonator filter. It is evident that the spurious performance of Stepped impedance resonators filter is far superior to the uniform impedance ceramic waveguide filter and ceramic waveguide filter with TEM resonators.

Table.1 shows the comparison of Q-factor and volume of ceramic uniform impedance waveguide resonator, ceramic TEM waveguide resonator and ceramic stepped impedance waveguide resonators at a fundamental frequency of 1842 MHz. The data clearly shows the reduction in sizes of ceramic TEM resonators and stepped impedance resonator but with lower unloaded Q-factor.

Fig. 4 Ceramic Stepped Impedance Resonator (i) Top View (ii) Side View

Fig. 5 Ceramic Waveguide Filter with Stepped Impedance Resonators (i) Top View (ii) Side View

Fig. 6 Spurious Performance Comparison of Ceramic Uniform Impedance Waveguide Filter, Ceramic TEM Resonators Filter and Ceramic Stepped Impedance Resonators Filter

TABLE 1. Q Factor, Volume and Spurious performance of Resonators

V.CONCLUSION

The design of monolithic integrated ceramic waveguide filter with better spurious performance has been presented in this paper. A six pole integrated ceramic waveguide filter with TEM resonators was designed which offered significant spurious improvement and miniaturization without having much effect on selectivity of dielectric waveguide filter. Another integrated ceramic stepped impedance resonator filter was designed which offered excellent spurious performance and further miniaturization of the filter at an expense of lower Q-factor. Simulated results were also in consonance with the theoretical concepts. Future publication will include experimental results and address other design aspects including tuning, interfaces, temperature performance, manufacturability tolerances, power handling etc.

ACKNOWLEDGMENT

The authors would like to thank Royal Academy of Engineering and Radio Design Ltd for sponsorship of Prof. Ian Hunter and Sukkur Institute of Business Administration for sponsoring Sharjeel Afridi and Muhammad Sandhu.

REFERENCES

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[2]A. R. Weily and A. S. Mohan, "Microwave filters with improved spurious performance based on sandwiched conductor dielectric resonators," in Microwave Conference, 2000 Asia-Pacific, 2000, pp. 520-524.

[3]I. C. Hunter and M. Y. Sandhu, "Monolithic integrated ceramic waveguide filters," in Microwave Symposium (IMS), 2014 IEEE MTT-S International, 2014, pp. 1-3.

[4]H. Hee-Yong, p. Nam-Sin, C. Yong-Ho, Y. Sang-Won, and C. IkSoo, "The design of band-pass filters made of both dielectric and coaxial resonators," in Microwave Symposium Digest, 1997., IEEE MTT-S International, 1997, pp. 805-808 vol.2.

[5]G. L. Matthaei, Microwave filters, impedance-matching networks, and coupling structures: McGraw-Hill, 1964.

[6]P.M Iglesias, I.Hunter "Non-Uniform Q Factor Distribution in Microwave Filters",Microwave Conference (EuMC), 2012, pp. 1182-1185.

[7]M. Morelli, I. Hunter, R. Parry, and V. Postoyalko, "Stop-band improvement of rectangular waveguide filters using different width resonators: selection of resonator widths," in Microwave Symposium Digest, 2001 IEEE MTT-S International, 2001, pp. 1623-1626 vol.3.

[8]M. Morelli, I. Hunter, R. Parry, and V. Postoyalko, "Stop-band performance improvement of rectangular waveguide filters using