диафрагмированные волноводные фильтры / d0d73355-4342-4a1d-8ed6-a177537691cc

Abstract - This paper presents an ultra compact waveguide bandpass filter that exhibits a pseudo-elliptic response. The transmission zero created in the upper stopband to form a rapid roll off is produced through a bypass coupling with higher order modes. A 3rd order filter is designed at the centre frequency of 9.4 GHz with a 5.3% fractional bandwidth. The proposed structure's size is 38% smaller than one of a 3rd order E-plane extracted pole filter with comparable response. Additionally, this configuration allows larger span of different bandwidths. The filter has been fabricated and tested using E-plane waveguide technology, which has benefits of being inexpensive and having mass producible capabilities. Measurements of such a fabricated filter validate the simulated results.

Index Terms - Inline filters, E-plane filters, extracted pole filters, waveguide filters.

I. INTRODUCTION

Waveguide filters, widely used in fixed wireless communication, as well as for radar and satellite applications, offer a unique set of benefits in terms of low-loss and high power handling capability. However, continuous advancements in such communication systems have placed stringent demands for compactness of these structures. The most famous approach to achieving size reduction of these structures came with the invention of dual mode waveguide filters [1], which reduces the number of resonators required for realization of a filter by half. Recent examples of such filters have been presented in [2]-[3]. Disadvantages of currently available dual mode filters are that they can be complex as well as time consuming and costly to produce.

One method for realising waveguide filter resonators is to use a planar circuit mounted E-plane strip which was first introduced by Konishi and Uenakada [4]. It has ever since become popular for filter applications due to its low-cost and mass producible characteristics. However, conventional filters formed out of such planar mounted half wavelength resonators again pose a disadvantage in terms of size. Another approach that leads to size reduction is the miniaturization of resonators. Such attempts at achieving compactness for this type of structures were made using cross-coupled E-plane resonators [5], S-shape resonators [6], and extracted pole sections (EPS) [7]. However, the widths of septa required for realizing low coupling coefficients between adjacent resonators, is another factor that leads to increase in size. This paper therefore addresses this issue and proposes an ultra compact

pseudo-elliptic waveguide filter for applications where space is at a premium.

II. PROPOSED RESONATORS

The behaviour of the proposed resonators can be explained by the model of a singlet. A singlet is a structure which generates a pole and a transmission zero due to a bypass coupling between the source and the load [8]. The proposed realisation of a singlet within a waveguide structure using E-plane inserts is shown in Fig. la. The inserts consist of a single metallic septum and a single fin with one end open and the other connected to a wider sidewall of the waveguide. The septum and the fin are longitudinally centred one to another and placed with equal offsets from the central E-plane. The coupling scheme of a singlet is shown in Fig. 1b, where the synthesis of such a module can be carried out using the technique shown in [9].

Resonator

Fig. 1. E-plane singlet: (a) arrangement of E-plane inserts within a waveguide housing; and (b) coupling schematic of a singlet taking into account spurious resonance.

A coupling matrix of the circuit in Fig.l is given by

and can be determined either analytically or through optimisation for the given specification. The spurious resonance above the passband is represented by the third node of the coupling schematic with the normalized frequency shift of -b2 = 135.078. S-parameter responses of the coupling scheme can be computed by applying equations

The corresponding coupling matrix is given by

Additional parasitic couplings within the structure (Cl3, C24, and C36), though comparatively low, have also been accounted for in the coupling matrix.

transmission zero: lOA GHz) with source-load coupling has been designed in CST Microwave Studio™ and fabricated using the E-plane technology, which utilizes a pair of copper inserts within a standard WG-16 (22.86xlO.16 rum2) rectangular waveguide housing. The inserts (Fig. 3) with the dimensions given in Table I have been plotted on a copper foil with 0.2 rum thickness. S-parameters have been measured using the Agilent E8361A vector network analyzer and shown in Fig. 5 together with the simulated results.

0

Frequency (GHz)

Fig. 5. Comparison between coupling schematic, EM simulation and measurement S-parameter responses of the proposed filter.

The measured results show good agreement with that of the simulated. The insertion loss for the filter of around 1.0 dB that can be observed is mainly due to signal leakage through the imperfect waveguide housing. A slight shift in the transmission zeros can also be observed. Further contributions to these are imperfections in alignment of the two inserts within the waveguide housing and tolerances encountered during fabrication process such as the limitation to accuracy which the plotter reaches. Results can be further improved through meticulous use of the available tools. A photograph of the fabricated filter prototype is shown in Fig. 6.

Fig. 6. Experimental 3rd order filter.

V. CONCLUSION

An ultra compact E-plane waveguide filter using parallel coupled quarter wavelength resonators has been proposed. The structure is 70% more compact in comparison to a conventional E-plane filter with comparable response and 38% smaller than a 3rd order extracted pole filter. Additionally, the filter has improved upper stop band selectivity due to a transmission zero generated through source-load coupling. S-parameter response of the fabricated prototype shows reasonably good agreement with that of the simulated, even considering low accuracies of the fabrication device used.

ACKNOWLEDGEMENT

This work was supported by the EUErasmus Mundus Action 2 project EURWEB.

REFERENCES

[I]A.E. Atia, A.E. Williams, "Narrow-Bandpass Waveguide Filters," Microwave Theory and Techniques. IEEE Transactions on , vo1.20, no.4, pp.258,265, Apr 1972

[2]c. Tomassoni, S. Bastioli, R. Sorrentino, "Generalized TM Dual-Mode Cavity Filters," Microwave Theory and Techniques. IEEE Transactions on, vo1.59, no.12, pp.3338,3346, Dec. 2011.

[3]S. Bastioli, C. Tomassoni, R. Sorrentino, "A New Class of

Waveguide Dual-Mode Filters Using TM and Nonresonating Modes," Microwave Theory and Techniques, IEEE Transactions on, vo1.58, no.12, pp.3909,3917, Dec. 2010.

[4]Y. Konishi, and K. Uenakada, "The Design of a Bandpass Filter

With Inductive Strip Planar Circuit Mounted in Waveguide,"

IEEE Trans. Microwave Theory Tech., vol. MTT-22, pp. 869873,Oct.l974.

[5]E. Otli, R. Vahldieck, S. Amari, "Novel E-plane filters and diplexers with elliptic response for millimeter-wave applications," Microwave Theory and Techniques. iEEE Transactions on, vo1.53, no.3, pp.843,851, March 2005

[6]N. Suntheralingham and D. Budimir, "Enhanced Waveguide

Bandpass Filters Using S-shaped Resonators," Int. J RF and Microwave CAE, vol. 19, no. 6, pp. 627-633, 2009.

[7]O. Glubokov, D. Budimir, "Extraction of Generalized Coupling Coefficients for Inline Extracted Pole Filters With Nonresonating Nodes," Microwave Theory and Techniques. iEEE Transactions on, vo1.59, no.12, pp.3023-3029, Dec. 2011.