диафрагмированные волноводные фильтры / 1c252710-3074-4139-ac8a-fde45e5fe993

E-PLANE DOUBLE RIDGE WAVEGUIDE FILTERS AND DIPLEXERS FOR

COMMUNICATION SYSTEMS

G. Goussetis and D. Budimir

Wireless Communications Research Group, Department of Electronic Systems, Westminster University, London W1W 6UW, UK, Tel: +44 20 79115139; Fax: +44 20 75804319 Email: gousseg@wmin.ac.uk, budimid@wmin.ac.uk

ABSTRACT

Modeling, design and experimental verification of E-plane ridge waveguide filters are presented. The proposed filters have been integrated in a fully double ridge waveguide diplexer including a double ridge waveguide T- junction. Mode matching method based tools have been developed for the electromagnetic simulation of ridge waveguide filters and T-junctions. Design procedures for the filters and diplexers are also presented. Experimental testing for the ridge waveguide filter and comparison with commercial finite element method simulator for a diplexer structure confirms the accuracy of the developed tools.

INTRODUCTION

Point-to-point and point-to-multipoint digital communication systems is a rapidly expanding market. Such systems commonly employ filters and diplexers in microwave and mm-wave transceivers as channel separators. Hence there is an increasing demand for low cost, low dissipation loss and compact filters and diplexers.

Waveguide E-plane technology is an attracting solution as it combines the above properties with high power capabilities, while at higher frequencies has reduced physical size. Ridge waveguide in particular, Fig. 1(a), is well known to further combine the advantages of lower cutfoff frequency of the dominant mode, wider bandwidth free from higher modes and low characteristic impedance. Previous work [Goussetis and Budimir (1)] has demonstrated that when introduced in the resonators of conventional E-plane filters, Fig. 1(b), ridge waveguide can improve the selectivity. Furthermore, based on the improved characteristics of ridge waveguide, [Linag et al (2)] proposed ridge waveguide T-junction, Fig. 2(a), for wide band applications (Fig. 1b).

This paper therefore presents E-plane double ridge waveguide filters and proposes a fully ridge-waveguide diplexer that incorporates ridge waveguide T-junction. The designed diplexer offers broadband operation and improved selectivity without introducing any further complexity than that involved in the fabrication of standard E-plane diplexers [Arndt et al (3)]. Experimental results for ridge waveguide filters and analysis and design of the proposed diplexer are presented.

METHOD

The generalized transverse resonance field matching technique can be applied to rigorously solve the field propagation in a double ridge waveguide [Bornemann (4)]. The knowledge of the EM field expressed as superimposition of orthogonal modal field distribution on both sides of a surface discontinuity allows the mode matching technique to be applied in order to obtain the generalised scattering matrix (GSM). Cascaded surface discontinuities can then be combined with the finite lengths of waveguide between them. Hence a fast and accurate simulator of the proposed filter structure is obtained.

A filter design procedure for this type of filters is based on half wavelength resonators and K-inverter couplings prototype [Budimir (5)]. Based on mode matching simulation, the S-matrix for each metal post is easily determined. The scattering matrix of this reciprocal 2-port can be converted into its equivalent impedance inverter value, as in [Levy (6)]. An iterative procedure will therefore determine the length of each metal septum, whose equivalent K- inverter value is equal to the prototype’s. The resonator lengths are then determined accordingly [Levy (6)]. A final optimization procedure is then applied.

The generalized scattering matrix of a double ridge waveguide T-junction, as shown in Fig. 2(a), can be obtained upon application of the three-plane mode matching technique (3PMM) [Linag et al (2)]. The perpendicular arm of the T-junction is shorted at three different planes and the remaining 2-port structure, a cascade of 5 ridge waveguide sections, is simulated using mode-matching method. The 3-port scattering matrix can then be determined following mathematical manipulation of the three 2-port scattering matrices.

The S-parameters of the diplexer are obtained by combining the T-junction S-matrices with the filters’ S-matrices. The configuration of Fig. 2(b) is used, where the common port of the diplexer is on the perpendicular arm of the T- junction. The diplexer design aims to generate an open circuit at channel 2 port at the passband of channel 1 and vice versa. The two filters are designed separately at the required bands and the distances between the T-junction and the filters are optimized in order to achieve acceptable reflections at all ports.

NUMERICAL AND EXPERIMENTAL RESULTS

A. Filters

In order to demonstrate the accuracy of the developed simulator, a five-resonator E-plane ridge waveguide filter in WG16 housing was designed, fabricated and tested. Mode matching method with 20 TE and 10 TM modes was used. The filter was fabricated using brass waveguide housing and a copper metal insert which was realized using spark erosion. Its photograph is shown on Fig. 3. Filter’s dimensions together with the measured and simulated response are shown in Fig. 4. Very good agreement between theory and experiment was observed.

B. Diplexer

To illustrate an example of using E-plane waveguide filters with improved stopband performance and E-plane ridge waveguide T-junction in the diplexer configuration, a double ridge waveguide diplexer was designed. The insertion loss of a double ridge waveguide diplexer with center frequencies at 38.05 and 38.75 GHz and channels’ bandwidth 250 MHz is shown in Fig. 5.

To verify the accuracy of the developed method, a double ridge waveguide diplexer with one resonator at each channel was designed. Fig. 6 shows the simulated insertion loss by MM/3PMM and commercial finite element method (FEM) simulator. The two methods are in good agreement.

CONCLUSION

E-plane double ridge waveguide filters diplexers have been presented. Mode matching method and three plane mode matching technique were used to model the filters and the T-junction respectively. CAD tools for this type of filters and diplexers have been developed. A five-resonator X-band double ridge waveguide bandpass filter has been designed, fabricated and tested. Measured response shows good agreement with theory. Validation of using the E- plane ridge waveguide T-junction and filters in the diplexer configurations is illustrated by design example at 38GHz. Commercial finite element method simulator has been used to validate the accuracy of the developed diplexer simulator.

ACKNOWLEDGEMENT

The authors wish to acknowledge the financial support of the Engineering and Physical Sciences Research Council (GR/K58634), UK

REFERENCES

1.G. Goussetis and D. Budimir, "Ridge waveguide filters with improved stopband performance", 2000 European Microwave Conference Dig., vol. 2, pp. 310-313, 2000

2.X.-P. Linag, K. A. Zaki and A. E. Atia, “A rigorous three plane mode-matching technique for characterizing waveguide T-junctions, and its application in multiplexer design”, IEEE Trans. Microwave Theory and Tech., vol. MTT-39, no. 12, pp. 2138-2147, December 1991

3.F Arndt, J. Bornemann, D. Grauerholtz, D. Fasold and N. Schroeder, “Waveguide E-plane Integrated-Circuit Diplexer”, Electronic Letters, Vol. 21, pp. 615-617, July 4, 1985

4.J. Bornemann, “Comparison between different formulations of the Transverse Resonance Field-Matching Technique for the three-dimensional analysis of metal-finned waveguide resonators”, International Journal of Numerical Networks, Devices and Fields, vol. 4, pp. 63-73, 1991

5.J.D. Rhodes, “Microwave circuit realizations”, in Microwave Solid State Devices and Applications, D.V. Morgan and M.J. Howes, Eds. England: Peregrinus, 1980, pp.49-57

6.R. Levy, “A generalized design technique for practical distributed reciprocal ladder networks”, IEEE Trans. Microwave Theory and Tech., vol. MTT-21, no. 8, pp. 519-526, August 1973

Fig. 5: Simulated response of a ridge waveguide diplexer at

Fig. 6: Comparison between MM/3PMM (a) and FEM (b) for an X-band diplexer structure (1-resonator at each channel)