диафрагмированные волноводные фильтры / f34742a3-7dd9-4872-ad17-89c7cb24b29b

Abstract—This letter presents an ultra compact extracted pole -plane filter. The proposed structure can achieve up to 65% size reduction in comparison with a standard extracted pole filter designed at 9.5 GHz centre frequency with a 3% fractional bandwidth. The filter has been fabricated and tested using -plane waveguide technology. Measurements on a fabricated filter confirm the accuracy of the design method.

Index Terms—-plane filters, extracted pole filters, inline filters, waveguide filters.

WAVEGUIDE filters offer a unique set of benefits for modern microwave and millimetre-wave applications in terms of low-loss and high power handling capability. How-

ever, current advancements in mobile communication systems have placed stringent demands for compactness of these structures.

-plane technology, well known for its low-cost and mass producible characteristics, was first introduced by Konishi and Uenakada [1] and has since become one of the most popular technologies for waveguide filters implementation. However, one of the disadvantages of -plane filters is their considerable size. One of the approaches that lead to size reduction is the miniaturization of resonators. Such attempts at achieving compactness for this type of structures were made using SRRs [2], S-shape resonators [3], and extracted pole sections (EPS)

[4].Another contribution to increase in size is partly due to the widths of septa required for realizing low coupling coefficients between adjacent resonators. The most recent development was the introduction of an EPS which has the capability of generating two poles and a transmission zero using a single section

[5].This greatly aided in improving lower stop band selectivity, however not much improvement towards compactness has been made. This letter therefore addresses this issue and proposes an ultra compact -plane waveguide EPS filter for applications where space is at a premium.

II. EXTRACTED POLE SECTIONS

Let us consider an -plane insert creating an EPS within a rectangular waveguide, shown in Fig. 1(a). The insert consists of

Manuscript received May 09, 2013; accepted May 22, 2013. Date of publication July 15, 2013; date of current version August 05, 2013.

D. Budimir is with the Wireless Communication Research Group, Faculty of Science and Technology, University of Westminster, London, W1W 6UW, U.K. (e-mail: d.budimir@wmin.ac.uk).

N. Mohottige and O. Glubokov are with the Wireless Communication Research Group, Electronics and Computer Science, University of Westminster, London, W1W 6UW, U.K.

Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LMWC.2013.2270431

Fig. 1. Extracted pole sections: (a) -plane waveguide insert for standard EPS; (b) -plane waveguide insert for ultra compact EPS; (c) Equivalent circuit model for ultra compact EPS; (d) Arrangement of -plane insert within waveguide housing.

input and output metallic septa, two uniform waveguide sections and a metallic rectangular fin with one end open and the other connected to the top ground plane. This structure is capable of producing a single pole and a single transmission zero at the upper stop band [4].

Extending length of EPS, the structure has shown to produce two poles and a single transmission zero at the lower stop band [5], however it sacrifices size as well as upper stop band performance in order to achieve this trait.

This drawback can be overcome by the introduction of two extra fins as depicted in Fig. 1(b), which improves performance as well as achieve significant size reduction. Fig. 1(c) shows a schematic model of the proposed ultra compact filter, with the input/output septa represented by inverters. -blocks represent fins as series LC resonators which are spaced through sections of transmission lines with phase length and characteristic impedance . The -matrix for a single extracted pole section (Fig. 1(a)) without septa can be computed as a system of cascaded 2-port networks

(1)

The initial over-all -matrix of the entire ultra compact filter can be approximately obtained (without taking into consideration of the additional couplings, represented by dashed lines) as

input and output of the circuit. Therefore, the resonant condition

Fig. 2. Solutions to poles and zeros of the ultra compact EPS.

Fig. 3. Comparison of -parameter responses of the standard -plane two-pole EPS, the proposed ultra compact EPS and its equivalent circuit.

TABLE I

DIMENSIONS (IN MM) OF THE INSERT FOR FILTER (SEE FIG. 3)

Fig. 4. Simulated and measured -parameters of the proposed filter.

Fig. 5. Photograph of fabricated filter.

capacitive coupling between them (presented in Fig. 1(c) by capacitance and ) and mutual inductances result in the splitting of the upper stop band transmission zeros ( and ). The equivalent circuit has been simulated using Agilent ADS software tool. The response of the equivalent circuit is also presented in Fig. 3 by taking into account these additional couplings. It is also visible from the plot that the upper stop-band performance has improved for the new structure at the cost of lower stop-band performance. This decrease is due to the shift in towards higher frequencies after the introduction of the new fins.

The length of the fin required for realising the transmission zeros can be found as and the width of the input/output septa as well as the fins were obtained during optimization process to satisfy filter requirements. Final dimensions of the two -plane inserts are summarized in Table I. It is clear that the proposed structure is 65% smaller in comparison with

the standard -plane two pole EPS [5]. Higher order filters based on the proposed structure can be realised through having cascaded sections, or by having extra adjacent fins spaced by transmission line sections between the two septa. In which case a fourth order filter based on the proposed structure is 35 mm, which is 48.5% compact than filters using resonant posts presented in [6].

IV. EXPERIMENTAL VERIFICATION

An ultra compact EPS filter (with centre frequency: 9.45 GHz; passband: 9.35–9.65 GHz; return loss: 20 dB; transmission zeros: 8.8 and 10.2 GHz) has been designed in CST Microwave Studio and fabricated using the -plane technology which utilizes a single copper insert within the conventional rectangular waveguide. The insert (Fig. 3) with the dimensions given in Table I has been plotted on a copper foil with the thickness 0.1 mm. Then it has been placed within the standard waveguide housing WG-16 (22.86 10.16 ).

-parameters have been measured using the Agilent E8361A vector network analyzer. The simulated -parameter response of the EPS is shown in Fig. 4 together with the experimental data. Insertion loss of the filter was around 1.4 dB. The measured results show good agreement with the simulated. A slight shift in the transmission zeros can also be observed. This is mainly due to the tolerances encountered during fabrication process and the limitation to which the plotter reach. Results can be further improved through meticulous use of the available tools. A photo of the fabricated filter is shown in Fig. 5.

V. CONCLUSION

An ultra compact EPS filter has been proposed in this letter. The proposed filter is 65% smaller in comparison with the standard two pole EPS and produces three transmission zeros which can be realized both at the upper and the lower stop-bands. The new structure has been designed, fabricated and tested. Considering the inaccuracies involved in practical realization of the proposed filter, the agreement between simulation and measurement is very good. In addition, a higher order filter based on these structures retains the merit of compact size.

REFERENCES

[1]Y. Konishi and K. Uenakada, “The design of a bandpass filter with inductive strip planar circuit mounted in waveguide,” IEEE Trans. Microw. Theory Tech., vol. MTT-22, pp. 869–873, Oct. 1974.

[2]A. Shelkovnikov and D. Budimir, “Left-handed rectangular waveguide bandstop filters,” Microw. Opt. Technol. Lett., vol. 48, no. 5, pp. 846–848, 2006.

[3]N. Suntheralingham and D. Budimir, “Enhanced waveguide bandpass filters using S-shaped resonators,” Int. J. RF Microw. CAE, vol. 19, no. 6, pp. 627–633, 2009.

[4]O. Glubokov and D. Budimir, “Extraction of generalized coupling coefficients for inline extracted pole filters with nonresonating nodes,”

IEEE Trans. Microw. Theory Tech., vol. 59, no. 12, pp. 3023–3029, Dec. 2011.

[5]O. Glubokov and D. Budimir, “Novel inline waveguide -plane filters using dual-mode extracted pole section,” in Proc. 41st Eur. Microw. Conf. (EuMC), Oct. 10–13, 2011, pp. 99–102.

[6]C. Tomassoni and R. Sorrentino, “A new class of pseudo-elliptic waveguide filters using resonant posts,” in IEEE MTT-S Int. Dig., Jun. 17–22, 2012, p. 1,3.