Compact Microstrip Dual-Band Quadrature Hybrid Coupler for Mobile Bands

Transcription

1 Compact Microstrip Dual-Band Quadrature Hybrid Coupler for Mobile Bands Vamsi Krishna Velidi, Mrinal Kanti Mandal, Subrata Sanyal, and Amitabha Bhattacharya Department of Electronics and Electrical Communications Engineering, Indian Institute of Technology Kharagpur, Kharagpur 730, India {vvamsi.iitkgp, {ssanyal, Abstract This paper presents a new simple method for miniaturization of the dual-band quadrature hybrid coupler that can operate at two arbitrary frequencies for mobile applications. The proposed miniaturization is achieved by using equivalent high impedance parallel lines in place of low impedance lines. The performance of the proposed coupler is compared with a conventional dual-band design. Full-wave electromagnetic simulation is used to confirm the design approach for the hybrids operating in mobile bands. The frequency response of the proposed hybrid is similar to other conventional dual band designs. Q I. INTRODUCTION UADRATURE-HYBRIDS are 3 db directional couplers with a 90 0 phase difference in the outputs of the through and coupled arms [] and are often made in microstrip form. In general the quadrature hybrid is one of the most popular passive circuits used for microwave and millimeter-wave applications []. It is an important circuit element in microwave integrated circuits and can be used as a power divider/combiner or a part of mixer. The significant properties of branch line couplers [7] are:. The coupling between the two lines is through joining branch lines of finite lengths. This gives the flexibility in designing.. The couplers are capable of handling high RF powers. 3. They are better suited for tight coupling (coupling coefficient less than 0 db) than for weak couplings. A prototype quadrature coupler is composed of four quarter-wave length transmission line segments forming a loop structure, and hence occupies a large area on printed circuit boards []. Several methods have been reported in the literature concerning miniaturization, bandwidth enhancement for the quadrature, but only a few exist about the dual-band design [-7] fabricated on different substrates. Especially for mobile systems with cellular and personal communications system (PCS) bands, the couplers that are able to be functional at multiple frequencies are of great interest [3]. The topology of a three-branch-line coupler (3-BL) for dual band (.4 and 5 GHz) is presented in [3], but the design occupies very large area for the dual band (0.9 and.0 GH) mobile applications. Branch-line coupler design based on the use of lumped distributed elements and other techniques were proposed in the literature [-6] for dual-band applications. However, some are exhibiting the drawbacks like limited operating bandwidth (less than 0 MHz), sub-optimum return/insertion loss performance, occupies a much larger substrate area than the conventional design. In this paper, the concept of replacing low impedance lines by their equivalent parallel lines is applied to achieve miniaturization. For performance comparison, a conventional dual band coupler design [] is also simulated. The present method can be applied for all substrates and it is more effective for the thick substrates with low dielectric constant values where the required line widths are too high. II. CONVENTIONAL DESIGN OF DUAL-BAND QUADRATURE HYBRID A. Basic structure of the novel design The basic structure of the conventional dual-band quadrature hybrid coupler proposed by Cheng and Wong [] is shown in fig., Fig.. A novel dual-band quadrature hybrid coupler [].

2 B. Coupler Analysis The basic structure shows that a transmission line with an electrical length of θ and characteristic impedance A connected to the pair of shunt elements (jy) behave like a quarter wavelength line of characteristic impedance T at two frequencies f and f if the following condition is satisfied, A T nδπ sin for n,3,5,., and A T nδπ sin (a) (b) (a) (b) for n, 4,6,, here f f δ (3) f + f and θ is the electrical length corresponding to the mean frequency of f and f. The impedance values of the branches,, 3 are III. PROPOSED QUADRATURE HYBRID USING HIGH IMPEDANCE LINES Since the standard conventional dual band designs [-3] have low impedance, quarter wave length lines, they occupies large area on the microstrip at the mid band frequency. On the other hand the dual band design employs matching stubs hence the area required on the microstrip is large. Also the prototype design consists of low impedance lines in the series arms and requires large microstrip width on the substrate. However, the low impedance lines can be represented by the equivalent parallel combination of two or more high impedance lines. The proposed design utilizes this idea for the miniaturization of the coupler. A. Coupler Design The transmission line segment with characteristic impedance and electrical length θ can be effectively replaced by two parallel line segments of higher characteristic impedances and of the same electrical length θ. Fig..shows the representation. The impedance values can be calculated from the simple relation (7) + Fig.. Representation of parallel lines with proper spacing. In general, (7) will be satisfied if, > and hence it is an advantage in terms of microstrip width, as high impedance lines will have smaller width than that of the low impedance lines. This facilitates meandering of lines by providing sufficient spacing between the lines to avoid the coupling effects. The proposed design considers two equal impedance parallel lines to represent the equivalent low impedance series arms of the coupler. Fig.3. shows the configuration of the proposed coupler with high impedance lines in the series arms. 0 δπ cos (4) 0 δπ cos (5) δπ δπ sin tan (6) Fig.3. Final structure of the proposed coupler using high impedance lines.

3 Fig.4. Full-wave EM Simulation: Return loss S (db). Fig.5. Full-wave EM Simulation: Insertion loss S (db). Fig.6. Full-wave EM Simulation: Insertion loss S 3 (db). As the impedance of each line in the series arms is, the line width is very small compared to the conventional design []. Now, the lines can be meandered conveniently to achieve the compact structure. The performance of this structure is good only when there is no mutual coupling between the lines. Hence, meandering must be done by providing proper separation between the lines. The other two arms and stubs are kept unchanged with their characteristic impedances, 3 respectively. IV. SIMULATED RESULTS AND FABRICATION A. Simulation A dual-band microstrip quadrature hybrid coupler operating at 900/000 MHz is deigned and simulated. The calculated fractional bandwidth from (3) is δ 0.38 and the midfrequency is.45 GHz. From (4)-(6), the impedance values of the design are 4.7 Ω, 60.4 Ω, and Ω. Fig.7. Full-wave EM Simulation: Isolation loss S 4 (db). For the proposed coupler, the series arms are parallel lines and each line has the impedance of 85.4 Ω. All electrical lengths are of quarter wavelength. The microstrip widths corresponding to the impedances are shown in Table.. The dual band coupler with mid-frequency.45 GHz can be fabricated on a.58 mm thick FR4 substrate with dielectric constant ε R 4.3 and loss tangent of 0.0. Coupler physical dimensions are obtained by using the full-wave simulator IE3D R. Fig 4-7 gives the simulated magnitude responses of the both the conventional [] and the proposed dual band coupler, in which the center frequencies of the two operating bands are found to be approximately same. Return loss is found to be better than 0 db at the center frequencies of both operating bands. The insertion loss for the both the ports are approximately same as the novel design. However, the isolation is better than 30dB at the first operating band and it is 6. db at the next operating band.

4 TABLE I IMPEDANCE NITS AND WIDTHS COMPARISON OF PROPOSED AND CONVENTIONAL DESIGN TABLE I SIMULATION RESULTS OF THE PROPOSED DUAL BAND HYBRID Conventional design Proposed design Series branch (Ω) W(mm) Shunt branch (Ω) W(mm) Open stub (Ω) W(mm) Band f Band f Return loss -0 db db Isolation factor S db -6. db Phase difference (S 3 - S ) The simulated responses of the proposed coupler are found at the two operating bands 940 and 04 MHz. The simulated responses show that there is a little degradation of the insertion loss and the isolation particularly at the second operating band; however the bandwidths remain the same approximately. B. Fabrication The microstrip geometry of the proposed dual band quadrature hybrid coupler is shown in fig.0. This is found to be miniaturized coupler. Fig.8. Full-wave EM Simulation: phase response of the conventional design. Fig.9. Full-wave EM Simulation: phase response of the proposed coupler. Fig 8-9 gives the simulated phase responses of the both the novel design [] and the proposed dual band coupler. The phase difference between S 3 and S for the proposed design is approximately same as the conventional design (less than 0 variations) at both the frequency bands. The simulated results are summarized in Table II for both the magnitude and phases at the two operating bands. Fig.0. Microstrip layout of the proposed coupler. The proposed coupler utilizes meandering of the lines, which is an advantage for miniaturization. As the series branches are replaced by their equivalent high impedance parallel lines, they occupy less width and hence meandered. The widths of the parallel lines are only. mm through out. The separation between the lines, which are meandered, is kept.3 mm for which the mutual coupling can be neglected. The open stubs are connected conveniently with proper bends.

5 With the geometry shown in Fig.0, the proposed coupler occupies a rectangle area, which is smaller than that of the area occupied by the conventional design. The comparison between the areas occupied by both the couplers is shown in Table III. TABLE I RECTANGULAR AREA COMPARISON OF THE PROPOSED AND CONVENTIONAL DESIGN Rectangular area (mm ) Conventional Design (with bended stubs) Proposed % Size reduction % The size of the proposed coupler is between approximately 48 % of the novel design. This level of miniaturization is expected to be achievable for other frequencies and substrates. V. CONCLUSION In this paper, a new and very simple method for compact dual-band quadrature hybrid design is proposed without using any additional elements like stubs, lumped elements etc. or any other techniques except meandering available in literature. The miniaturization is achieved by using high impedance lines properly meandered with required separation to neglect the mutual coupling. The design method uses very simple formula and considers technology constraints and the area occupied by the circuit. The method is suitable for the couplers operating at both the single band and dual band frequencies. For performance comparison, a conventional dual band coupler [] is also simulated. The size of the proposed coupler compared to conventional design is between 48 %. The proposed coupler has frequency responses (and hence bandwidths) similar to other dual band designs. REFERENCES [] D.M. Pozar, Microwave Engineering, nd ed. New York: Wiley, 998. [] K.-K. M. Cheng and F.-L. Wong, A novel approach to the design and implementation of dual-band compact planar 90 branch-line coupler, IEEE Trans. Microw. Theory Tech., vol. 5, no., pp , Nov.004. [3] C. Collado, A. Grau and F.D. Flaviis, Dual band planar quadrature hybrid with enhanced bandwidth response, IEEE Trans. Microw. Theory Tech., vol. 54, no., pp , Jan [4] Santanu Dwari and S.Sanyal, Size reduction and harmonic suppression of microstrip branch-line- coupler using defected ground structure, Microwave and Optical technology Letters, vol.48, no. 0, pp , Oct [5] T. Hirota, A. Minakawa, and M. Muraguchi, Reduced-size branch-line and rat-race hybrids for uniplanar MMICs, IEEE Trans. Microwave Theory Tech., vol. 38, pp , Mar [6] K.-K. M. Cheng and F.-L. Wong, Dual band rat race coupler design using tri-section branch-line, Electronic Letters, vol. 43, no. 6, March. 007 [7] K.C.Gupta, A. Singh, Microwave Integrated Circuits, st ed. Wiley, 974.