J. of Electromagn. Waves and Appl., Vol. 7, No. 5, 77 78, 3 A SMALL SIZE 3 DB /8 MICROSTRIP RING COUPLERS A. Mohra Microstrip Department Electronics Research Institute Cairo, Egypt A. F. Sheta Electronic Engineering Department College of Engineering, King Saude University P.O. Box 8, Riyadh, Saudi Arabia S. F. Mahmoud Electronic Engineering Department Kuwait University P.O. Box 5969, Safat 36, Kuwait Abstract New small size 3 db microstrip ring couplers suitable for microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs) are presented. Area saving more than 85% of that of the conventional ring coupler can be achieved using the proposed configurations. Small size couplers based on the proposed configurations and designed at 9 MHz have been implemented on RT/duroid 588 dielectric material. The theoretical and experimental results show favorable comparison. Introduction Ring Coupler Using T-Shape Section Equivalent to Quarter Wave Line 3 Ultra-Miniaturized Hybrid Ring Coupler Configurations 3. Using Circuit Equivalent
78 Mohra, Sheta, and Mahmoud 3. Coupled Line Short Circuited at Its Diagonal 3.3 Implementation of the Three Quarter Line in Serpentine Form Design Cases and Experimental Results. Coupler Design Based on T-Shape Equivalent to λ/ Line (Fig. ). Coupler Design Based on T-Shape Equivalent to λ/ Line and the Implementation of 3λ/ Line in Form of Serpentine Shape (Fig. 8).3 Coupler Design Based on T-Shape Equivalent to λ/ Line and -Shape Equivalent to 3λ/ Line (Fig. 6) 5 Conclusion References. INTRODUCTION Hybrid couplers are used as components, in almost every RF system, such as power combiners and dividers, de(modulators), balanced mixers, image rejection mixers, balanced amplifiers and feed network in antenna arrays. The /8 hybrid coupler is preferred in some applications, namely, mixers, modulators, and isolated power splitters since the isolation between its input ports may be independent of the value of the two balanced impedance loads []. The circumference of the conventional /8 hybrid ring coupler is.5 λ g (see Fig. ). At the lower frequency of the microwave band such as mobile frequencies such size is too large for system integration and MMICs applications, since a large circuit area results in high chip cost. Several design techniques have been proposed to reduce the coupler size. A quarter wavelength pair of coupled lines short-circuited at their diagonal ends has been used to replace the three quarter wavelength line []. The circumference of such coupler has been reduced to λ g. However this technique requires a very tightly coupled line section that is difficult to fabricate with simple microstrip technology. The design of 3 db reduced size hybrid ring based on λ g /6 or λ g /8 sections has been studied [3]. The circumference reported is.5 λ g. Another approach to reduce the ring coupler size requires; () using a small section of transmission line with a specified characteristic impedance instead of the λ g / line; and () replacing the three quarter wavelength line by a one-quarter-wavelength line with phase inverter [, ]. Based on this approach the.5 λ g circumference has been reduced to.67 λ g
A small size 3 db /8 microstrip ring couplers 79 3 λ g / λ g / λ g / Z o =7.7 Ω 3 λ g / Figure. Layout of the conventional ring coupler based on λ g / line. []. Significant increase in bandwidth to exceed one octave has been obtained. The circuit is composed of a coplanar strips (CPS) ring and coplanar wave-guides (CPW) feed lines. In this case air bridges, which are potentially expensive, are needed. A crossover of the two strips on the ring is also required to achieve 8 phase shift (phase inverter). Design theory of a reduced size ring coupler using phase inverter in different topologies has been studied []. In this paper, new miniaturized /8 ring coupler configurations are introduced. The replacement of each quarter wavelength line by its T -shape equivalence described earlier by the authors [5] for hybrid quadrature couplers will be introduced in Section. This approach leads to more than 7% area saving, ending up with.75 λ g circumference. Further size reduction can be achieved by replacing the three quarter wavelength line either by: () its circuit equivalent, () a pair of coupled lines short circuited at their diagonals, or (3) the same line length in serpentine shape. These configurations will be described in section 3. The proposed structures are suitable for hybrid MICs and MMICs applications. With this approach more than 85% area saving with respect to the conventional type can be achieved. A coupler operating at.9 GHz, has been designed and implemented in three different configurations. The design, simulation, and experimental results are presented in Section, and are followed by concluding remarks in Section 5.
7 Mohra, Sheta, and Mahmoud λ θ / ( =9 o ) Z o θ, Z θ, Z (a ) (b) Figure. (a) λ/ transmission line, (b) T-shape circuit equivalent to λ/ line. θ Z. RING COUPLER USING T-SHAPE SECTION EQUIVALENT TO QUARTER WAVE LINE Conventional ring couplers utilize three sections of λ g / line and a section of three quarter wavelength with.5 λ g circumference as shown in Fig.. The length of the quarter wavelength line can be reduced by making use the T-shape circuit equivalent shown in Fig. [5]. The equivalence between (a) and (b) of Fig. results in tan θ = M and tan θ = K (cot θ tan θ ) () Where M = Z /Z o and K = Z /Z. Z o is the characteristic impedance of the quarter wavelength line. Z, θ, Z, and θ are the characteristic impedances and electrical lengths of the series line and stub respectively, as shown in Fig.. Fig. 3 shows the variation of the stub length θ against M or θ T for different values of K. Where θ T =θ = cot (M). Applying the T-circuit equivalent in Fig. leads to the ring coupler layout shown in Fig.. It should be noted that θ must be less than the electrical length of the ring radius, R, to avoid overlapping between the six stubs. This imposes the inequality: θ <R=6θ /π degrees (θ and θ in degrees) () The unrealizable values of the circuit parameters, when θ is becomes greater than the ring radius, R, are marked by the dashed region in Fig. 3. The stub widths should be taken into consideration while selecting its lengths. 3. ULTRA-MINIATURIZED HYBRID RING COUPLER CONFIGURATIONS Further size reduction can be obtained by some arrangements to replace the three quarter wavelength line by a small size circuit
A small size 3 db /8 microstrip ring couplers 7 θ =Ring radius 6 θ (Degrees) 5 3 K= Unrealizable region K= K=6 K=8 K=.5.5 3 3.5 M=Z /Z o 9 53. 36.87 3.9 θ T = θ Figure 3. Design curves for the circuit in Fig. (b) to be equivalent to λ/ line in Fig. (a). 3 θ Z θ Z Figure. Ring coupler layout based on the T-shape circuit equivalent in Fig.. / λ ( =7 θ o ) θ, Z θ, Z θ, Z Z o θ θ (a ) Z (b) Z Figure 5. (a) 3λ/ transmission line (b) -shape circuit equivalent to 3λ/ line.
7 Mohra, Sheta, and Mahmoud equivalent: The following three configurations offer different ways of doing so. 3.. Using Circuit Equivalent The length of the 3/λ g line in the conventional coupler can be reduced by using its circuit equivalent circuit shown in Fig. 5. Using the familiar ABCD matrix, the appropriate circuit parameters can be adjusted to have similar response as the three quarter wavelength. Let the ABCD matrices of the three quarter wavelength line in Fig. 5(a), a series line of electrical length θ and characteristic impedance Z, and an open circuit stub of electrical length θ and characteristic impedance Z be denoted, respectively, by jz o cos θ jz sin θ A a = j, A b = j sin θ, cos θ Z o Z and A c = j tan θ (3) Z The equivalence in Fig. 5 can be achieved through the equality: A a = A b A c A b A c A b () The resultant circuit layout takes the shape shown in Fig. 6. Overlapping between circuit stubs should be avoided while looking for the circuit parameters. Such constrained equivalence has been solved using Matlab program..37 8.95 3 All dimensions are in mm 6 9 6 3...3 3.8.3 Figure 6. Layout of a compact coupler based on the T -shape circuit in Fig. (b) and the circuit equivalent in Fig. 5(b).
A small size 3 db /8 microstrip ring couplers 73 3 Figure 7. Layout of a compact coupler based on the coupled line short circuited at its diagonal. 3.. Coupled Line Short Circuited at ItsDiagonal With the aid of ABCD parameters it can be shown that, a coupled line of length l, short circuited at its diagonal is equal to a transmission line of the same length l in series with 8 phase shift network []. Thus the three quarter wavelength line (7 ) can be replaced by a pair of quarter wave coupled lines, short-circuited at their diagonals. Therefore, the ring coupler in Fig. can be reduced to the coupler in Fig. 7 if the designed parameters selected from Figure 3 such that, the ring diameter is set equal to 9. The appropriate circuit parameters that achieve this goal are: M =.9, Z = 6.3 Ω, θ =3.56. Meanwhile, Z and θ can be selected from Fig. 3. However, it is difficult to implement this configuration using the simple microstrip technology due to the tight coupling required. 3.3. Implementation of the Three Quarter Line in Serpentine Form In order to avoid the design complexity in configuration (3.) and fabrication complexity in (3.) the three quarter wavelength line can be implemented in serpentine shape as shown in Fig. 8. This will allow about 5% area saving relative to the reduced size coupler in Fig.. The design parameters can be chosen similar to those in configuration case (3.). Separation between lines should be large enough to avoid parasitic coupling.. DESIGN CASES AND EXPERIMENTAL RESULTS In order to confirm and validate the proposed configurations, three 3- db couplers are designed, simulated, and measured at 9 MHz. The couplers were fabricated on an RT/duroid 588 substrate with ε r =. and thickness of.78 mm. The IE3D software has been used to simulate
7 Mohra, Sheta, and Mahmoud 3 Figure 8. Layout of a compact coupler the three quarter wavelength line formed in serpentine shape. the designed couplers. The measurements have been performed using HP 85 vector network analyzer:.. Coupler Design Based on T-Shape Equivalent to λ/ Line (Fig. ) For realization using the available simple etching process, the smallest line width is limited to about µm. This is a line of 58 Ω characteristic impedance on the substrate. Thus M = Z /Z o is fixed at.3 (for Z o =7.7Ω). From (), θ =.5. The other circuit parameters can be selected either from equation () or Fig. 3. Taking K =3.6, we get Z = 5 Ω and θ =3. The obtained coupler diameter is 6 mm compared to 8 mm of the conventional one. The simulated and measured results are shown in Figures 9(a) and 9(b) respectively. From.8 to.6 GHz, the measured scattering parameters are S <.5 db, S = 3.78 ±.55 db, S 3 =.895 ±.335 db, S (isolation) < 6.3 db. The measured phase difference between ports and when fed at port varies from 89 at.8 GHz to 65.5 at.6 GHz. Frequency shift of MHz has been observed. The relative bandwidth is 3%. Due to the frequency shift noted, the coupler area will be slightly greater than the theoretical estimation (9.%). The coupler is now implemented on 3.66% of the area of the conventional coupler at the measured center frequency ( GHz)... Coupler Design Based on T-Shape Equivalent to λ/ Line and the Implementation of 3λ/ Line in Form of Serpentine Shape (Fig. 8) The same parameters in section. have been used, but in this case the 3λ/ line has been implemented in the form of serpentine shape. This reduces the area to its half compared to design in section.. The simulated and measured performances are given in Figs. 9(c)
A small size 3 db /8 microstrip ring couplers 75 db db - S S 3 - S S 3 - S - S -3-3 (a) S.7.8.9. Freq. GHz -3-3 S.7.8.9.. (b) Freq. GHz db - S 3 db - S S 3 S - -3 S S 3 - -3 S S 3 -.7.8.9. (c) Freq. GHz - Freq..7.8.9.GHz (d) db S 3 db S 3 - S - S S S - 3 S -3.7.8.9.. (e) Freq. GHz - -3 3.7.8.9.. (f) S Freq. GHz Figure 9. Simulation and measured results of 3-dB ring coupler, designed at 9 MHz (a), (c), and (e) are the simulated results performed by IE3D program (b), (d), and (f) are the measured results performed by HP85 vector network analyzer.
76 Mohra, Sheta, and Mahmoud and 9(d), respectively. The measured bandwidth is 3 MHz centered at 9 MHz. Within this band the scattering parameters are S < 3 db, S = 3.33 ±.7 db, S 3 = 3.5 ±.37 db, S < 5 db. The measured phase difference between ports and when fed at port varies from 8 at.75 GHz to 58.5 at.5 GHz. The area used is about 5% of the area of conventional coupler..3. Coupler Design Based on T-Shape Equivalent to λ/ Line and -Shape Equivalent to 3λ/ Line (Fig. 6) In this design, the selection of the -shape equivalence parameters, Fig. 5, and T-shape equivalence, Fig., should be optimized carefully in order to avoid overlapping between circuit stubs. Equation (3) has been solved using the Matlab program and the resultant dimensions are shown in Fig. 6. The simulated and measured results are shown in Figs. 9(e) and 9(f) respectively. The measured bandwidth is 3 MHz centered at GHz. Within this band the scattering parameters are S < 3 db, S = 3.5 ±.55 db, S 3 = 3. ±.6 db, S < 5 db. The measured phase difference between ports and when fed at port varies from at.85 GHz to 8 at.5 GHz The actual area is.7% of the area of the conventional coupler. 5. CONCLUSION New 3-dB small size ring coupler configurations have been presented. Design curves have been introduced. In order to demonstrate the advantages of the design approach three couplers designed at 9 MHz have been simulated and implemented. Good agreement is established between theory and measured results. The area saving, relative to the conventional coupler, exceeds 67% and reaches 85% for the coupler in Fig. 8. The measured relative bandwidth of this coupler is 33.33% and no less than 3% for the other two. The three implemented couplers in this paper do not need any lumped elements or via hole grounding and consequently have the advantages of low cost, simple fabrication, and excellent design accuracy. These hybrids are suitable for MICs and MMICs applications. REFERENCES. Wang, T. and K. Wu, Size reduction and band broadening design technique of uniplanar hybrid ring coupler using phase inverter for M(H)MICs, IEEE Trans. Microwave Theory Tech., Vol. MTT- 7, 98 6, 999.
A small size 3 db /8 microstrip ring couplers 77. Mar, S., A wide band stripline hybrid ring, IEEE Trans. Microwave Theory Tech., Vol. MTT-6, 36, 968. 3. Kim, D. I. and G. S. Yang, Design of new hybrid-ring directional coupler using λ g /8 or λ g /6 sections, IEEE Trans. Microwave Theory Tech., Vol. 39, 779 783, 99.. Murgulescu, M. H., E. Moisan, P. Legaud, E. Penard, and I. Zaquine, New wideband.67 λ g circumference 8 hybrid ring coupler, Electron. Lett., Vol. 3, 99 3, 99. 5. Sheta, A. F., A. Mohra, and S. F. Mahmoud, A new class of miniature quadrature couplers for MIC and MMICs applications, Microwave Opt. Technol. Lett., Vol. 3, 5 9,. Ashraf Shouki Mohra was born in Dakahlia, Egypt, in 963. He received the B.Sc. degree in Electronics and Communications from Zagazig University, in 986. He received the M.Sc. and Ph.D. degrees in Electronics and Communications from Ain Shams University, Cairo, Egypt, in 99 and, respectively. He is currently with the Electronics Research Institute, Ministry of scientific research and technology. His current research interests include microstrip antennas, computer aided design of planar and uniplanar of microwave and millimeter wave circuits, and monolithic microwave integrated circuits. Abdel Fattah Sheta received the B.Sc. degree in Communications and Electrophysics Department from Alexandria University, Egypt in 985, and the M.Sc. degree in Electronic Engineering Department, Cairo University, Egypt in 99. In,996, he received the Ph.D. degree in Microwave circuits Analysis and Design from ENST, Universite de Bretagne Occidentale, France. Between 996 and 998, he was based at National Telecommunication Institute (NTI), Cairo, Egypt, where he conducted research in passive and active microwave circuits. In 998 he joined Electric Engineering Department, Fayoum Branch, Cairo University, as Assistant professor. Currently he is an assistant professor in Electrical Engineering Department, King Saud University, Riyadh, Saudi Arabia. His current research interests include microstrip antennas, planar and uniplanar microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs). Samir F. Mahmoud graduated from the Electronic Engineering Department, Cairo university, Egypt in 96. He received the M.Sc. and Ph.D. degrees in the Electrical Engineering Department, Queens University, Kingston, Ontario, Canada in 97 and 973. During the academic year 973 97, he was a visiting research fellow at
78 Mohra, Sheta, and Mahmoud the Cooperative Institute for Research in Environmental Sciences (CIRES). Boulder, CO, doing research on Communication in Tunnels. He spent two sabbatical years, 98 98, between Queen Mary College, London and the British Aerospace, Stevenage, where he was involved in design of antennas for satellite communication. Currently Dr. Mahmoud is a full professor at the EE Department, Kuwait University. He had a sabbatical leave at Queen s University, Kingston, Ontario, Canada, during the academic year. His research activities have been in the areas of antennas, geophysics, tunnel communication, e.m wave interaction with composite materials and microwave integrated circuits. Prof. Mahmoud is a Fellow of IEE.