A Wideband Compact Broadside Coupler-based Impedance Transformer
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A Wideband Compact Broadside Coupler-based Impedance Transformer with 6:1 BandwidthWei Jia LuTemasek LaboratoriesNational University of SingaporeSingapore tsllwj@.sgKian Sen AngSchool of EEENanyang Technological UniversitySingaporeakiansen@.sgKoen MouthaanDepartment of ECENational University of SingaporeSingaporek.mouthaan@.sgAbstract —A compact broadband impedance transformer using a broadside coupler is presented. The tradeoff between good return loss and design parameters that can be realistically implemented is considered during the design. To implement the coupler and the required high impedance transmission line, a combination of stripline and microstrip is used. The circuit transforms 50 Ω to 100 Ω from 1 GHz to 6 GHz. The measured return loss is larger than 15 dB from 1.1 GHz to 6.4 GHz. In general, the measured results agree well with the simulated results.Keywords-Impedance transformer; broadside coupling; passive circuitI. I NTRODUCTIONImpedance transformers are used in impedance matching, power splitting and power combining circuits. Commonly used impedance transformer include conventional quarter-wavelength impedance transformer, coupled line impedance transformer, tapered impedance transformer, balun impedance transformers and coaxial line impedance transformer [1]-[4]. The conventional quarter-wavelength impedance transformer is easy to implement but is typically narrowband. The bandwidth can be increased by using multiple sections of quarter wavelength line or long tapered transmission line at the expense of a larger physical footprint. Coaxial line impedance transformers such as Guanella [5] and Ruthroff [6] are able to provide a large bandwidth. These transformers typically operate in radio frequencies and the use of coils and ferrite cores limits the smallest physical size achievable. As such, compact impedance transformers are useful in practical applications with space constraint.A compact broadband impedance transformer with three reflection zeros was proposed in [7]. The transformer employs a coupler with a high coupling coefficient and a high impedance transmission line loop as shown in Fig. 1(a). A 50 to 110 Ω stripline impedance transformer was demonstrated using edge-coupled transmission lines. It achieved better than 25 dB return loss from 0.6 GHz to 1.6 GHz. As the realizable bandwidth is dependent on the maximum realizable coupling, the achieved bandwidth or impedance transformation ratio islimited by the realizable gap between the coupled lines.Fig. 1 (a) Schematic diagram of the proposed quarter-wavelength impedance transformer [7] (b) Quarter-wavelength impedance transformer using broadside coupler (c) Cross-sectional view of the impedance transformer.Proceedings of the 42nd European Microwave ConferenceAn alternative method to implement the circuit using theVertically Installed Planar (VIP) coupler is presented in [8]. The method is able to achieve strong coupling without being restricted by the minimum gap size between two transmission lines. However, the drawback of this topology is the difficulty in determining the electrical length of the loopback transmission line. In addition, the vertical component of thestructure is more difficult to fabricate and deploy whencompared to the solution presented in [7].In this paper, the impedance transformer is implementedusing a stripline broadside coupler and a microstrip transmission line. For the same line width, a microstrip can provide higher characteristic impedance compared to stripline. Furthermore, for a given physical length, microstrip has a longer electrical length since its effective permittivity is smaller. As such, the physical length of the loopback transmission line is longer than the coupled-line section, which allows easy practical implementation of the loopback transmission line. II. S TRUCTURE O F T HE I MPEDANCE T RANSFORMER Fig. 1(a) shows the schematic diagram of the impedance transformer. It transforms the impedance from Z 1 to Z 2. Fig. 1(b) shows our proposed implementation of the circuit and Fig. 1(c) is the cross sectional view of the impedance transformer. The stack up has four metal layers and three substrate layers. The top and bottom metal layers are the ground. The input and output transmission lines and part of the coupled line section are implemented on the second metal layer. The loopback transmission line and the other components of the coupled line section are implemented on the third metal layer. As the coupled line and loopback transmission line are both quarterwave in length, for the convenience of implementation, the loopback transmission line is realized partially in microstrip. It was done by removing a rectangular window of the top metal layer and the top and middle substrate, as shown in Fig. 1 (b). A vertical interconnect joins the loopback transmission line on the third metal layer to the second metal layer. The quarterwavelength is determined at the center operating frequency.III. C HARACTERISTICS O F T HE I MPEDANCE T RANSFORMER The impedance transformation ratio and the bandwidth are controlled by the characteristic impedance, Z 0, of the loopback transmission line and the even mode impedance, Z 0e , and odd mode impedance, Z 0o , of the coupled lines. Given the desired impedance transformation ratio and the bandwidth, the design curves in [4] can be used to determine the required Z 0o , Z 0e and Z 0. However, when the bandwidth and the impedance transformation ratios are large, it may be difficult toimplement it if one refers to the design curves. Simulationusing Agilent’s Advanced Design System (ADS) shows that Z 0 and Z 0o have greater impact on the bandwidth and the return loss of the circuit while Z 0e mainly affects the return loss. A very large return loss at some frequencies will result in extreme dimensions which cannot be realistically achieved. Therefore, a compromise between return loss and practicallyachievable printed circuit dimensions is necessary.Fig. 2 and Fig. 3 show the simulated S-parameters of the same impedance transformer but with different sets of design parameters. Fig. 2 has a better return loss but requires an impedance of 250 Ω, which is difficult to realize. Fig. 3 has amoderate return loss performance but with a more realisticFig. 2 ADS simulation of a 50 Ω to 100 Ω impedance transformer with parameters: Z 0 = 250 Ω, Z 0e = 155 Ω and Z 0o = 27.5 Ω.Fig.3 ADS simulation of 50 Ω to 100 Ω impedance transformer with different parameters: Z 0 = 155 Ω, Z 0e = 174 Ω and Z 0o = 21 Ω.impedance requirement of 155 Ω. As such, the following section will focus on this design. IV. F ULL W AVE S IMULATION A 50 Ω to 100 Ω impedance transformer with a centre frequency of 3.5 GHz is designed and simulated. The bandwidth of the impedance transformer is 142.9%. It covers 1 GHz to 6 GHz. The required Z 0, Z 0e and Z 0o are 155 Ω, 174 Ω and 21 Ω respectively.The physical structure of the impedance transformer issimulated in Ansys HFSS. Further optimization has been done to take into consideration the effect of the verticalinterconnect. The circuit is implemented using Rogers 5880.The top and bottom substrate are 62 mils while the middlesubstrate is 5 mils. The slot on the top substrate is 17.1 mm by 4 mm. The section of circuit used for impedancetransformation is 13.7 mm in length. Using [9], the width ofFig. 4 HFSS simulation result for |S 11| and |S 21|.Fig. 5 Different layers of the fabricated quarter-wavelength impedance transformer.Fig. 6 Photo of the fabricated quarter-wavelength impedancetransformer.Fig. 7 Measured |S 11| and |S 21| (solid line) and simulated |S 11| and |S 21| (dot dash line) with Z 1 = 50 Ω and Z 2 = 100 Ω.Fig. 8 Measured |S 12| and |S 22| (solid line) and simulated |S 12| and |S 22| (dot dash line) with Z 1 = 50 Ω and Z 2 = 100 Ω.the required coupled line is about 0.8 mm. The width of the loopback transmission line is 0.3 mm. The distance between the coupled line and the transmission line is 1.7 mm. The screws that hold the PCBs together are modeled as cylindrical perfect electric conductors.Fig. 4 shows the HFSS simulation results for |S11| and |S21|. Three dips are observed in the |S11| response. The return loss is better than 15 dB from about 0.9 GHz to 5 GHz and better than 10 dB from 5 GHz to 5.8 GHz. The bandwidth of the impedance transformer is narrower than the designed bandwidth. The dip observed at 5.8 GHz is probably caused by a cavity resonance. Additionally, the vertical interconnection used to join the loop back transmission line and the top layer of the coupled line, is also a source of error.V.M EASUREMENT A ND D ISCUSSIONFig. 5 and Fig. 6show the fabricated circuit. The overall dimension of the circuit is 45 mm by 29.5 mm. A length of 50 Ω transmission line is added to the input and output ports respectively for calibration purposes. M2 screws are used to hold the three substrates together. The top and bottom grounds are soldered to the ground of the SMA connectors. The loopback transmission line is connected to the top layer of the coupled line by soldering.The circuit is then measured using a HP8510C Vector Network Analyzer (VNA). A TRL calibration set for the coaxial to stripline transition was designed and used to calibrate out the effect of the 50 Ω connectors and the coaxial to stripline transitions. As the test ports of the VNA are 50 Ω, the data collected was inserted into ADS where 100 Ω load terminations can be readily used.Fig. 7 and Fig. 8 show the simulated and measured performance of the impedance transformer. When terminated with 100 Ω at port 2, the fabricated impedance transformer achieves a return loss larger than 15 dB from 1.1 GHz to 4.6 GHz and from 4.8 GHz to 6.4 GHz. The circuit still has a return loss better than 10 dB between 4.4 GHz and 4.8 GHz despite an observed resonance.A shift in resonance frequency is observed in the operation band of the fabricated impedance transformer towards lower frequencies as compared to the simulation results. The explanation is the presences of stray capacitance introduced by the vertical interconnect. Further studies using optimization on these parts are necessary for improvement of the performance.VI.C ONCLUSIONA compact broadband impedance transformer using a broadside coupler and loopback transmission line is presented. The method overcomes the minimum gap limitation encountered in couplers using edge coupling. The device is able to achieve a 6:1 bandwidth with a length of a quarter wavelength, which is significantly smaller than the traditional multi-section counterpart. The device will be useful in broadband power dividers and broadband matching for antennas. The drawback of the circuit is the difficulty in making the vertical interconnect between the top and bottom layer PCBs in a laboratory. PCBs made in an industrial process, which incorporates plated through vias and blind vias, will solve this problem.A CKNOWLEDGMENTThe authors would like to thank Mr Ray Fang and Madam Lee Siew Choo from the National University of Singapore for their technical support in the TRL calibration set design and the circuit fabrication.R EFERENCES[1] E.G. Cristal, “Meander-line and hybrid meander-line transformer,”IEEE Trans. Microw. Theory Tech., vol. MTT-21, pp. 69-75, Feb 1973. [2]G. L. Matthaei, L. Young, and E. M. T. Jones, “Short-step Chebyshevimpedance transformers,” IEEE Trans. Microwave Theory Tech., vol.MTT-14, pp. 372-383, Aug. 1966.[3] A. Podcameni, “Symmetrical and asymmetrical edge-coupled-lineimpedance transformers with a prescribed insertion loss design,” IEEE Trans. Microwave Theory Tech., vol. MTT-34, pp. 1-7, Jan 1986.[4] A. Grebennikov, RF and Micrwave Transmitter Design, 1st ed.Hoboken, NJ: Wiley, 2011, ch. 4.[5] C. L. Ruthroff, “Some Broadband Transformers,” Proc IRE, Vol 47,August 1959, pp 1337-1342.[6]G. Guanella, “New Method of Impedance Matching in Radio-Frequency Circuits”, Brown Boveri Review, September 1944, pp. 327-329.[7]K. S. Ang, C. H. Lee, and Y. C. Leong, “A broadband quarter-wavelength impedance transformer with three reflection zeros within passband,” IEEE Trans. Microwave Theory Tech, vol. 52, no. 12, pp.2640-2644, Dec. 2004.[8]W. Lu, K. S. Ang, and K. Mouthaan, “A Broadband Quarter-wavelength Impedance Transformer Using Vertically Installed Planar Coupler,” Microwave Symposium Digest (MTT), 2011 IEEE MTT-S International, vol., no., pp.1, 5-10 June 2011[9]Cohn, S.B., "Characteristic Impedances of Broadside-Coupled StripTransmission Lines," Microwave Theory and Techniques, IRE Transactions on , vol.8, no.6, pp.633-637, November 1960.。
