Microwave Interconnects Via Electromagnetic Coupling

ABSTRACT Two transitions are described which couple coplanar waveguide on one substrate surface (a motherboard) to either coplanarwaveguide or microstrip on another substrate surface (a chip). No wire bonds are necessary. A coupled transmission linemodel, along with a full wave analysis, is used to predict the behavior of these transitions. Experimental results show goodagreement with predictions in cases where the coupler length to width ratio is not too small. 1. IntroductionCurrently, many microwave and millimeter wave modules (receiver front ends, for example) are made of submodules whichare hybrid or MMIC components such as low noise amplifiers or phase shifters. These submodules are most often connectedto each other using wire bonds. It is desirable to minimize this type of connection since it is a labor intensive process and candegrade the circuit performance at millimeter wave frequencies'.In this paper, we describe the theory and experiment for wire bond -less interconnects between a microwave transmission lineon one substrate surface (possibly a motherboard) and a microwave transmission line on a different substrate surface (an MMICchip). The interconnect occurs via electromagnetic coupling instead of physical coupling. Figure 1 shows a configuration inwhich a "chip" substrate having microstrip circuitry is connected to a coplanar waveguide (CPW) feed line on a "motherboard"substrate by merely placing one substrate on top of the other. Figure 2 shows a different configuration in which a chip havingCPW circuitry is connected to a CPW feed line on a motherboard. In Figure 2(a), the feed line is located on the underside ofthe motherboard. Alternatively, Figure 2(b) shows the feed line on the top of the motherboard.In what follows, each transition is modeled as a set of coupled lines with two of the four possible ports terminated in opencircuits. A full wave analysis is described and then used to design a CPW /CPW transition at C -band and a microstrip /CPWtransition at X -band. Finally, potential designs for millimeter wave uses are discussed.2. AnalysisThe structures shown in Figures 1 and 2 each consists of a four -port coupled line section with one port connected to aninput line on one surface, one port connected to an output line on another surface, and the remaining ports terminated in opencircuits. Figures 3 and 4 show a cross section of the coupling region for the microstrip /CPW coupler and the CPW /CPWcoupler, respectively. Since, in general, there is no up -down symmetry inherent in these problems, asymmetric coupled lineanalysis must be used. Also, the cross sectional size of the couplers is a significant fraction of wavelength, therefore a full waveanalysis is necessary in order to accurately determine the propagation constants and impedances of the couplers. It shouldbe noted that, because of the up -down symmetry in the CPW /CPW coupler shown in Figure 2(a), a simpler even /odd modeanalysis may be used2.2.1. Coupled Line AnalysisFigure 3 shows the current configuration for the two asymmetric modes which are used in the analysis of the microstrip /CPWcoupler. It should be noted that for the it mode, the total current on either side conductor is at least an order of a magnitudesmaller than the total current on the center conductor. It is for this reason that no direction is indicated for the side conductorcurrent. Using a quasistatic analysis, Tripathi3 has derived an impedance matrix which can be applied to this problem,