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RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 71 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. exactly. Figure 4.10 shows the resulting layout. The 100 Ωresistor and its connecting lines will have some effect on the performance as shown in figure 4.11, where the notch of S 22 is shifted in frequency as a result. The performance is however still satisfactory, since a 20 dB isolation or return loss is normally sufficient. Figure 4.11. Performance of stripline Wilkinson hybrids of figure 4.9. The line length L1 of TL1 and TL2 can be tuned to ensure that the best isolation occurs at 1.0 GHz. There will be some coupling between TL1 and TL2, since they are in close proximity. The effect of this coupling can be investigated using an EM simulator like AXIEM, which is included with AWR DE. In chapter 7, the section on EM Simulation, shows how AXIEM can be used to analyse circuits. A good indication of the coupling between lines TL1 and TL2 can simply be obtained by replacing them with a coupled line element M2CLIN as shown in the right part of figure 4.9. The length of the coupled line is the same as TL1 or TL2. The line width is 0.9519 mm like TL1 or TL2. The spacing is set to 4.836 mm, corresponding to the spacing between TL1 and TL2. The layout is the same as figure 4.10, but now the effect of the coupling between TL1 and TL2 is included. Figure 4.11 shows a comparison between the performance of the layout of figure 4.10, when TLIN elements are used for TL1 and TL2 of left figure 4.9 are replaced with a coupled line M2CLIN of right figure 4.9, initially with a width correction factor lcf=0. The coupling does not change but the isolation (S 32 ) and the return loss of S 11 and S 22 are reduced. If needed, the line widths used for M2CLIN can be changed slightly, by varying lcf, to improve these values. Optimising the width of the 70.71 lines, results in a width correction factor, lcf = 0.05 mm, and results in S 22 = -59 dB at 1 GHz, as shown in the width-corrected WC-M2CLIN curves in figure 4.11. A similar improvement in S 22 can be made by slightly tuning the width of the lines for the circuit in the left of figure 4.9. Figure 4.11 does show that unintended coupling between lines can seriously degrade the performance of a circuit, but that steps can easily be taken to overcome any performance degradation. RF Electronics: Design and Simulation 71 www.cadence.com/go/awr

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