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RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 76 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 4.16 can now be used as the basis for a PCB layout. Often it is necessary to bend the Microstrip lines to fit the circuit in the available space, as shown in figure 4.17. Each of the transmission lines are a quarter wavelength long. If in figure 4.16, lines TL2 and TL5 are disabled, resistors R1 and R3 are disabled and resistor R2 is made a short circuit (0 ), then the circuit becomes a transmission line of various impedances connecting port 1 to port 2 with an open circuited quarter wavelength stub consisting of TL7 connected to it. This line will thus present itself as a short circuit at R2 at the centre frequency. A similar circuit is shown on the right of figure 4.30. The transfer function from ports 1 to 2 will thus have a notch exactly at the centre frequency of the hybrid, where TL7 is a quarter wavelength. The length of the folded sections making up TL7 in figure 4.16, can be adjusted to be exactly a quarter wavelength by changing the lengths of the sections to ensure that the notch in the transfer function from ports 1 to 2 occurs exactly at the centre frequency of the hybrid. This process is repeated for the other three different transmission lines in the hybrid. Variables are used to ensure that the folded transmission lines corresponding to the sets of lines TL1 and TL2, lines TL3 and TL5 and lines TL6 and TL7 are exactly the same length and impedances. Figure 4.18. Hardware for 90 MHz to 270 MHz Wilkinson Transmission-line hybrid. Finally, the line lengths and widths are optimised to provide the final layout and meet the required specifications. Figure 4.18 shows the hardware for this layout and figure 4.19 shows the simulated frequency response for the hybrid. The details of the PCB layout are included in the CADENCE AWR DE project file for figures 4.18 and 4.19. The measured performance is shown in figure 4.20. This agrees closely with the calculated values. The hardware has an isolation that is better than 26.9 dB over the entire 90 to 270 MHz frequency range and the return loss is better than 22.0 dB. Figures 4.9 and 4.11 show that just a 50 m change in track width results in significant changes in the return loss. Manufacturing tolerances on the dielectric constant of the laminate and track width imperfections as part of the PCB milling process resulted in the return loss and isolation not being as good as simulated. The insertion loss of the coupled path is also 0.24 dB RF Electronics: Design and Simulation 76 www.cadence.com/go/awr

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