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RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 74 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The bandwidth of the Wilkinson hybrid can be increased by cascading two resistor linked transmission lines as shown in figure 4.14. The characteristic impedances of the lines and the resistor values can be optimised to give the required isolation over the specified bandwidth. There is a compromise between bandwidth and isolation. A wide bandwidth will result in a low isolation and a smaller bandwidth can result in a better isolation. For the hybrid shown in figure 4.14, the isolation was specified as -30 dB. The required values for Z1, Z2, R1 and R2 are then determined by optimisation using AWR DE. Figure 4.15. Performance of a 2-stage wideband Wilkinson hybrid. From figure 4.15, it can be seen that a wide bandwidth hybrid results. The impedance for the first section has increased to 83 compared with 70.71 for the conventional hybrid. The higher impedance will result in an increased insertion loss of the hybrid, due to thinner PCB tracks being required. A compensated hybrid configuration will reduce the impedance values and thus reduce the insertion loss of the hybrid as well as having a wider bandwidth still. Example 4.2: 90 MHz to 270 MHz Wilkinson Hybrid This process can be extended to produce Wilkinson hybrids with good isolation and return loss on all ports over a 2:1 or even 3:1 frequency range. As an example, a Wilkinson hybrid covering the frequency range from 90 MHz to 270 MHz is to be designed. The target specification is a better than 30 dB isolation over the 90 to 270 MHz frequency range and better than 30 dB return loss for all the ports from 95 MHz to 260 MHz. The design process requires two steps: 1) The circuit is designed and optimised, using ideal transmission lines for the hybrid. 2) Once the specifications are achieved for this ideal circuit, then the ideal transmission lines are replaced with Microstrip lines, with the same electrical line length and impedance. Figure 4.16 shows the schematic for a suitable design. The variables for the centre frequency F0, the transmission line impedances Z1 to Z4 and the isolation resistors R1 to R3 are all enabled for optimisation. The initial values for Z1 and Z2 are chosen to be 42 RF Electronics: Design and Simulation 74 www.cadence.com/go/awr

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