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RF Electronics Chapter 2: Computer Simulation Page 13 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. As can be seen from figure 2.10, return loss (S 11 ) is only -1.4 dB near the cut off frequency of the filters so that the input is poorly matched. By comparing the blue curve of the low- pass filter by itself, with the cyan curve of the low-pass filter as part of the diplexer, it can be seen that there is a significant interaction between the low-pass and high-pass filters. There is a similar degradation in performance of the high-pass filter, as shown by the red and magenta traces. One can use the optimisation capability to improve the performance of the diplexer at the crossover frequency range. The first task is to specify the optimisation goals. For this design, the requirements are that S 11 is less than -20 dB over the entire VHF and UHF TV frequency range. Figure 2.11. Specification of optimisation goals. These optimisation goals are entered in the optimisation goal function as shown in figure 2.11. Double click on the elements to be optimised and select the "optimize" checkbox in the element options window, for the component or the variable linked to a component as shown in figures 2.7 and 2.8, to ensure that the element can be optimised. In addition, some attenuation limits at 100 MHz and -35 dB and 625 MHz and -35 dB, for S 21 and S 31 can be added, to prevent the components being changed so that all frequencies are passed by both the high-pass and low-pass networks. Finally, the S 11 limit is set to -45 dB to ensure a near perfect match over the entire frequency band. Often the optimisation limits are changed during the optimisation process, to ensure the best performance to be obtained for the final network. For most circuits a higher return loss is normally acceptable, however this network will optimise in one-step to meet these limits. The weighting of the pass-band optimisation limits for S 21 and S 31 are made 1000 since the typical errors are much smaller than the error in S 11 , which is given a weighting of 1 to give comparable errors at the start, thus ensuring that all limits receive sufficient optimisation effort. Select Simulate Optimise to start the optimisation. For this optimisation, "Simplex Optimisation" is a good optimisation method to use and the optimisation will reach its goal in one-step. Figure 2.12 shows the optimisation starting performance with the optimisation goals and the Optimizer panel. A logarithmic frequency scale is used since that will clearly show the straight lines of the 80dB/decade stopband attenuation. Figure 2.13(top) shows the optimisation criteria and the performance after the optimisation has been completed. It can be seen that the diplexer meets all the required specifications and that it is even possible to meet a -45 dB return loss on S 11 . Less than 300 iterations of the "Simplex Optimisation" are required to obtain the final values. Changing the S 11 optimisation limit to -60 dB, allows S 11 to be reduced even further, as shown in Figure 2.13(bottom). RF Electronics: Design and Simulation 13 www.cadence.com/go/awr

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