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RF Electronics Chapter 2: Computer Simulation Page 15 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. For the -70dB optimisation of S11, the stopband attenuation limits need to be reduced slightly from -28 dB to -25 dB. It then becomes a choice whether a better S11 or a higher stopband attenuation is more important. Zn1 reduces more but Zn4 does not change much when going from -45 dB for S11 to -70dB. This diplexer is also used in Application Note AN-M-OPT-DPX-2020.2.6.pdf [8] to investigate the speed of operation of different optimization routines in AWR DE V14. When the cost function does not decrease, it is worthwhile to change some conditions since the optimisation may be stuck in a local minimum rather than the global minimum for the cost function. Changing the weight for one or more of the goals or changing the optimisation function may get the optimisation out of the local minimum. The "random local" optimiser is good for a complex optimisation function that will exist close to the global optimum condition. Other optimisation methods, such as Simulated Local Annealing or Discrete Local Search can also be tried to ensure the optimisation is not stuck in a local minimum. It may even be necessary to alternate between several optimisation strategies and change optimisation limits and weights in order to ensure that a good performance is obtained. For this example, a Discrete Local Search will not produce a good optimisation, however for complex problems [9] it gives better results than the Simplex Optimisation. It is thus important to try different optimisation techniques to obtain the best results. A good overview of each optimisation method can be found in section 2.6.9 of the AWR DE Simulation and Analysis Guide and in Application Note AN-M-OPT-DPX-2020.2.6.pdf [8]. The component values of the filter after optimisation with a -70 dB S 11 return loss for the low-pass sub-circuit are shown in figure 2.14 and those for the high-pass sub-circuit are shown in figure 2.15. Note that the component values are significantly different from the starting values using the Butterworth filter table values. The component values are very practical, so that the diplexer can easily be made. Figure 2.14. Final low-pass filter for diplexer. It is possible to replace the inductors using Microstrip lines as shown in figures 7.10 and 7.11 of Chapter 7. Using Microstrip lines allows this diplexer to be made on a low cost FR3 or FR4 PCB using only capacitors and Microstrip lines. If this is done then the length and impedances of the Microstrip lines must be further optimised to meet the same specifications as this diplexer, since figures 7.10 and 7.11 show that Microstrip lines RF Electronics: Design and Simulation 15 www.cadence.com/go/awr

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