AWR eBooks

RF Electronics Chapter 7: RF Filters Page 235 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 7.43 shows a close-up of the centre of the first resonator, and shows TL10, MT4 and TL1 of figure 7.41. It can be seen that without the tapered transition a short circuit would be produced. The filter from figure 7.41 as designed using the above procedure has a centre frequency of 981 MHz, and is 1.9% low. The bandwidth is 62 MHz and is 12 % low. The resonator lengths and coupling gaps are optimised, using the optimisation routines in Cadence AWR DE, to achieve the correct centre frequency and bandwidth. Since the design procedure gives a result very close to the specifications, the optimisation procedure is quick. The resulting frequency response is shown in figure 7.44. The insertion loss of the filter is 1.9 dB. The return loss can be improved if needed by setting optimisation goals on S 11 and using the optimiser to automatically fine tune the elements. Figure 7.43. Close-up of part of figure 7.42, showing TL10, MT4 and TL1. Figure 7.44. Frequency response of the filter from figure 7.41 after optimisation. Figure 7.45. PCB layout for coupled-line filter with tap coupling into and out of the filter. For the filter in figures 7.41 to 7.43, the input coupling gap is 0.14 mm. This gap is close to the limit of reliable manufacturing capability. To avoid this limitation, tap coupling is used to connect to the input and output resonators, as shown for the 100 MHz bandwidth at a 1.7 GHz centre frequency filter shown in figure 7.45. For wider bandwidth filters, the input and output coupling gaps are very small and tap coupling is best. For very narrow bandwidth filters, the connection point for tap coupling becomes too close to the effective ground point of the first resonator, so that line coupling as shown in figure 7.41 is more practical. RF Electronics: Design and Simulation 235 www.cadence.com/go/awr

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