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RF Electronics: Design and Simulation

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RF Electronics Chapter 7: RF Filters Page 244 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The left filter in figure 7.54 shows a 3-resonator 142 MHz helical filter with a bandwidth of 6 MHz from Temwell [9]. Each resonator is 8mm wide and 12mm high. Temwell specialise in helical filters and make commercial filters from 45 MHz to 2.6 GHz. The right filter in figure 7.54 is an older, 3-resonator, 137 MHz helical filter with a bandwidth of 2 MHz originally made by Toko in Japan. The resonators are 12mm wide and 18mm high. Temwell [9] now supply both filters in figure 7.56. The measured dimensions of these filters are the outside dimensions of the cavities. The ratio of the internal dimensions thus satisfies equation 7.17. Interdigital Filters With interdigital filters, the resonators are grounded at alternate ends. This reduces the coupling between the resonators and as a result, a compact filter can be constructed that has no walls between the individual resonators. A drawing of an interdigital filter is shown in figure 7.57a. H L E C Tuning Screw Resonator Coupling Loop Figure 7.57a. Interdigital filter dimensions, top view. Figure 7.57b. Interdigital filter dimensions, side view. For a small bandwidth (<10%) M. Dishal [11] provided the following equations for designing interdigital filters: 6 . 0 h E Eqn. 7.25 048 . 0 91 . 0 37 . 1 h d h C K Log Eqn. 7.26 RF Electronics: Design and Simulation 24 4 www.cadence.com/go/awr

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