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RF Electronics Chapter 7: RF Filters Page 236 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The actual filter, when produced, will have a centre frequency and bandwidth that will be slightly different from the simulated results. These differences are due to; the computer simulation not being completely accurate, variations in PCB substrate materials, etching, or milling errors in producing the PCB and so on. In many instances, the filter performance will be good enough. When the filter specifications are critical, a second iteration of the filter can be produced, where the simulation is biased to correct for the discrepancy between the measured and simulated results. Hairpin Filters The filters of figure 7.42 and 7.45 are long and skinny and are difficult to mount securely. To obtain a better form factor for the filter, the resonators can be folded as shown in figure 7.46. Coupled resonator filters where the resonators are made up from quarter wavelength transmission lines will have additional passbands when the wavelength of the resonators are three quarter of a wavelength long, five quarter of a wavelength long and so on. These harmonic resonances are normally undesirable. To avoid these harmonic stubs can be added. Figure 7.46. Hairpin filter with harmonic stubs. The hairpin filter in figure 7.46 has two second and third harmonic suppression stubs incorporated as part of the design. The design procedure consists of firstly designing and optimising a basic hairpin filter as outlined in the previous example. Second and Third harmonic suppression stubs are then designed as a separate circuit, as shown in figure 7.47. The second harmonic stubs are one-quarter wavelength long at the second harmonic and the third harmonic stubs are one-quarter wavelength long at the third harmonic. The RF Electronics: Design and Simulation 236 www.cadence.com/go/awr

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