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RF Electronics Chapter 7: RF Filters Page 255 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Fine Tuning the Filter To shift the centre frequency, the length of all the resonators is changed slightly using the 'tuning simulation' capability of Cadence AWR DE. In addition, it is desirable to fine- tune the filter to obtain the lowest insertion loss. This is best achieved by ensuring that the filter has a low return loss. Setting the optimiser constraints on S 11 to be less than -20 dB and carrying out the first stage of the optimisation process of the filter to meet this return loss as well as the passband specifications results in the following tapping points: Resonator 1 50 Tap = 0.5147 Resonator 2 36 Tap = 0.5969 Resonator 3 36 Tap = 0.4304 Resonator 4 36 Tap = 0.5969 Resonator 5 50 Tap = 0.5147 Figure 7.72 shows the resulting frequency response of the filter after this first stage of optimisation. For a wideband filter implementation like this, the spurious response at the harmonics, particularly that at 2 GHz is unacceptable. To provide better attenuation at those frequencies, stubs need to be added to the filter. Those stubs will distort the frequency response of the filter. In order to minimise the effect of the stubs on the centre frequency of the filter, it is desirable to use two sets of stubs and vary the spacing between the two sets of stubs to provide a low return loss at the centre frequency of the filter. In addition, it is desirable to make the harmonic stubs slightly different lengths, to widen the bandwidth over which good harmonic attenuation is obtained. In addition, some change in the harmonic response is obtained by changing the length of the coupling lines. Comparing figures 7.71 and 72, for a filter with a quarter wavelength coupling line, with that of figures 7.82 and 85, for a filter with a one-eight wavelength transmission line for the coupling illustrates this effect. Figure 7.72. Frequency response of the filter after stage 1 optimisation. RF Electronics: Design and Simulation 255 www.cadence.com/go/awr