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RF Electronics Chapter 7: RF Filters Page 257 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 7.74 shows the frequency response of the final filter after optimisation, it also shows the final optimisation limits used. To determine the reliability of the design technique, four filters were constructed. The measured frequency response of those filters is also shown in figure 7.74. It can be seen that the design technique results in highly repeatable filters, whose measured performance agrees remarkably with the results from computer simulation. The agreement between the measured and simulated results for these filters is much better than those for the hairpin filter of figure 7.46, or the interdigital filter of figure 7.65 and 7.66. It should be noted that the right most resonator is significantly larger than the other resonators. This is required to compensate for the effects of the harmonic stubs on the passband response. Microstrip Filter Comparison When designing a Microstrip Bandpass filter, it can be designed as: an interdigital filter, a Combline Filter, a hairpin filter or a direct-coupled resonator filter [14]. Each of these filters have advantages and disadvantages. To highlight the comparative performance of these filters, a 5-resonator filter with a centre frequency of 1 GHz and a 70 MHz bandwidth was designed as using each of these four filter types. The filters were optimised using the same optimisation limits. Each filter has 10 mm tracks connecting to the tapping point to allow a connector to be soldered to that track. The same RO4003 substrate was used and all resonators are 3 mm wide. For each of the filter types, a simple test circuit like those shown in figures 7.60, 7.61 or 7.68 is used. The tapping and coupling test circuit for the Combline filter, is shown in figure 7.75 and that for the hairpin filter is shown in figure 7.76. The input tapping for all these four circuits is adjusted to obtain frequency responses like figures 58 and 59, but with the bandwidths as per equations 29 and 30 for the relevant filter. Since the centre frequency, bandwidth and the number of resonators for these filters is the same as those of figures 58 and 59, exactly the same responses should be obtained. The coupling tuning, can be done most accurately when the end resonator loading is reduced as done in figures 7.75 and 7.76. Figure 7.75. Resonator loading and coupling test circuit for Combline filters. PORT P=1 Z=50 Ohm MLSC ID=TL8 W=Wr mm L=0.1 mm MLSC ID=TL7 W=Wr mm L=0.1 mm MLIN ID=TL6 W=W50 mm L=5 mm W W 1 2 3 4 MCLIN ID=TL2 W=Wr mm S=Scrq mm L=Lct-0.1 mm W W 1 2 3 4 MCLIN ID=TL1 W=Wr mm S=Scrq mm L=Lcr-Lct-W50-0.1 mm PORT P=2 Z=1000000 Ohm MLIN ID=TL5 W=Wr mm L=W50 mm MSUB Er=3.38 H=0.8128 mm T=0.035 mm Rho=0.7 Tand=0.0027 ErNom=3.38 Name=RO/RO1 Scrq=10 1 2 3 MTEEX ID=TL10 MLEFX ID=TL3 W=Wr mm L=0.1 mm MLEFX ID=TL9 W=Wr mm L=0.1 mm Scrc=1.8 MLIN ID=TL6 W=W50 mm L=5 mm MLSC ID=TL9 W=Wr mm L=0.1 mm MLSC ID=TL8 W=Wr mm L=0.1 mm MLIN ID=TL5 W=Wr mm L=W50 mm W W 1 2 3 4 MCLIN ID=TL2 W=Wr mm S=Scrc mm L=1 mm W W 1 2 3 4 MCLIN ID=TL1 W=Wr mm S=Scrc mm L=Lcr-0.1-W50-1-0.1 mm MSUB Er=3.38 H=0.8128 mm T=0.035 mm Rho=0.7 Tand=0.0027 ErNom=3.38 Name=RO/RO1 MLEFX ID=TL10 W=Wr mm L=0.1 mm 1 2 3 MTEEX ID=TL4 MLEFX ID=TL3 W=Wr mm L=0.1 mm PORT P=2 Z=1000000 Ohm PORT P=1 Z=50 Ohm RF Electronics: Design and Simulation 257 www.cadence.com/go/awr

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