Miniaturized impedance inverter 2.4 GHz filter with optimal transmission zeroes

A compact bandpass microstrip filter with high selectivity capability is fully investigated in this article. The basic characteristics, circuit models, and design formulas are explained completely. This technique provides more freedoms for putting the transmission zeroes with negative coupling structures in appropriate frequencies and leads to highly selective, low profile and compact filters. To validate the procedure, 3 design examples based on impedance inverter (K), admittance inverter (J) and λ/4 resonators are presented. All of the filters operate at 2.4 GHz with the fractional bandwidth more than 10% but different values in source–resonator–load couplings. The latter is obtained by tilting the middle J‐inverter toward the main line. For tilt‐angle equal to 90°, more than 50 dB null‐depth has been achieved in filters stop‐band near 8% of passband edges showing the appropriate figure of merit for this symmetrical filter. Simulated results performed by CST Microwave Studio and measured results are in good agreement with the theoretical peers.

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