Fourth-order Electrical Tunable Microstrip LC Cross-coupled Bandpass Filter

Electronically tunable/reconfigurable bandpass filters with varied passband are the key components for multi-band/mode radio systems. Cross-coupling topology is usually used to generate transmission zeros for improving the passband selectivity. In this paper, we propose and develop a novel fourth-order electrical tunable bandpass filters with cross coupled topology based on microstrip LC tunable resonators. Electrical coupling structure and magnetic coupling structure are designed to build the cross coupled topology. SMV 1405 varactor diode is used to tune the resonator. The effect of the parasitic resistance of the varactor diode to the insertion loss of the filter is studied. The filter is simulated and measured. The measurement shows that, a pair of transmission zeros are generated beside the passband due to the cross-coupling topology. The filter can be tuned with a central frequency range of 2 GHz to 2.45 GHz, -3 dB fractional bandwidth of 5.19% to 6.75%, and insertion loss of 3.15-6.85 dB. The maximum IIP3 is 28 dBm under 30 V bias, and the minimum IIP3 is 4.5 dBm under 0 V bias.