Low Voltage Tunable Narrow Bandpass Filter using Cross-Coupled Stepped-Impedance Resonator with Active Capacitance Circuit

This paper proposes a tunable bandpass filter using cross-coupled SIRs(stepped-impedance resonators) with active capacitance circuit. To use low varactor bias voltage, the resonator with 36.8 degree( 0.1 lambda) electrical length is used. For good insertion loss, active capacitance circuit is used. For high selectivity and constant bandwidth, cross coupling is utilized and the SIR configuration is used for enhanced stop-band performances. The proposed tunable bandpass filter with 2.54% of fractional bandwidth at 1.85 GHz was fabricated and tested. The measured results show 1.8GHz-1.95GHz tunable range, constant bandwidth, high selectivity, enhanced stop-band and low insertion loss. Bias voltage of the varactors is up to 3.3 V.