A Tunable Bandpass Filter Using the Stepped Impedance Resonator with Harmonic Suppression

Stepped impedance resonator with harmonic suppression and capability of frequency tuning have been incorporated into a bandpass filter design. A tapped-in U-shaped feed structure is introduced in the design to suppress harmonic and to reduce the passband insertion losses. The passband flexibility is accomplished by using a pair of tuning varactors. A simple method to implant a zero in the frequency response is also studied herein to further deepen the pass-band skirt. A 163% (4 GHz) stopband bandwidth is observed under over 20 dB band rejection. A measured minimum insertion loss of 1.27 dB is observed for the circuit with absence of the tuning varactors. Using the varactors, this filter has achieved a 21.9% frequency tuning range with all the insertion losses less than 5 dB.