Balanced dual-band bandpass filter using microstrip resonator loaded with lumped-elements

We present a balanced dual-band bandpass filter (BPF). The proposed filter is established by using half-wavelength resonators loaded with a lumped series resonator. To verify the operation of the proposed resonator, the differential-and common-mode equivalent circuits are extracted and studied. Moreover, to enhance the common-mode suppression, a lumped resistor is added to the half-wavelength resonator with the lumped series resonator. Based on the proposed circuit, the desired differential-mode dual-band response is easily realized and the level of common-mode noise is minimized simultaneously. Two-pole balanced dual-band BPF with center frequencies of 1.3 GHz and 2.75 GHz is designed and the measured results agree well with simulated results.