Balanced bandpass filter with reflectionless common-mode suppression

A compact balanced bandpass filter using two types of LC resonator and quarter-wavelength resonator is proposed in this paper. By taking the advantage of the property of quarter-wavelength resonator and symmetry of the proposed circuit, a bandpass response can be achieved while the band stop response can be achieved at the same frequency band. Parallel LC resonator shunt to the ground is used for the differential bandpass response and series LC resonator shunt to the ground via a resistor is used for the common-mode suppression response. For the purpose of a non-reflected suppression of the common mode noise, a resistor matching to the port impedance is connected to the end of a series LC resonator. By doing so, a single frequency point common-mode noise suppression can be achieved at the desired band. The proposed circuit is analyzed and experimented carefully. The validity of this structure is demonstrated by the experiment result on a 2.4GHz filter.