Narrowband coupled-line bandstop filter with absorptive stopband

This paper presents a new type of absorptive bandstop filter (ABSF) design that is targeted for absorbing the local-oscillator-to-RF leakage of mixer in a heterodyne receiver. The proposed ABSF is designed to absorb the input power at its Port 1, and it is achieved by introducing one lossy resonator to a conventional narrowband coupled-line bandstop filter. Suitable equivalent circuit models and closed-form design equations are established to enable the direct synthesis of proposed ABSFs. Specifically, a proposed microstrip ABSF with a stopband center frequency f₀ of 2 GHz is demonstrated. The maximal stopband rejection is 34.8 dB with an input return loss of 27.7 dB, which implies that 99.8% of the input power is dissipated by the proposed ABSF.