L-section matching with notches and its application for composite lowpass filter with spurious signal suppression

L-section matching circuits are used for various applications. Such circuits can be advanced to provide notch responses to suppress undesired spurious signals. The design guide that allows integration of notch responses in the L-section matching circuits is presented. With given impedance at the targeted matching frequency and notch frequencies, a matching circuit with the spurious signal suppressing notches can be simply designed based on the closed-form equations provided. A composite lowpass filter (LPF) based on the image parameter method is composed of two matching sections and two filter sections. Replacing conventional matching sections with the proposed L-section matching circuits, an LPF with spurious signal notches can be realised. The proposed matching circuit and filter with spurious suppression notches will bring an added dimension of design freedom for modern communication systems that demand stringent out-of-band requirements.