Quasi-Elliptic Coupled-Line-Based Balanced Bandpass Filters with Ultra-Wide Stopband Characteristics

This paper presents the RF design of new types of balanced quasi-elliptic bandpass filters (BPFs). They are based on modified capacitive-loaded coupled line configurations and exhibit: i) highly-selective quasi-elliptic power transmission response in the differential (DD) mode of operation with ultra-wide stopband suppression and ii) ultra-wide common (CC) mode suppression. The proposed filter concept is presented by means of linear circuit analysis. For proof-of-concept demonstration purposes, a microstrip prototype operating at a center frequency of 3.45 GHz was designed, manufactured and measured. The proposed filter exhibits the following RF performance characteristics: 3-dB fractional bandwidth= 4.3%, > 20 dB out-of-band rejection from DC-3.24 GHz and 3.64–16 GHz, and >11 dB of CC mode suppression from DC to >16 GHz. Good agreement is observed between simulated and measured S-parameters.