Quasi-Input Reflectionless Bandpass Filter with Quasi-Elliptic Response and Controllable Transmission Zeros Using Coupled Lines

This paper presents a detailed RF design methodology for quasi-elliptic bandpass filter (BPF) with quasi-input reflectionless behaviour in both their passband and stopband regions. The proposed input reflectionless BPF consists of microstrip coupled lines and transmission lines, comprising three sections: a BPF, a transmission zero (TZ) and an input matching sections. A quasi-elliptic transfer function is achieved by generating TZs at lower and upper sides of passband without affecting the passband response or input reflectionless behaviour. The TZs can be located close to the passband for achieving sharper power absorption ratio profiles within the stopband-to-passband transitions. The theoretical foundation and design methodology of the proposed quasi-elliptic input reflectionless BPF are described. To validate the proof-of-concept, a prototype was manufactured and measured at center frequency of 3.5 GHz. The measured results are fair agreement with the EM simulations and theoretically predicated results.