Transformed radial stub cell embedded resonator for high performance filter applications

Parasitic passbands of the traditional resonators or bandpass filters are drawbacks for the resonator or filter to achieve wide stopband. A bandpass filter topology designed based on the embedded transformed radial stub cells is proposed for the first time. The implemented filter with center frequency ƒc=1.73 GHz and absolute bandwidth of 200MHz (fractional bandwidth of 1.2%) using transformed radial stub cell embedding demonstrates more than 45dB rejection up to 10.4ƒc i.e. 18 GHz. The measured group delay is around 3.6∼4.3 nS. The sizes of the designed fourth order bandpass filter is only 0.42 λg × 0.26 λg, (λg is the guide wavelength at center cutoff frequency) without using any lumped elements.