Miniaturized DBR filter: Formulation and performances improvement

This paper presents the general fundamental equations for a miniaturized narrow bandpass DBR filter, where the miniaturization is achieved by loading the transmission lines with capacitors while increasing their characteristic impedance. The derived fundamental equations show the interesting performances of this miniaturized filter when compared with a classical one. Not only high miniaturizing percentage, but also better out-of-band rejection could be achieved. Hybrid classical and miniaturized DBR prototypes with a 1 GHz center frequency, based on a microstrip technology, were fabricated and measured. The obtained results show miniaturization of 75% and better filter out-of-band rejection. The increase of the insertion loss is solved by distributing the capacitance along the stub length.