Compact Ultra-Wideband Microstrip Bandpass Filter Using SIR Approach

Planar band pass filters (BPF) with low insertion loss, compact size, high selectivity, wider bandwidth and good stop band rejection are required for the next generation mobile and satellite communication systems. In the case of the planar filter solution, electrical performance with regard to both out-of-band rejection and in-band transmission losses, as well as cost, reliability and dimensions have to be considered. By comparison, waveguide filters are bulkier, costlier and have narrow bandwidth. UWB filters covering the range of 3.1 to 10.6 GHz require high order designs that lead to high insertion loss and large circuit size for communication applications. Also, flat group delay over the entire pass band response needs to be ensured for distortion-free transmission. Traditional filter design approaches fail to meet these requirements due to tight coupling and uncontrolled non-linear frequency dispersion over the wide bandwidth of interest. The UWB BPF realized using lossy material results in high insertion loss of 6.7 dB [1]. Filter configurations using cascaded ring resonator sections with open stubs have bandwidth limitations compared to the desired requirements [2]. Coupled lines using backside aperture on ground plane for enhancing coupling and inter-digital coupled lines with multimode resonators were also used for realizing the ultra-wideband performance [3, 4]. Typically, these structures require tight fabrication tolerances and are sensitive to the proximity affect of the metal enclosures when packaged with the RF integrated circuits. Electromagnetic band gap structures and defected ground plane structures [5] are also reported but can be difficult to integrate into a planar system.