A Selective Wideband Bandpass Filter With Wide Stopband Using Mixed Lumped-Distributed Circuits

In this brief, a wideband bandpass filter with extremely wide spurious-free stopband and high selectivity is presented. The filter consists of a wideband bandpass cell cascaded with sharp-rejection wide-stopband low-pass filters (WSLPFs), both designed using a mix of lumped and distributed elements. The proposed wideband bandpass cell, consisting of two series LC resonators and shunt-connected open-ended microstrip stubs, produces a three-pole passband with two transmission zeros on its opposite sides. The LC resonators provide suppression of spurious transmission peaks arising due to the main stub, which is further enhanced by the auxiliary stubs in the wideband cell and the WSLPFs. The design technique imbibes the effects of the self-resonating frequencies of the lumped components, and thus enable the filter to achieve the stopband extension irrespective of their size and make. A prototype of the proposed filter, designed at 1.25 GHz with 62.4% bandwidth, shows the stopband extended up to 18.6 times the center frequency with minimum rejection of 19 dB. The selectivity factor of the passband, defined as the 3-dB to 30-dB bandwidth ratio, is 0.65. The maximum filter dimension is less than a quarter of the wavelength at the center frequency, providing a miniature design.