High-Selectivity Wideband Filter With Ultra-Wide High-Attenuation Stopband Region Using Mixed Lumped-Distributed Circuits

This paper presents design of a wideband bandpass filter exhibiting high passband selectivity and ultra-wide stop-band with high rejection levels. The design is based on mixed lumped and distributed elements. A wideband cell, consisting of two series LC tank circuits coupled through an open-ended microstrip stub, provides a quasi-elliptic passband with wide bandwidth. The tank circuits suppress the harmonic passbands of the open stub to a certain extent, which is enhanced by integrating sharp-rejection low-pass sections at the access ports of the wideband cell. The novel design of the low-pass sections enable achieving high out-of-band rejection levels. The composite filter exhibits ultra-wide stopband on account of multiple transmission zeros introduced by the constituent circuits. A prototype filter, designed at the center frequency of 0.528 GHz with 3-dB bandwidth of 72.5%, shows rejection better than 42 dB from 1.12 GHz to 6.2 GHz (138.8 %). The measured selectivity factor, defined as the ratio of 3-dB to 30-dB bandwidths, is 0.735. The size of the filter is 0.128 by 0.096 times the wavelength at the center frequency.