High-Performance Inline Bandpass Filters Using Stub-Loaded Resonators with Internal and External Frequency-Variant Couplings

In this paper, two compact four-pole filter configurations based on cascaded identical stub-loaded resonators are proposed for high-selectivity and harmonic-suppression characteristics. Both the designs employ inter-resonator frequency-variant couplings to insert transmission zeros near the upper passband edge. The stub-loaded resonators are designed with mutually-coupled loaded stubs to produce two transmission zeros below the passband. The quasi-elliptic characteristics, achieved by these zeros, enhance the filters' selectivity. Multiple transmission zeros (greater than the filter order) are achieved by realizing frequency-variant external couplings in the form of tapped feeds and novel T-shaped gap-coupled structure; thereby achieving higher out-of-band rejection and harmonic suppression. The developed prototypes for 2300–2400 MHz band show excellent selectivity factor, defined as the ratio of 3-dB to 20-dB bandwidths, of 0.68. One of the prototypes achieves harmonic suppression up to 3.6 times the center frequency with minimum rejection of 19.7 dB. The size of the larger filter is 0.27 by 0.17 times the wavelength at the center frequency.