New Ultra-Wide Stopband Low-Pass Filter Using Transformed Radial Stubs

A new type of low-pass filter (LPF) with ultra-wide band rejection and compact size by using a proposed transformed radial stubs (TRSs) is introduced and investigated. The operating mechanism of the filter is investigated based on proposed equivalent-circuit model. The implemented LPF with 1-dB cutoff frequency fc of 3 GHz demonstrates stopband rejection up to 8 fc i.e., 24 GHz. The skirt selectivity is achieved up to -144 dB per octave through multiple-zero generation using TRSs. The measured passband insertion loss is less than 1.5 dB. I/O loading cells are used to further extend the deep stopband to more than 13 fc i.e., above 40 GHz. The size of the four-cell LPF is only 0.31 λg × 0.24 λg, (λg is the guide wavelength at center cutoff frequency) without using any lumped elements.