Embedded Transformed Radial Stub Cell for BPF With Spurious-Free Above Ten Octaves

An embedding cell approach for design of a filter with ultrawide-band rejection and compact size is introduced and investigated. A bandpass filter (BPF) topology design based on the transformed radial stub (TRS) cell embedded resonator (CER) is proposed for the high-performance filter design. The implemented 1.7-GHz BPF with a fractional bandwidth of 1.2% using the TRS CER demonstrates more than 45 dB rejection up to 10.8f⁰ i.e., 18 GHz. The implemented 1.9-GHz BPF with fractional bandwidth 21% demonstrates 40-dB rejection from 4 to 19 GHz (10f₀). The measured passband insertion loss is 2.7 dB, and group delay is around 2.5 nS. The size of the 1.9-GHz fourth-order filter is only 0.408g×0.33 λ_g, (λ_g is the guide wavelength at center cutoff frequency) without using any lumped elements.