Design of Selective Linear Phase Cross-Coupled Dielectric Resonator Filters

In this paper, a highly selective linear phase filter is designed and implemented in cross-coupled form using dielectric resonators in Ku band to be used as a channel filter in IMUX of a satellite receiver. Extreme delay flatness is required: 1.5 nS delay ripple over 60 % of the passband and 12 nS near the band edges. The initial design is carried out as 10 resonator symmetric cross-coupled filter with all ideal inverter couplings realizing two finite transmission zeros (FTZ) satisfying also the delay flatness requirements. Design is carried out on Filpro, then optimized in ADS and then on CST. The positive inverters (magnetic couplings) are realized as irises while the negative inverters (electric couplings) are realized in ridged waveguide forms which is the main novelty of this paper. However when the inverters are converted into iris and ridge waveguide forms a tilt is observed in delay exceeding the delay ripple limits. The tilt is then corrected by redesigning the filter by inserting diagonal cross-couplings also. Measured results confirm the theory.