Parametric investigation of multilayer ring resonator filters

This paper reports a parametric investigation of a compact UWB bandpass filter based on multilayer ring resonators (MRR). With the strip width, loop length and gap size fixed, the scattering parameters of the filter were simulated using CST electromagnetic simulator for various cases of differing substrate thickness and permittivity, and against the number of resonators and their orientation. Our results show that when the substrate permittivity was increased, the bandwidth and the center frequencies lowered. The bandwidth can be restored, however, by increasing the ring orientation from 90° to 180°. Decreasing the substrate thickness, however, increases the bandwidth and the return loss but the thickness below than 0.2 mm decreases the return loss. The substrate thickness also affected the insertion loss characteristics - thicknesses of more than 0.8 mm would give too much insertion loss in the passband. The return loss improved by increasing the number of rings. The output feedline position also affect the position and number of transmission zeros. Odd ring numbers will give transmission zeros at either side of the passband depending on the coupling sense, while even ring numbers can either produce zeros on both sides or none at all.