Whispering gallery traveling wave interferometer for low phase noise applications

Traveling wave filters utilizing sapphire resonators excited at whispering gallery modes (WGR) have a good perspective for application as a microwave interferometer in the field of ultralow phase noise microwave sources. Our paper examines in detail a classical approach of a traveling wave (TW) disk dielectric WGR coupled with a dielectric waveguide. The TW WGR design is obtained by means of direct electrodynamic 3D FEA. The permittivity properties of WGR were assumed to be the same as for sapphire while its tgδ was artificially increased ten times to simplify the FEA procedure. During analysis, an S-parameter matrix of the structure was investigated. It was found that in a TW regime the transmission coefficient of the TW WGR has a bridge-type frequency response similar to that which is realized with the resonator reflected wave extraction using a ferrite circulator. The input reflection coefficient of the structure is found to have a relatively low magnitude. On the other hand, the analysis revealed strong dependence of the TW regime realization on dimensions of the primary dielectric waveguide and coupling parameters.