Ring filters and their application to new measurement technique on inherent-ring-resonance frequency

A ring filter is proposed as a wide-banded filter. It consists of a ring and two short stubs that are connected at the 90° and 270° points of the ring. Since the characteristic impedances of all the transmission line sections have an effect on the bandwidths of the ring filters, their relations are studied. On the basis of the study, one having more than 100% fractional bandwidth is selected and tested. The measured results are in good agreement with the simulated ones. As an application of the ring filter, a new and simple method to determine an inherent-ring-resonance frequency is introduced. The ring filter may be seen in such a way that two filters are connected in parallel. Therefore, if the two powers of the two filters are out of phase at the output, the excited power cannot be delivered. That can be used for determining the inherent-ring-resonance frequency and its condition is derived. Also, an effective loss tangent at a frequency of interest may be obtained correctly using the method suggested. To verify the method, two ring filters have been fabricated in microstrip lines and the measured results also show good agreement with the predicted ones.