1. Field of the Invention
The present invention generally relates to the field of communication systems. More particularly, the present invention relates to a dual mode planar filter for use in high-frequency signal processing devices used in communication systems.
2. Description of the Related Art
High frequency resonating filters are essential in the field of high-frequency communication systems. In particular, the field of mobile communication systems requires filters able to efficiently use the frequency band. Further, in base stations for mobile communications, filters having little loss, compact size and durability against a large electric power are desirable.
A wide variety of high-frequency resonating filters are known in the art.
For instance, in U.S. Pat. No. 5,136,268, a dual mode microstrip resonator usable in the design of microwave communication filters is disclosed. The substantially square resonator provides paths for a pair of orthogonal signals, which are coupled together using a perturbation located in at least one corner of the resonator. The perturbation can be introduced by notching the resonator or by adding a metallic or dielectric stub to the resonator.
The Applicant has observed that the above described filter can have problems due to the fact that electric current tends to concentrate at the corners of the resonator to considerably increase resistance loss therein. This can lead to a degradation of the Q-value of the resonator and therefore and increased loss in the filter.
In U.S. Pat. No. 5,172,084, planar dual mode filters are formed by a conductive resonator having circular symmetry and two pairs of symmetrically oriented planar conductive leads. The conductive leads are aligned colinearly with two orthogonal diameters of the circular conductive resonator. A perturbation located on a region axis oriented symmetrically with respect to the two pairs of conductive leads couples electromagnetic modes which are injected into the resonator by the planar conductive leads. Higher order filter circuits can be realized by combining multiple filters. The filters are amenable to printed circuit (microstrip or stripline) fabrication using superconductors for the conductive elements.
However, the Applicant has also observed that these type of filters can have problems due to the fact that an excessive concentration of electric current can occur at the edges of the perturbation, leading to a degradation of the Q-value of the resonator and increased loss in the filter.
In U.S. Pat. No. 6,239,674, a resonator having high Q-value is disclosed. The resonator has a compact structure with little loss caused by the conductor's resistance. The conductor of elliptical shape forming the resonator has two points along its circumference at which both of the two orthogonal resonating modes of the resonator are excited equally.