The invention relates to a dielectric double-mode resonator used in radio-frequency filters.
High-frequency filters, such as radio-frequency filters, are used to implement high-frequency circuits in the base stations of mobile networks, mobile phones and other radio transceivers. Possible radio-frequency filter applications include the adapter circuits and filter circuits of transmitter and receiver amplifiers.
In telecommunications applications in particular, good performance in a desired operating range, temperature stability and a small size are required of radio-frequency filters. These properties can be achieved using dielectric resonators, the frequency properties of which, such as the resonance frequency, can be influenced by the structure of the resonator, the physical dimensions of the resonator and the resonator material, for instance.
The operation of a dielectric resonator is based on the reflection of electromagnetic waves from the boundary between a material having a high dielectric constant and a material having a low dielectric constant, such as air. A simple dielectric resonator is formed of a disc-like structure made of dielectric material, whose outer sheath and the air surrounding the outer sheath form a boundary reflecting electromagnetic waves. The disc-like structure can be replaced by a thick planar structure, in which the thickness of the plane is in the same range as the lengths of the sides of the plane. The structures described above can be used to form a typical one-mode resonator that produces as its first mode the TE01xcex4 resonance mode, also called basic mode, that is produced when a radio-frequency electromagnetic field is directed to the resonator.
The disc-like structure is typically made by compressing powdery ceramic material into a desired form in a mould, after which the compressed article is sintered at a high temperature.
The size of a high-frequency filter can be significantly reduced using a double-mode resonator as the resonating element. A double-mode resonator has two primary modes and secondary modes, the resonances of the primary modes being utilized in a high-frequency filter and the impact of the secondary modes being eliminated by external filters, for instance. The resonance modes can be generated for instance by combining two one-mode resonators in such a manner that a connection is established between the one-mode resonators. The connection is established for instance by means of two substantially similar disc-like structures, in which the discs are positioned crosswise. The double-mode resonator is then formed of two structural resonators, each of which functions unconnected as a separate resonator, but which can have common structural parts. This type of double-mode resonator can be made in the same manner as a one-mode resonator, but a drawback of the obtained double-mode resonator is a poor separation of the secondary modes from the primary mode of the filter, which has a weakening effect on the frequency response of the filter. The separation of the primary modes from the secondary modes can be improved substantially by making openings in the disc-like structure, whereby an empty space is formed between the crosswise-positioned disc-like structures. The manufacturing of a double-mode resonator of this kind is, however, not possible by one-stage compression molding, and complex milling techniques are required.
In prior-art solutions, the above double-mode resonator equipped with an empty space is formed of three parts in such a manner that one of the structural resonators of the double-mode resonator is formed of a uniform disc-like structure having an opening and the other structural resonator is formed by joining side sections to the sides of the uniform disc-like structure to form the side walls of the opening of the second structural resonator. The first structural resonator is then formed of the uniform disc-like structure having an opening and the second structural resonator is formed of a total of three parts: two side sections and a section of the uniform disc-like structure.
In one prior-art solution, the double-mode resonator is formed of two structural resonators that differ from each other, the difference being caused by the structures of the parts forming the double-mode resonator: the first structural resonator is made up of a uniform structure, whereas the second structural resonator comprises three parts having boundaries between them that separate the second resonator and affect the frequency response of the second resonator. The frequency response of the double-mode resonator is then very sensitive to errors occurring during the installation of the parts and to the effects of the fastening mechanism of the parts.
It is an object of the invention to implement a dielectric double-mode resonator in such a manner that the manufacturing of the double-mode resonator becomes simple and reliable.
This is achieved by a dielectric double-mode resonator of a radio-frequency filter that comprises a block structure comprising at least two resonator structures, each having at least one resonance mode, the block structure also comprising a cavity wall limiting a cavity at least partly inside the block structure and the cavity affecting the resonance modes of the at least two resonance structures. The block structure of the dielectric double-mode resonator of the invention comprises, set against each other: a first block that comprises at least part of the at least two resonator structures and at least part of the cavity wall, and a second block that comprises at least part of the at least two resonator structures and at least part of the cavity wall.
Preferred embodiments of the invention are set forth in the dependent claims.
The invention is based on the fact that the dielectric double-mode resonator is formed of the two pre-compression-molded and sintered blocks, each comprising at least part of two resonator structures and at least part of the cavity wall of the double-mode resonator. The use of two blocks forms a significant manufacturing engineering advantage in relation to the prior art, because the invention streamlines the assembly of the double-mode resonator. In addition, operational advantages of the double-mode resonator are achieved, because the boundaries between the blocks affect homogeneously the frequency properties of both resonator structures, whereby said boundaries mainly affect the resonance frequencies, but the impact on the coupling of the resonance modes is low.