Wave transmission lines and networks – Resonators – Dielectric type
Patent
1996-06-12
1997-11-25
Lee, Benny
Wave transmission lines and networks
Resonators
Dielectric type
333235, H01P 710
Patent
active
056916779
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to the field of microwave resonators and specifically a tunable resonator for microwave oscillators and filters.
DESCRIPTION OF THE RELATED ART
As known, the more conventional microwave resonators consist of simple cavities enclosed by metal walls. With the appearance of low-loss ceramic materials it has become possible to use in the microwave resonators dielectric bodies of varying forms of which the most widely used is cylindrical. The operation of dielectric resonators, also termed DR below, is based essentially on the reflection phenomenon which an electromagnetic wave undergoes when it strikes the separation surface between two materials having different dielectric constants.
Theoretically, it is not necessary to enclose the dielectric resonators in metal walls because the resonance frequencies of the excited modes depend principally on the geometrical form and dimensions of the resonator. In practice however, to avoid irradiation of electromagnetic energy and to obtain physically usable devices the DRs are positioned in closed metal cavities.
The use of ceramic materials with high dielectric constant has made very advantageous the use of dielectric resonators in the realisation of microwave filters and oscillators. Indeed, since because of the high dielectric constant the electromagnetic field tends to remain confined mostly with the DRs, it has been possible to reduce the sizes and obtain greater miniaturisation of the circuits. In addition, the low temperature coefficients of the ceramic ensure greater temperature stability in comparison with circuits employing conventional resonators.
In view of the above, a microwave filter provided by using dielectric resonators in accordance with the known art comprises generally a metal cavity in which are located one or more cylindrical dielectric resonators arranged in accordance with an appropriate direction. Coupling between the filter and external circuits is achieved by means of various devices, e.g. coaxial probes, loops, irises, wave guide sections, etc., whose position and orientation are designed to optimise performance for the resonant mode used.
It is also known that in industrial applications of filters it is often essential to be able to change the resonance frequency of the individual dielectric resonators with a tuning operation simple to implement, e.g. to be able to recover the resonance frequency changes caused by machining tolerances.
For this purposes two different tuning methods are known for dielectric resonators.
A first method consists of modifying the volume of the metal cavity containing the dielectric resonators at points where the energy density of the resonant mode is high. The resulting deformation of the electromagnetic field present outside the DR causes a change of resonance frequency of the resonant modes excited in the resonators. From the theory it is known that the resonance frequency of an electromagnetic mode in a cavity increases when the volume of the cavity is reduced by a quantity dV if in the volume dV the energy of the electric field predominates in relation to the magnetic field and decreases in the contrary case. The amount of the frequency variation is proportional to dV and to the difference between the local electrical and magnetic energies. This amount depends thus on the mode considered and the point where the cavity deforms.
In practice, the change in volume of the cavity is achieved by introducing into the cavity metallic material in the form of screws or plates such as for example in the resonator described in U.S. Pat. No. 5,008,640 in which the tuning is changed by introducing screws in the side wall of the metal cavity.
The main disadvantage of this first tuning method lies in the fact that in order for the tuning achieved to be sufficient it is necessary to act where the energy density of the mode to be tuned is highest. This in the generality of cases is not always easy nor effective. A second disadvantage is that the current induced on th
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Snyder, R.V., "Dielectric Filters with Wide Stopbands", IEEE Transactions on Microwave Theory and Techniques, p.2100-2103(Nov. 92).
"Dielectric Resonators", ed. Darko Kajfez and Pierre Guillon, ARTECH House Inc., 1986, index and pp.3,138,161-164, 1986.
De Maron Lino
Urciuoli Riccardo
Gambino Darius
Greenberg Laurence A.
Italtel SpA
Lee Benny
Lerner Herbert L.
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