The present invention relates to a ceramic microwave filter having improved edge steepness, which has at least two resonators in a one-piece ceramic body and coupling structures associated with the resonators, for capacitively coupling the ceramic filter to an external radio-frequency circuit.
DE 197 41 147 A1 describes a ceramic microwave filter comprising a ceramic body in which two mutually parallel inner conductor bores with a round cross section are constructed as resonators. The ceramic body itself comprises two sintered green compacts pressed together, and is therefore seen to that extent as being one piece although the two green compacts form different dielectrics. The ceramic body also has coupling structures which are formed by electrical isolation of the metallic coating which is otherwise applied over the whole surface to the ceramic body. This isolation can be undertaken mechanically or by an etching technique or photo-resist technique.
Furthermore, DE 197 41 177 A1 describes a ceramic microwave filter which is formed from a standard ceramic body having coupling structures constructed on its surface and two inner conductor bores with a circular cross section as resonators, these two inner conductor bores having steps which serve to steepen the flanks of the forward characteristic of the ceramic microwave filter. The ceramic body of this ceramic microwave filter is also metallized on all sides with the inclusion of the inner surfaces of the inner conductor bores, the coupling structures being electrically isolated from this metal coating. Such ceramic microwave filters are preferably used as mobile radio filters in mobile radio telephones (mobiles).
Mobile radio sets and, specifically, mobile radio telephones are intended to have as high an output power as possible in order to be able to ensure a maximum output power in mobile radio sets, transmitting filters should be distinguished by insertion losses that are as low as possible, on the one hand, and transmission noise in a receiving band should be minimized by transmitting filters with a high selectivity, on the other hand. Thus, overall there is a need for a transmitting filter which has a high selectivity in conjunction with low insertion losses.
In the case of ceramic microwave filters, there is a fundamental relationship between insertion loss or the quality of the filter and the volume of the ceramic body of the filter. The larger the ceramic body made from a highly dielectric ceramic, the lower the insertion loss, since the quality of the line resonators coupled in the ceramic body is proportional to the volume of the ceramic body. This means that a reduction in the insertion loss can be achieved only via an increase in the volume of the ceramic body. However, this is opposed by the requirement that the ceramic microwave filters should be fashioned as small as possible for the purposes of mobile radio, in order likewise to be able to fashion the size of a mobile radio telephone to be as small as possible. It is to be noted that the selectivity can be H achieved only by increasing the pole number or by a complicated coupling of the resonators of the ceramic microwave filters with coils and capacitors to form a band-stop filter.
It is accordingly an object of the invention to provide a ceramic microwave filter having improved edge steepness which is distinguished by a lower insertion loss at the band edge, and thus by a correspondingly high edge steepness.
With the foregoing and other objects in view there is provided, in accordance with the invention, a ceramic microwave filter having a characteristic curve with an improved edge steepness. The ceramic microwave filter includes a one-piece ceramic body with at least two resonators and with coupling structures associated with the resonators for capacitively coupling the ceramic filter to an external RF circuit. At least one counter-oscillator is configured in the ceramic body approximately in parallel with the resonators.
If appropriate, it is also possible to provide two or more counter-oscillators, it being preferred for each resonator to be assigned a counter-oscillator. The resonators themselves can be rectangular in cross section, since given such a cross section, the unloaded quality, and thus the insertion loss of the ceramic microwave filter, can be further improved.
The ceramic body is completely metallized except for its raised end face, the coupling structures being isolated in the usual way from this metallic coating.
The two resonators which, as explained, preferably have a rectangular cross section, are excited capacitively via the coupling structures. The counter-oscillators are weakly excited, in order to render the insertion loss of the frequency at which the xe2x80x9ccounter-notchxe2x80x9d forms in the characteristic curve as highly selective as possible. Specifically, at the same time the insertion loss at the band edge is then low, with the result that the edge steepness is correspondingly high.
The edge steepness and selectivity can be further increased by use of a second counter-oscillator when the xe2x80x9ccounter-notchxe2x80x9d is selected to be approximately the same for both resonators, Another possibility consists in providing the counter-oscillator such that both filter edges of the ceramic microwave filter are steepened.
It is also possible, in principle, to use more than two resonators, although the insertion loss then rises slightly. However, a further improvement in the edge steepness can also be achieved here by using appropriate counter-oscillators.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a ceramic microwave filter having improved edge steepness, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.