1.  Field of the Invention
The present invention relates in general to a dielectric ceramic composition suited for low temperature firing, and a method of preparing the same In particular, the invention is concerned with a dielectric ceramic composition for microwave applications, which can be fired at a relatively low temperature, and which is suitably used for a dielectric resonator having internal conductive layers, of a stripline type filter, for example, and with a method of preparing such a dielectric ceramic composition The present invention is also concerned with a dielectric resonator obtained by using such a dielectric ceramic composition, a dielectric filter including a plurality of such dielectric resonators, and with a method of producing the dielectric resonator or dielectric filter. 
2.  Discussion of the Prior Art
In a modern microwave telecommunication system such as a portable or automobile telephone system, there is widely used a coaxial type dielectric filter using a ceramic composition having a high dielectric constant.  The coaxial type dielectric filter has a plurality of coaxial type resonators connected to each other.  Each resonator is a cylindrical dielectric block which has inside and outside conductors formed on inner and outer circumferential surfaces of the block, respectively.  This type of dielectric filter has a limitation in reducing the size and thickness thereof due to its construction.  In view of this, there is proposed a stripline type filter of a tri-plate structure, which incorporates internal conductive layers or strips within a dielectric substrate.  In this stripline type filter, a patterned array of conductors in the form of strips are integrally embedded in the dielectric substrate so as to provide a plurality of resonators.  The thus constructed stripline type filter is comparatively compact and thin. 
In fabricating such a stripline type dielectric filter having the internal conductive layers or strips as described above, a dielectric ceramic composition must be co-fired with the internal conductive layers.  Since known dielectric ceramic compositions have a considerably high firing temperature, there is a limit to conductive materials which can be used for the internal conductive layers, thus making it difficult to employ an Ag-contained material having a relatively low conductivity resistance For instance, the firing temperature of the dielectric ceramic composition must not exceed 1000.degree.  C. when Ag-Pd or Ag-Pt alloys are used for the internal conductive layers, and, in particular, must be controlled to be around 900.degree.  C. when the conductive layers are formed solely of Ag having a low conductivity resistance.  To meet the needs, it is required to provide a dielectric ceramic composition which can be fired at a sufficiently low temperature while assuring excellent microwave characteristics. 
Among various dielectric ceramic compositions which have been proposed, a dielectric ceramic composition which contains oxides of Ba, Ti, RE (rare earth metals) and Bi is known as having a high specific dielectric constant, a large unloaded Q, and a small temperature coefficient of the resonance frequency.  This composition, however, has a problem in its firing temperature as high as 1300.degree.-1400.degree.  C., and various attempts have been made to reduce the firing temperature, by the addition of oxides of Pb, for example. 
An example of such dielectric ceramic composition is disclosed in U.S. Pat.  No.  3,811,937, wherein a calcined mixture of BaO, TiO.sub.2 and a rare earth oxide is blended with 8 to 30% by weight of a glass formulation containing CdO, PbO and Bi.sub.2 O.sub.3.  The thus prepared composition is fired at a temperature between about 982.degree.  C. and 1150.degree.  C. Another example of dielectric ceramic composition as disclosed in JP-A-59-214105 contains BaO, TiO.sub.2 and Nd.sub.2 O.sub.3 as major components, which are mixed with powders of PbO, Bi.sub.2 O.sub.3, SiO.sub.2 and ZnO. This composition is fired at a temperature between 1050.degree.  C. and 1100.degree.  C. A further example of composition as disclosed in JP-B2-4-16884 contains BaTiO.sub.3, Nd.sub.2 O.sub.3, TiO.sub.2 and Bi.sub.2 O.sub.3 as major components, to which Pb.sub.3 O.sub.4, B.sub.2 O.sub.3, SiO.sub.2 and ZnO are added in respective suitable amounts.  This composition is fired at a temperature between 1000.degree.  C. and 1050.degree.  C. A still further example of dielectric ceramic composition as disclosed in JP-A-2-44609 contains BaTiO.sub.3, Nd.sub.2 O.sub.3, TiO.sub.2, Bi.sub.2 O.sub.3 and Pb.sub.3 O.sub.4 as major components, to which 2CaO.multidot.3B.sub.2 O.sub.3, SiO.sub.2 and ZnO are added This composition is fired at a temperature between 1000.degree.  C. and 1050.degree.  C.
The known dielectric ceramic compositions as described above, which can be fired at a relatively low temperature, still has a firing temperature of around 1000.degree.  C. or higher, and thus cannot be used with internal conductors formed solely of Ag having a low conductivity resistance, or alloys consisting principally of Ag.  In fact, these compositions can be used only with internal conductors formed of Ag-Pd alloys including a relatively high content of Pd having a large conductivity resistance.  The known low firing temperature dielectric ceramic compositions, which contain a relatively large amount of Pb oxides, also have some problem upon handling thereof, in view of the toxicity of the Pb oxides. 
While some known techniques are available for lowering the firing temperature of a dielectric ceramic composition down to around 1000.degree.  C., there have been unknown such techniques as permitting the firing at a temperature lower than the melting point of Ag, i.e., 962.degree.  C., desirably at 950.degree.  C. or lower, more desirably at around 900.degree.  C.