Dielectric filter

Details Dielectric filter Dielectric filter

1999-05-21

2001-04-24

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Wave filters including long line elements

C333S205000, C333S219000, C333S235000

Reexamination Certificate

active

06222430

ABSTRACT:

BACKGROUD OF THE INVENTION
This invention relates to a dielectric filter suitable for use in high-frequency radio communication devices such as mobile phones and particularly to a miniature chip-type dielectric filter which is constructed by laminating a number of dielectric layers with electrodes sandwiched therebetween.
There is an increasing demand, in the field of mobile phones and other handy radio communication devices, for further miniaturized high frequency filters, while such filters are required to meet both a high frequency selectivity and a low insertion loss. There has been proposed as a high-frequency filter which can meet the above requirements a multilayer ceramic filter which comprises stripline electrodes as resonators (i.e., a so-called stripline filter). For such multilayer ceramic filters, reference should be made, for example, to PCT WO 96/19843 and Japanese Patent Application Laid-Open No. 312503/95. A multilayer ceramic filter of this type is advantageous in that since the dielectric constant of the ceramic material used therein is high the effective wavelength of the signal becomes shorter, so that the required length of its resonators can be shorter. As a result, the size of the filter can be reduced and the insertion loss is relatively low.
The manufacturing cost of a dielectric filter of the above-described type which uses a dielectric material of a high dielectric constant is determined mainly by the cost of the dielectric material. Therefore, in order to reduce the manufacturing cost of a dielectric filter of this type, it may be considered that the size of the filter is further decreased to thereby reduce the amount of the dielectric material used. However, since the size of the filter itself directly relates to its frequency characteristic (particularly, its centre frequency), the reduction of cost by decreasing the filter size is limited.
It may further be considered as another way of reducing the manufacturing cost of a dielectric filter of this type that the cost of the dielectric material itself is lowered. However, since principal characteristics of a filter such as a cut-off characteristic and insertion loss are determined by the dielectric material used, it is necessary for the manufacture of a filter to use a dielectric material whose dielectric constant and dielectric loss have accurately been adjusted or controlled. Consequently, the reduction of the manufacturing cost by this approach is also limited.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a miniature dielectric filter which can be manufactured at reduced cost.
It is another object of the present invention to provide a dielectric filter which has a lower insertion loss in the pass band.
It is a further object of the present invention to provide a dielectric filter which exhibits a better attenuation in the cut-off band.
It is a still further object of the present invention to provide a dielectric filter which is electromagnetically less affected by the surroundings.
In order to achieve the above objects, according to the present invention, a dielectric filter comprising a first dielectric layer, which contains at least a pair of strip-line resonators to be electro-magnetically coupled to each other, and second and third dielectric layers, which are disposed in an opposed relation to each other with the first dielectric layer sandwiched therebetween, have substantially the same dielectric constant K1 and each contains at least one shield electrode, is characterized in that fourth and fifth dielectric layers having substantially the same dielectric constant K2 are interposed between the first and second dielectric layers and between the first and third dielectric layers, respectively, and in that the dielectric constant K2 of the fourth and fifth dielectric layers is selected to be less than any one of the dielectric constant K1 of the second and third dielectric layers and the dielectric constant K3 of the first dielectric layer. In this case, the dielectric constant K2 should preferably be less than at least one third of the smaller one of the dielectric constants K1 and K3.
The dielectric filter of the above-described structure can be manufactured at lower cost by forming the fourth and fifth layers with a cheap dielectric material of a low dielectric constant such as a ceramic material containing glass to a large extent, a lacquer or the like, as compared to the conventional filter in which all the dielectric layers are formed with expensive dielectric materials of high dielectric constant. It should be noted that the use of dielectric layers of high and low dielectric constants to form a dielectric filter has been known from Japanese Patent Application Laid-Open No. 214204/97. However, the dielectric layer of lower dielectric constant of such known dielectric filter is not interposed between the dielectric layers of higher dielectric constant. In the present invention, although the relationship between the dielectric constants K1 and K3 may be any of K1=K3, K1>K3 and K1<K3, the relation K1=K3 is most preferred since the number of kinds of materials to be used is low. In this case, in certain filters for specific purposes, the dielectric constants K1 and K3 may typically be selected to be in the range from 70 to 100 and the dielectric constant K2 may typically be selected to be in the range from 5 to 25.
In the dielectric filter of the above structure according to the present invention, the fourth and fifth dielectric layers of the dielectric constant K2 which is lower than that of the first dielectric layer are disposed outwardly of the first dielectric layer having the dielectric constant K3 and containing the resonator electrodes, so that the electric field around the resonator electrodes more concentrates in the first dielectric layer, particularly between both resonator electrodes, as compared to the case where the fourth and fifth dielectric layers have high dielectric constant. In this case, the lower the dielectric constant K2 is as compared to the dielectric constant K3, the more the electric field concentrates in the first dielectric layer. As a result of such concentration, the electric field between both resonator electrodes becomes more straight, that is to say, there will be less electric field which is curved outwardly of edges of the resonator electrodes. Consequently, the electric field between the resonator electrodes is more evenly distributed without partially concentrating at edge portions of these electrodes, so that the conduction loss usually caused by such partial concentration of electric field at the edges of the electrodes is reduced, whereby the insertion loss of this filter in the pass band is decreased. For the same reason, the Q value of this filter is improved and the attenuation in the cut-off band is increased.
In the dielectric filter of the above structure, the dielectric constant K1 of the second and third dielectric layers is also selected to be greater than the dielectric constant K2 of the fourth and fifth dielectric layers which are adjacent thereto. As a result, the electric field around the shield electrodes are prevented from spreading further beyond edges of the shield electrodes, i.e., from spreading beyond the sides of this filter, so that the characteristic of this filter is less sensitive to electromagnetic effects in the outer area of the sides.
In the dielectric filter according to the invention, the pair of strip-line resonator electrodes may be provided on different planes within the first dielectric layer which are parallel to each other and spaced in a direction of thickness of the layer. Alternatively, the pair of strip-line resonators may be arranged in a spaced relation on a single plane which is located within and parallel to the first dielectric layer. Furthermore, in the filter according to the invention the first dielectric layer may further comprise capacitance electrodes capacitively coupled to the pair of strip-line resonators, respectively, and/or a

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