Stripline filter device having a coupling dielectric layer between two stripline resonators

A stripline filter device including two or more dielectric substrate assemblies each having a pair of superimposed dielectric substrates between which at least one resonator conductor is disposed, wherein the dielectric substrate assemblies are stacked with an intermediate dielectric layer being sandwiched between the adjacent substrate assemblies so as to couple the resonator conductors in a direction perpendicular to the plane of each substrate, and by suitably setting the material and/or the thickness of each intermediate dielectric layer the coupling quantity between the resonators can be adjusted to obtain a desired frequency bandwidth.

BACKGROUND OF THE INVENTION 
The present invention relates to a stripline filter device with means for 
adjusting a coupling quantity between pairs of dielectric substrates. 
Such a stripline filter device is known, which is used as a band-pass 
filter for a microwave range. The stripline filter is becoming watched 
because it is easily made thinner or smaller than block type filters and 
suitable for meeting a requirement for a size reduction of potable 
communication equipments such as potable telephones. 
An example of such a conventional stripline filter device is illustrated in 
FIG. 1. As will be seen in FIG. 1, a plurality of stripline resonator 
conducting layers A are disposed in an interdigitated form on a dielectric 
substrate B whose outer surface is provided with a ground conductor layer 
C with which one end of each resonator conducting layer A is connected. An 
example of such a stripline filter is disclosed in U.S. Pat. No. 
4,157,517. 
With this conventional stripline filter device, if the distance W between 
the adjacent resonator conducting layers A and A on the dielectric 
substrate B is too small, a coupling therebetween becomes strong to give 
too large resonance bandwidth. If the distance W between the adjacent 
resonator conducting layers A and A is set to a larger size so as to 
obtain a desired resonance bandwidth, the filter completed increases in 
size. In order to overcome this problem it has been proposed to provide a 
groove between the adjacent resonator conducting layers for forming a gap 
therebetween which decreases the quantity of the coupling. In this 
connection, for example, Japanese Patent Kokai Nos. 61-100002 and 
61-201501 are referred. However, this method has a disadvantage that 
troublesome machining is required. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a small size 
stripline filter device capable of overcoming the problems or 
disadvantages of the conventional stripline filter arrangements, in which 
a coupling quantity can be easily adjusted. 
According to the present invention, there is provided a stripline filter 
device which comprises at least two dielectric substrate assemblies 
intended to be stacked to each other, each dielectric substrate assembly 
including a pair of superimposed dielectric substrates, at least one 
stripline resonator of a predetermined pattern interposed between the 
paired substrates and a ground conductor provided at least on the main 
portion of outer surfaces other than the surfaces on which said stripline 
resonator is arranged, and an intermediate dielectric layer having a 
dielectric constant smaller than that of each dielectric substrate, which 
is interposed between the dielectric substrate assemblies for adjusting a 
coupling quantity between the stripline resonators in the stacked 
dielectric substrate assemblies. 
Preferably, the intermediate dielectric layer may be provided with a 
connecting ground conductor by which the ground conductors of said stacked 
substrate assemblies are connected to each other. 
The thickness and/or material of the intermediate dielectric layer may be 
determined so that a desired coupling quantity is obtained. 
The stripline resonators interposed between the respective paired 
dielectric substrates are coupled with each other in a stacked direction 
so that a required filtering function is obtained. By the provision of the 
intermediate dielectric layer having a dielectric constant smaller than 
that of each dielectric substrate between the adjacent dielectric 
substrate assemblies, the quantity of coupling between the stripline 
resonators of the different paired dielectric substrates is reduced to 
provide a desired narrow frequency band. It is, therefore, possible to 
suitably adjust the magnetic coupling between the stripline resonators by 
selecting the dielectric constant and/or the thickness of each 
intermediate dielectric layer. 
The present invention will now be described by way of example with 
reference to the accompanying drawings:

DETAILED DESCRIPTION 
Referring to FIGS. 2 and 3, there is shown a stripline filter device 
according to an embodiment of the present invention. The illustrated 
stripline filter device comprises two dielectric substrate assemblies 1 
and 1'. For convenience, the similar elements of assembly 1' will not be 
discussed separately and instead will be identified with the same numerals 
as used with assembly 1, but with a prime (') added thereto. Assembly 1 
includes a pair of dielectric substrates 2a and 2b. The dielectric 
substrate 2a is provided with a ground conductor 3a on both lateral 
surfaces and rear surface except the front surface thereof. The dielectric 
substrate 2b is provided with a ground conductor 3b on the outer surface 
thereof which is exposed to the atmosphere when being assembled except the 
front surface thereof. 
On the surfaces of the dielectric substrates 2a and 2b to be superimposed 
to each other there are respectively arranged stripline resonance 
conductors 4a and 4b each having a predetermined pattern, which are 
positioned to be integrated to form a resonator when the dielectric 
substrates 2a and 2b are superimposed. Rear ends of the resonance 
conductors 4a and 4b are connected with the ground conductors 3a and 3b to 
form short-circuit terminals, respectively, while front ends of the 
resonance conductors 4a and 4b are extended toward the front edge of the 
respective substrates to form open-circuit terminals, respectively. Also, 
the ground conductors 3a and 3b are extended to both lateral edge portions 
on the surfaces to be superimposed of the dialectic substrates 2a and 2b 
so as to form connecting edge conductors 5a and 5b, respectively. The 
ground conductor 3a is also extended to both lateral edge portions on the 
opposite surface of the dielectric substrate 2a so as to form connecting 
edge conductors 6a. 
For example, the resonator (4a and 4b) in one substrate assembly 1 is 
connected with an input or output terminal (not shown) via a connecting 
capacitor not shown, while the resonator (4a' and 4b') in the other 
substrate assembly 1' is connected with an output or input terminal (not 
shown) via a connecting capacitor not shown. 
In FIGS. 1 and 2, reference numeral 7 denotes an intermediate dielectric 
layer for reducing the coupling quantity between the resonators in the 
substrate assemblies 1 and 1'. This dielectric layer 7 is made of 
dielectric material having a dielectric constant smaller than that of each 
substrate 2a or 2b and as shown in FIG. 1 is provided with ground 
conductors 8 which is formed on both the lateral edge portions of the 
dielectric layer 7. 
When being assembled the dielectric layer 7 is sandwiched between the 
substrate assemblies 1 and 1' and the ground conductors 3a' and 3b in one 
substrate assembly 1 is electrically connected with the ground conductors 
3a and 3b' in the other substrate assembly 1' by means of the ground 
conductor 8 in the sandwiched dielectric layer 7. 
The substate assemblies 1 and 1' are assembled by sandwiching the 
intermediate dielectric layer 7 therebetween, and then is integrally 
sintered. For this end, it is desirable that each dielectric substrate and 
the intermediate dielectric layer may be made of dielectric material which 
can be sintered at the same temperature. For example, the substrates 2a 
and 2b may be made of BaO--Nd.sub.2 O.sub.3 --TiO.sub.2 --Bi.sub.2 O.sub.3 
having a dielectric constant of 90, and the intermediate dielectric layer 
7 may be made of dielectric material having a dielectric constant of about 
5-50 which is obtained by mixing Mg, Ca, Ba, Ti or the like into the 
material used for each substrate. In this connection, alternatively or 
additionally the magnetic coupling quantity can be adjusted by properly 
setting a thickness of the intermediate dielectric layer 7. 
Instead of sintering the substrate assemblies 1 and 1' and the intermediate 
layer 7 may be combined with each other by using cream solder or the like 
after forming the ground conductors, the connecting conductors and the 
resonance conductors thereon. 
With the arrangement mentioned above, the resonator conductors in the 
substrate assemblies 1 and 1' are coupled in a stacked direction and it is 
possible to perform an adjustment in such a direction that the coupling is 
obstructed by the intermediate dielectric layer 7, thereby securing a 
predetermined coupling quantity and a required frequency bandwidth 
characteristic. 
In the illustrated embodiment the filter device comprises two dielectric 
substrate assemblies. However, three or more dielectric substrate 
assemblies may used to construct a filter device. Also, no ground 
conductor may be formed on each of the surfaces to be superimposed of the 
adjacent substrate assemblies. Alternatively, when each substrate assembly 
is prepared, the ground conductor previously formed on the surface of each 
substrate which is to be abutted on the dielectric layer may be removed. 
Furthermore, the illustrated arrangement may be modified by using an 
interdigitated or comb type stripline resonator. 
As described above, according to the present invention two or more 
dielectric substrate assemblies each having a pair of superimposed 
dielectric substrates between which at least one resonator conductor is 
disposed are stacked with an intermediate dielectric layer sandwiched 
between the adjacent substrate assemblies so as to couple the resonator 
conductors in a direction perpendicular to the plane of each substrate. 
Therefore, by suitably setting the material and/or the thickness of each 
intermediate dielectric layer the coupling quantity between the resonators 
can be adjusted to obtain a desired frequency bandwidth. 
As compared with the conventional arrangement in which a groove is formed 
between the resonators for adjusting a frequency response, the filter 
device according to the present invention can be easily manufactured, and 
also permits the distance between the resonator conductors to be reduced 
by selecting of the material and/or thickness of the intermediate 
dielectric layer. Therefore, the present invention can provide a stripline 
filter device having a thickness which can reduce as thin as possible and 
can fully meet for the requirement of size reduction for potable 
communication equipments such as portable telephones. 
It is to be understood that the above-mentioned embodiments are only 
illustrative of the application of the principles of the present 
invention. Numerous modifications and alterations may be made by those 
skilled in the art without departing from the spirit and scope of the 
invention, and the appended claims are intended to cover such 
modifications and alterations.