Method of adjusting a frequency response in a stripline filter device

A method of adjusting a frequency response in a stripline filter device having a pair of stacked dielectric substrates with a plurality of stripline resonator conductors being sandwiched therebetween, wherein the frequency adjusting is performed for each resonator conductor under the condition that the resonator conductor(s) adjacent to one to be determined is electrically connected via a fine strip member to the ground layer, thereby correctly tuning the frequency response of each resonator conductor without any influence of the other resonator conducting layers.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of adjusting a frequency response 
in a stripline filter device which may be used as a band-pass filter for 
example. 
Such a stripline filter device is known, which is utilized as a band-pass 
filter for a microwave range. An example of such a conventional stripline 
filter device is illustrated in FIG. 1. As will be seen in FIG. 1, it 
comprises a lower dielectric substrate 1 and an upper dielectric substrate 
2 which are stacked to each other. Each of the dielectric substrates 1 and 
2 may be of dielectric ceramic material having a high dielectric constant 
and a lower dielectric loss such as BaO--TiO.sub.2, BaO--TiO.sub.2 -rare 
earth or the like. The lower dielectric substrate I is provided with an 
external ground conducting layer 3 on the peripheral portion and bottom 
surface thereof. Similarly, the upper dielectric substrate 2 is provided 
with an external ground conducting layer 4 on the peripheral portion and 
upper surface thereof. On the upper surface of the lower dielectric 
substrate 1 are disposed a plurality of stripline resonator conducting 
layers 5, 6 and 7 which form a filter element. Each resonator conducting 
layer has one end or an open circuit end (5a, 6a and 7a) spaced from the 
ground conducting layer 3 and the other end or a short circuit end (5b, 6b 
and 7b) connected to the ground conducting layer 3. The open circuit ends 
5a, 6a and 7a of the respective resonator conducting layers 5, 6 and 7 are 
alternately disposed so as to form an interdigitated configuration. The 
upper dielectric substrate 2 is fixed on the lower dielectric substrate 1, 
and the ground conducting layers 3 and 4 of the respective dielectric 
substrates are connected to each other. 
As well known in the art, the filter device of this type has a frequency 
response which depends on the configuration and dielectric constant of the 
substrates, and the dimension of the resonator conductors. Upon the 
manufacturing of the filter device the dielectric constant Of the 
substrates and the size of the resonator conducting layers are strictly 
determined. However, it can not be avoided that there may occur any 
dispersions in the dielectric constant of the substrates and in the 
dimension of the resonator conducting layers. It is, therefore, necessary 
to adjust the frequency response of the filter device after being 
completed. 
The adjustment of the frequency response can not be performed by adjusting 
the length of the resonator conducting layers because they are embedded in 
the dielectric substrates. One solution to this problem has been proposed 
in U.S. Pat. No. 4,157,517. According to the adjusting method disclosed in 
this U.S. patent, the frequency of the filter is previously set at a lower 
level than a desired one, and the external conductor or ground conducting 
layer provided on the upper surface of the upper substrate is partially 
removed at regions adjacent the open circuit ends of the resonator 
conducting layers to reduce the capacitance between the external 
conducting layer and the respective resonator conducting layers and to 
increase the response frequency of the filter thereby making it possible 
to adjust the frequency. 
This previously proposed adjusting method are extremely useful for the 
frequency response characteristic of the stripline filter device. However, 
as the number of the resonator conducting layers to be provided is 
increased, the frequency response characteristics of the respective 
resonator conducting layers intricately interact with each other, thus 
involving much difficulty for individually discerning and properly 
adjusting each frequency response characteristic. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method of 
adjusting a frequency response of a stripline filter device in which the 
frequency response characteristics of respective resonator conducting 
layers can be individually discerned and can be properly adjusted. 
Another object of the invention is to provide a stripline filter device 
assembled by using the frequency response adjusting method according to 
present invention. 
According to one aspect of the present invention, there is provided a 
method of adjusting a frequency response of a stripline filter device 
which comprises a pair of dielectric substrates each having a peripheral 
and outer surfaces provided with an external ground conducting layer, and 
a plurality of stripline resonator conducting layers sandwiched between 
the dielectric substrates, each resonator conducting layer having a short 
circuit end connected to the ground conducting layer on one lateral 
surface of each substrate and an open circuit end spaced from the ground 
conducting layer on the opposite lateral surface of each substrate, 
wherein it comprises the steps of electrically connecting the open circuit 
end of one or more specific resonator conducting layers to the external 
ground conducting layer on the peripheral surface of each substrate by 
means of a fine strip member, assembling the dielectric substrate with the 
resonator conducting layers therebetween, adjusting the frequency response 
characteristics of the resonator conducting layers provided with no fine 
strip member at the open circuit ends thereof, and then adjusting 
sequentially the frequency response characteristics of the resonator 
conducting layers each provided with the fine strip member by 
disconnecting the associated fine strip member so as to separate the open 
circuit ends from the external ground conducting layer. 
By electrically connecting the open circuit ends of the specific resonator 
conducting layers to the external ground conducting layer via fine strip 
members, each of these resonator conducting layers respectively has no 
longer resonator function and then will act as an electrical barrier. 
With the method of the present invention, firstly, one adjusts the 
frequency response characteristics of the resonator conductor layers whose 
open ends are not connected to the external ground conducting layer via 
the fine strip members. In case all the resonator conductor layers are 
provided with the fine strip members for electrically connecting the open 
ends thereof to the external ground conducting layer, one cuts off the 
fine strip member from the desired resonator conductor layer and then 
adjusts the frequency response characteristic thereof. In this case, if 
the frequency is changed with result of the adjustment, the adjusted 
waveform of the resonator conductor layer is not affected by the adjacent 
resonator conductor layers because they have the fine strip members 
provided on the open ends thereof and the will function as the electrical 
barriers. This in turn allows a frequency response characteristic to be 
properly adjusted. The adjustment of the frequency of each resonator 
conductor layer may be carried out by removing partially the ground 
conducting layer on each substrate as is conventionally known. 
In this way, by sequentially removing the fine strip members provided on 
the open ends of the respective resonator conductor layers and adjusting 
the frequency response characteristics thereof, it is possible to tune the 
filter device for a desired frequency response. 
According to a second aspect of the present invention, there is provided a 
stripline filter device comprising a pair of dielectric substrates having 
a peripheral and outer surfaces an external ground conducting layer 
provided on the peripheral and outer surfaces of said each dielectric 
substrate, a plurality of stripline resonator conducting layers sandwiched 
between said dielectric substrates, each resonator conducting layer having 
a short circuit end connected to said ground conducting layer on one 
lateral surface of said each substrate and an open circuit end spaced from 
said ground conducting layer on the opposite lateral surface of said each 
substrate, and a fine strip member for electrically connecting the open 
circuit end of at least one of said resonator conducting layers with the 
external ground conducting layer, each fine strip member being 
disconnected when the frequency response of the resonator conducting layer 
associated therewith is adjusted 
The present invention will now be described by way of example with 
reference to the accompanying drawings.

DETAILED DESCRIPTION 
FIGS. 2 and 3 show a stripline filter for which the present invention is 
applied. 
The illustrated filter comprises a lower and upper dielectric substrates 11 
and 12 which are stacked to each other upon the assembling of the filter. 
Each of the dielectric substrates 11 and 12 may be of dielectric ceramic 
material having a high dielectric constant and a lower dielectric loss 
such as BaO--TiO.sub.2, BaO--TiO.sub.2 -rare earth or the like. The lower 
dielectric substrate 11 is provided with an external ground conducting 
layer 13 on the peripheral portion and outer surface thereof. Similarly, 
the upper dielectric substrate 12 is provided with an external ground 
conducting layer 14 on the peripheral portion and upper or outer surface 
thereof. 
On the upper or inner surface of the lower dielectric substrate 11 are 
provided a plurality of stripline resonator conducting layers 15, 16 and 
17 which form a filter element of an interdigital type. Each resonator 
conducting layer has one end or an open circuit end (15a, 16a and 17a) 
spaced from the ground conducting layer 13 and the other end or a short 
circuit end (15b, 16b and 17b) connected to the ground conducting layer 
13. The open circuit ends 15a, 16a and 17a of the respective resonator 
conducting layers 15, 16 and 17 are alternately disposed so as to form an 
interdigital type resonator. 
The resonator conducting layers 15 and 17 have lateral extensions 15c and 
17c, respectively. These lateral extensions 15c and 17c is connected to 
signal terminals not shown, respectively. 
The open circuit end 16a of the resonator conducting layers 16 is 
temporally and electrically connected to the ground conducting layer 13 
via a fine strip member 18. This fine strip member 18 is so constructed 
that it can be easily removed at a frequency adjusting procedure and does 
not affect the characteristic of the resonator conducting layers 16. 
Similarly, on the lower or inner surface of the upper dielectric substrate 
12 may also be provided a plurality of stripline resonator conducting 
layers 15, 16 and 17 which are disposed to have a reflected image relation 
with respect to the resonator conducting layers 15, 16 and 17 on the lower 
dielectric substrate 11. When being assembled the resonator conducting 
layers 15, 16 and 17 on the lower dielectric substrate 11 becomes into 
face-to-face contact with those on the upper dielectric substrate 12 
without occurring any gaps between the lower dielectric substrate 11 and 
the upper dielectric substrate 12. The ground conducting layers 13 and 14 
of the respective dielectric substrates are connected to each other. 
The upper dielectric substrate 12 is also provided with recesses or notches 
19 through which the lateral extensions 15c and 17c on the lower 
dielectric substrate 11 are extended so that they are prevented from bring 
into contact with the external ground conducting layers 13 and 14. 
With the filter device thus constructed, it is substantially unavoidable 
that there may occur any deviations in the dielectric constants of the 
used substrates and/or in the dimension of the resonator conducting layers 
upon the manufacturing, which results in that the frequency response of 
the completed filter may be deviated from an intended one. Therefore, the 
frequency response of the filter should be adjusted when being completed. 
As shown in FIG. 5, firstly a reflection characteristic signal having a 
waveform S11 from the resonator conducting layer 15 is measured via the 
lateral extension 15c. As shown in FIG. 6, if the measured waveform S11 
(shown by a dotted line) is different from a predetermined value (shown by 
a solid line), the adjustment is then carried out for that resonator 
conducting layer 15 in such a manner that the waveform S11 can be 
corrected into the curve shown by the solid line. This adjustment can be 
done by removing partially the external ground conducting layers 13 and 14 
on the substrates 11 and 12. 
That is, if the measured waveform S11 has a center frequency lower than the 
desired one as shown in FIG. 6, the external ground conducting layer 
provided on the peripheral surface of each substrate is partially removed 
at a portion (13a and 14a) which corresponds to the open circuit end 15a 
of the resonator conducting layer 15 so as to shift the center frequency 
toward a higher frequency zone. Contrarily if the center frequency of the 
measured waveform S11 is higher than the desired one the external ground 
conducting layer may be partially removed at a portion (13b and 14b) which 
corresponds to the short circuit end 15b of the resonator conducting layer 
15 so as to shift the center frequency toward a lower frequency zone. 
During this frequency adjusting procedure for the resonator conducting 
layer 15, the open circuit end 16a of the central resonator conducting 
layer 16 is held being connected to the external ground conducting layers 
13 and 14 via the fine strip member 18, and thus, the central resonator 
conducting layer 16 functions as an electrical barrier. As a result, the 
waveform of the frequency response characteristic of the resonator 
conducting layer 15 can be prevented from being subjected to any influence 
of the central resonator conducting layer 16 and the other side resonator 
conducting layer 17. In consequence, there can be obtained a genuine 
waveform for the resonator conducting layer 15, and thus, the frequency 
response characteristic of the resonator conducting layer 15 can be 
correctly performed. 
Next, as shown in FIG. 7, there is measured waveform S22 of a reflection 
characteristic signal from the resonator conducting layer 17 via the 
lateral extension 17c, and then the adjustment of the frequency 
characteristic therefor is performed in the same way as described 
hereinbefore so that the measured waveform S22 becomes identical with the 
desired one shown by a solid line in FIG. 8. 
Then, as shown in FIG. 4 through the external ground conducting layers 13 
and 14 a hole 20 is provided at the portion corresponding to one end of 
the fine strip member 18, thereby cutting off it. As a result, there can 
be materialized the state shown in FIG. 9, in which a reference numeral 
S21 designates a transmission characteristic of the filter. The hole 20 
may be provided by means of a laser beam trimming or a rotary whetstone. 
Finally, again one measures the reflection characteristic signal waveform 
S11 or S22, and then properly adjusts the resonance frequency 
characteristic of the central resonator conducting layer 16 in the same 
manner as described in the above. In this connection, since the adjustment 
has already been performed for the resonator conducting layers 15 and 17 
on both sides, readjustment therefor is not needed at all. 
In this way, the filter can be tuned to a desired frequency response. 
With the illustrated arrangement, the upper dielectric substrate 12 is 
provided with recesses or notches 19 for preventing the lateral extensions 
15c and 17c from bring into contact with the external ground conducting 
layers 13 and 14. However, these recesses 19 may be omitted if the lateral 
extensions 15c and 17c are extended so that they do not make contact with 
the external ground conducting layers 13 and 14. 
Further, the resonator conducting layers on the upper dielectric substrate 
12 may be omitted if necessary. 
Furthermore, the stripline pattern of the resonator conducting layers 15, 
16 and 17 may be formed as a comb type in which the open circuit ends and 
the short circuit ends thereof are disposed at the same sides, 
respectively. In that case, the centrally positioned resonator conducting 
layer should be connected via the fine strip member to the external ground 
conducting layer. 
The above description has merely referred to the stripline filter device 
having three resonator conducting layers as an embodiment of the present 
invention. It should be however understood that the scope of the invention 
is not confined to the number of available resonator conducting layers. 
If the stripline filter is provided with a pair of resonator conducting 
layers, then the open circuit end of one of these two resonator conducting 
layers remains being connected with the external ground conducting layer 
via a fine strip member. In that case, after adjusting the frequency 
characteristic of the other resonator conducting layer, the fine strip 
member provided on the open circuit end of one resonator conducting layer 
can be cut off and then the frequency adjustment can be performed for this 
resonator conducting layer. 
Furthermore, if there is provided a stripline filter device which comprises 
four or more resonator layers, it is possible to preliminarily provide all 
the resonator layers with fine strip members, and frequency adjustment for 
each resonator layer may be sequentially performed by cutting off the 
associated fine strip member. 
As described above, according to the present invention the frequency 
adjusting is performed for each resonator line under the condition that 
the resonator conductor(s) adjacent to one to be determined is 
electrically connected via the fine strip member to the ground layer, and 
thus the present invention has an advantage that during the frequency 
adjusting for each resonator line there can be avoided any influence of 
the other resonator conductor(s). 
Further, the present invention has also an advantage that it is possible to 
easily and correctly tune the frequency response of the filter device even 
if the number of the resonator lines is increased. 
It is to be understood that the present invention is not limited to the 
particular embodiments described and that numerous modifications and 
alterations may be made by those skilled in the art without departing from 
the spirit and scope of the invention.