Method of adjusting a frequency response in a three-conductor type filter device

Abstract: A method of adjusting a frequency response in a filter device of a three-conductor type having a pair of stacked dielectric substrates (11,12) with a plurality of strip­ line resonator conductors (15,16,17) being sandwiched therebetween, wherein the frequency adjusting of the filter is performed by partially removing the ground conducting layer (13) provided on the peripheral portion of each dielectric substrate (11,12) at regions (13a,b;14a,b) corresponding to the open circuit ends (15a,16a,17a), or to the open circuit ends (15a,16a,17a) and the short circuit ends (15b,16b,17b) of the resonator conducting layers (15,16,17).

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of adjusting a frequency response in a filter device of three-conductor type which may be used as a band-pass filter for example. It is known to provide a filter device of three-conductor type which is utilized as a band-pass filter for a microwave range. An example of such a conventional filter device is illustrated in Figs. 1 and 2. As will be seen in Figs. 1 and 2, 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 2 , BaO-TiO 2 -rare earth or the like. The lower dielectric substrate 1 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 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 dispertions 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 embeded in the dielectric substrates. One solution to this problem has been proposed in US Patent No. 4,157,517. According to the adjusting method disclosed in this US 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 4 provided on the upper surface of the upper substrate 2 is partially removed at regions 8 adjacent the open circuit ends of the resonator conducting layers 5, 6 and 7 to reduce the capacitance between the external conducting layer 4 and the respective resonator conducting layers and to increase the response frequency of the filter thereby making it possible to adjust the frequency. However, with this adjusting method, when the assembled filter is to be contained in an outer casing 9 after the adjustment of the frequency response is made, the removed regions 8 for the frequency adjustment of the upper surface of the upper dielectric substrate 2 come close to or come into contact with the upper wall of the outer casing 9 because the removed regions 8 are positioned on the upper surface of the upper dielectric substrate 2. Therefore, the stray capacitance may be changed from the adjusted value so that the frequency response may be deviated. For this reason, if the above mentioned adjusting method is applied, the outer casing should be so designed that it has an inner height larger than the height of the filter assembly and the upper surface of the upper dielectric substrate 2 is sufficiently spaced from the upper wall of the casing 9 as will be seen in Fig. 2. Recently, various equipments or elements adapted for use in a microwave range become thinner and it is thus demanded that the filter devices as well as the elements should be constructed in a thinner configuration or dimension. However, such a demand for a thinner construction cannot be satisfied by utilizing the above mentioned adjusting method in which a casing having a larger inner height is necessarily used. Document WO-A-8500929 discloses a microwave device comprising a bandpass filter, the device being formed from a block of dielectric material coated with a layer of electrically conductive material. This device is provided with a number of holes plated with electrically conductive material and forming resonator tubes. Such a filter device does not belong to the type (with embedded stripline resonator conducting layers) of the present invention. Consequently, the method of adjusting the frequency response in said device cannot be compared with the one of the present invention, although the cited document indicates the principle of removing the coating conductive material at some portions for tuning purposes. As an important difference, it should be observed that in the device of document WO-A-8500929, fine frequency adjustment requires partially removing material on the inside of the holes, for adjusting the length of resonator conducting layers. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method of adjusting a frequency response of a filter device of a three-conductor structure type in which there can be compensated any variation in the frequency which may occur when the filter is contained in a casing, without the inconveniences mentioned hereabove, and without removing internal conducting layers. According to the present invention, there is provided a method of adjusting a frequency response of a filter device of a three-conductor structure type comprising a pair of dielectric substrates which are stacked to each other upon the assembling of the filter, each having a peripheral and outer surfaces provided with an external ground conductor layer, and a plurality of stripline resonator conducting layers sandwiched between said dielectric substrates, each of the resonator conducting layers having a predetermined lenght and having a short circuit end connected to the external ground conductor layer, on one lateral surface of said each dielectric substrate, and an open circuit end positioned to be spaced from the external ground conductor layer on the other lateral surface of said each dielectric substrate which is opposite to said one lateral surface, wherein the method comprises the step of partially removing said external ground conductor layer on the peripheral surface of said each dielectric substrate at a portion which corresponds to the open circuit end of said each resonator conducting layer while maintaining said predetermined length of each resonator conducting layer, whereby tuning the filter device to a desired frequency response. According to one embodiment of the present invention, the method comprises a further step of partially removing said external ground conducting layer on the peripheral surface of said each substrate at a portion which corresponds to the short circuit end of said each resonator conducting layer so as to compensate for any overshoot of the tuning provided by said first step. According to a further embodiment of the present invention, the dielectric substrates each have a peripheral surface provided with a plurality of recesses, said method comprising the step of partially removing said external ground conducting layer on the peripheral surface of each said substrate defining the recesses at a portion which corresponds to the open circuit end of said each resonator conducting layer so as to increase the response frequency of the filter device to a desired level. By removing partially the external ground conducting layer on the peripheral surface of each substrate at a portion which corresponds to the open circuit end of each resonator conducting layer, the capacitance between each removed portion and the associated open circuit end of each resonator conducting layer is reduced. The present invention will now be described by way of example with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective partially cutaway view showing a prior art three-conductor type filter device; Fig. 2 is a longitudinal section showing the filter device of Fig. 1 contained in a casing; Fig. 3 is a perspective partially cutaway view schematically showing a filter whose frequency response is adjusted in accordance with one embodiment of the present invention; Fig. 4 is a longitudinal section showing the filter device of Fig. 3 contained in a casing; Fig. 5 is a perspective partially cutaway view schematically showing a filter whose frequency response is adjusted in accordance with another embodiment of the present invention; Fig. 6 is a longitudinal section showing the filter device of Fig. 5 contained in a casing; Fig. 7 is a perspective partially cutaway view schematically showing a filter whose frequency response is adjusted in accordance with a further embodiment of the present invention; Fig. 8 is a longitudinal section showing the filter device of Fig. 7 contained in a casing; Figs. 9 and 10 are graphes showing the frequency responses of the filter before the frequecy adjustment is made; Fig. 11 is a graph showing the frequency response of the filter adjusted in accordance with the present invention. DETAILED DESCRIPTION Figs 3 and 4 show a three-conductor type filter constructed in accordance with one embodiment of the present invention. The illustrated filter comprises 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 2 , BaO-TiO 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. In this connection, 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 dimensions of resonator conducting layers 15, 16 and 17 are determined so that the resonance frequency of the filter becomes slightly lower than the intended one as shown in Fig. 9. Each resonator conducting layer has one end as an open circuit end (15a, 16a and 17a) spaced from the ground conducting layer 13 and the other end as 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 upper dielectric substrate 12 is fixed on the lower dielectric substrate 11, and the ground conducting layers 13 and 14 of the respective dielectric substrates are connected to each other. The resonator conducting layers 15 and 17 have lateral extensions 15c and 17c, respectively. One of the lateral extensions 15c and 17c is connected to a signal input terminal, not shown, and the other extension is connected to a signal output terminal, not shown. With the three-conductor type filter thus constructed, in order to compensate any deviations in the dielectric constants of the used dielectric substrates 11 and 12 and in the length of each resonator conducting layer it is necessary to adjust the frequency of the filter after the dielectric substrates 11 and 12 are assembled with the resonator conducting layers sandwiched therebetween. To this end, 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 of each resonator conducting layer so as to reduce the capacitance between each removed portion and the associated resonator conducting layer. This removing operation may be performed by means of a cutting tool, a laser beam machining, a sand blasting or the like. In this way, the filter can be tuned to a desired frequency response. That is, as shown in Fig. 9, the filter has a center frequency f 1 which is slightly lower than a desired response frequency f 0 before the frequency adjustment is made. By removing the portions 13a and 14a of the external ground conducting layers 13 and 14 which correspond to the open circuit ends 15a, 16a and 17a of the respective resonator conducting layers 15, 16 and 17, the center frequency f 1 is shifted toward a higher frequency zone so that it becomes identical with the desired response frequency f 0 as shown in Fig. 11. Then, the three-conductor type filter thus adjusted to the desired frequency response is contained in a casing 18 as shown in Fig. 4. The casing 18 may be metal, and has an inner height equal to the height of the filter and a width larger than that of the filter. By selecting the dimension of the casing 18 in this way, the filter device can be constructed without any substantial increasing of the height, and the peripheral portion of the filter can be prevented from bringing into contact with the inner surface of the casing 18. In this case, even if the width of the casing 18 is set larger than that of the filter, the requirement for thinner dimension for electronic circuit elements can be effectively satisfied. Figs. 5 and 6 illustrate another embodiment of the present invention in which an additional adjusting means is provided for shifting the center frequency of the filter toward a lower frequency zone. As shown in Fig. 10, there may occur that the center frequency f 2 of the filter is shifted over the desired center frequency f 0 by the provision of the removed portions 13a and 14a on the external ground conducting layers 13 and 14 provided on the peripheral surface of the respective substrates 11 and 12 in accordance with the first embodiment. In order to compensate this overshoot, in this embodiment the external ground conducting layers 13 ans 14 provided on the peripheral surfaces of the substrates 11 and 12 are partially removed at portions 13b and 14b contacted with the short circuit ends 15b, 16b and 17b of the resonator conducting layers 15, 16 and 17. This removing operation may also be performed by means of a cutting tool, a laser beam machining, a sand blasting or the like as in the case of forming the removed portions 13a and 14a. Therefore, the capacitance between each removed portion and the associated resonator conducting layer is reduced, so that the center frequency f 2 is shifted toward a lower frequency zone so that it becomes identical with the desired response frequency f 0 as shown in Fig. 11. With the illustrated embodiments shown in Figs. 3 to 6, the upper dielectric substrate 12 may also be provided with a transmission line pattern of resonator conducting layers on the lower surface, which is disposed to have a reflected image relation with respect to the stripline pattern of the resonator conducting layers 15, 16 and 17 on the lower dielectric substrate 11. When being assembled the stripline pattern on the lower dielectric substrate 11 comes into face-to-face contact with the transmission line pattern on the upper dielectric substrate 12 without occurring any gaps between the lower dielectric substrate 11 and the upper dielectric substrate 12. Further, 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. Figs. 7 and 8 shows a further embodiment in which rectangular recesses 20 are provided on the portions of the dielectric substrates 11 and 12 which are opposite to the open circuit ends 15a, 16a and 17a and the short circuit ends 15b, 16b and 17b of the resonator conducting layers 15, 16 and 17. In this embodiment the adjustment of the frequency response can be performed by partially removing the portions of the ground conductor layers 13 and 14 provided on these recesses 19 as designated by 13c and 14c. When the filter is inserted into the casing 18 the removed portions 13c and 14c can be spaced from the inner end surfaces 18a and 18b of the casing 18, and thus the portions of the dielectric substrates 11 and 12 exposed through the removed portions 13c and 14c can be prevented from bring into contact with the associated inner end surface of the casing 18. Therefore, the capacitance between the ground conductor layer and the associated resonator conducting layer is not changed when the filter is inserted into the casing 18, and consequently, the frequency response of the filter can be stably maintained at the desired level without necessity of any readjustment. With this embodiment, alternatively, each of the inner end walls 18a and 18b of the casing 18 is outwards protruded at regions faced to the portions to be removed for the frequency adjustment so as to form inner recesses, thereby preventing the portions of the dielectric substrates 11 and 12 exposed through removed portions from bring into contact with the associated inner end surface of the casing 18. As described above, according to the present invention the frequency adjusting of the filter is performed by partially removing the ground conducting layer provided on the peripheral portion of each dielectric substrate at regions corresponding to the open chircuit ends, or to the open circuit ends and the short circuit ends of the resonator conducting layers. Therefore, since the outer conductor of the filter is not removed at regions which are to be abutted on the inner surface of a casing as in the case of the conventional filter device, the present invention has an advantage that there is no variation or deviation in the set frequency characteristic of the filter when the filter device is completed by inserting the filter into the casing. Further, the present invention has also an advantage that a frequency adjustment can be correctly made without increasing the thickness or height of the casing.

Claims: A method of adjusting a frequency response of a stripline filter device of a three-conductor structure type comprising a pair of dielectric substrates (11, 12) which are stacked to each other upon the assembling of the filter, each having a peripheral and outer surfaces provided with an external ground conductor layer (13, 14), and a plurality of stripline resonator conducting layers (15, 16, 17) sandwiched between said dielectric substrates (11, 12), each of the resonator conducting layers (15, 16, 17) having a predetermined length and having a short circuit end (15b, 16b, 17b) connected to the external ground conductor layer (13, 14) on one lateral surface of said each dielectric substrate (11, 12), and an open circuit end (15a, 16a, 17a) positioned to be spaced from the external ground conductor layer (13, 14) on the other lateral surface of said each dielectric substrate (11, 12) which is opposite to said one lateral surface, characterized in that the method comprises the step of partially removing said external ground conductor layer (13, 14) on the lateral surfaces of said each dielectric substrate (11, 12) at a portion (13a, 14a) which corresponds to the open circuit end (15a, 16a, 17a) of said each resonator conducting layer ( 15, 16, 17) while maintaining said predetermined length of each resonator conducting layer (15, 16, 17), whereby tuning the filter device to a desired frequency response. A method as claimed in claim 1, characterized in that the method further comprises the step of partially removing said external ground conducting layer (13, 14) on the lateral surfaces of said each substrate (11, 12) at a portion (13b, 14b) which corresponds to the short circuit end (15b, 16b, 17b) of said each resonator conducting layer (15, 16, 17) so as to compensate for any overshoot of the tuning provided by said first step. A method as claimed in claim 1, wherein the dielectric substrates (11, 12) each have a peripheral surface provided with a plurality of recesses (19), characterized in that said method comprises the step of partially removing said external ground conducting layer (13, 14) on the lateral surfaces of each said substrate (11, 12) defining the recesses (19) at a portion (13c, 14c) which corresponds to the open circuit end (15a, 16a, 17a) of said each resonator conducting layer (15, 16, 17) so as to increase the response frequency of the filter device to a desired level. A method as claimed in any of claims 1, 2 and 3, wherein said each removing step is performed by using a cutting tool, laser beam machining, or sand blasting.

Inventors: ITO KENJI, SHIMIZU HIROYUKI, ITO, KENJI, SHIMIZU, HIROYUKI, ITO, KENJI , B-TO 603, PARK CITY TORIMI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO. LTD.
Application year: 1990
Main classification: H01P1
IPC codes: H01P1, H01P11