Stripline resonator filter including cooperative conducting cap and film

The present invention relates to a filter employed in a radio communication apparatus and has as an object to prevent characteristics of the filter from degradation by increasing unloaded Q of strip line resonators of the filter. For achieving the above object, the invention provides a filter which comprises a substrate (4) having first and second strip lines (5), (6) formed on a top surface and mutually coupled through an electromagnetic field and an earth pattern (2) on a bottom surface, respectively, a dielectric layer (8) laminated on the top surface of the substrate and having capacitor patterns (9), (10) formed on a top surface thereof in opposition to the aforementioned first and second strip lines (5), (6), and a metal cap (1) fitted over the dielectric layer (8) having an electrically conductive surface at least on one of top and bottom surfaces, and an electrically conductive film formed on a portion of an outer peripheral surface of the aforementioned substrate (4) and connected to the earth pattern (2) on the bottom surface thereof, wherein at least a portion of an outer periphery of the metal cap (1) is led downwardly toward the electrically conductive film, and the portion led downwardly is connected to the electrically conductive film.

TECHNICAL FIELD 
The present invention relates to a filter employed in mobile communication 
apparatuses such as cordless telephones, portable telephones and the like 
as well as a method of manufacturing the same. 
BACKGROUND ART 
A structure of this type of filter known heretofore (e.g. from 
JP-A-03-71710) is shown in FIG. 13 and FIG. 14. In FIG. 13, numerals 70 to 
76 denote green sheets of a dielectric material, wherein the green sheets 
71 and 72 are provided with electrodes 77, 78, 79, 80 for capacitors. On 
the other hand, the green sheet 74 is provided with electrodes 81 and 82 
for coils, while the green sheet 76 is provided with shielding electrodes 
83 and 84. The green sheets 70-76 shown in FIG. 13 are laminated and 
subsequently fired at such a temperature at which the electrodes 77-84 
(e.g. of silver or copper) do not make disappearance, whereby these sheets 
are integrated in such a structure as shown in FIG. 14. In FIG. 14, 
numerals 85 and 86 denote input/output terminals. Thus, in the filter 
known heretofore, capacitors are formed by the electrodes 77-80 disposed 
in opposition, while coils are formed by the electrodes 81 and 82, wherein 
the filter is constituted by these capacitors and coils. 
A problem of the prior art filter described above is seen in that 
satisfactory filter characteristics can not be obtained because no-loaded 
Q of a resonator comprising the capacitor and the coil can not be made 
high. More specifically, referring to FIG. 13, since the green sheets 70 
to 76 are allowed to be fired only at a temperature at which the 
electrodes 77-84 can not disappear, significant dielectric loss is 
incurred, as a result of which a constant indicating low loss of the 
resonator (no-loaded Q) assumes a small value. Consequently, the filter 
comprising the resonators each having low unloaded Q suffers significant 
insertion loss in the pass-band with the characteristic in the attenuation 
band being damped. Thus, it is impossible to use the filter in such 
applications in which the requirement for the characteristic requirement 
is severe. 
DISCLOSURE OF INVENTION 
Accordingly, it is an object of the present invention to prevent the filter 
characteristics from degradation by increasing no-loaded Q of the 
resonator. 
For achieving the above object, there is proposed according to the present 
invention a filter, which comprises a substrate having first and second 
strip lines formed on a top surface and mutually coupled through an 
electromagnetic field and an earth pattern on a bottom surface, 
respectively, a dielectric layer laminated on the top surface of the 
substrate and having capacitor patterns formed on a top surface thereof in 
opposition to the aforementioned first and second strip lines, and a cap 
fitted over and above the dielectric layer and having an electrically 
conductive surface at least at one of top and bottom surfaces, and an 
electrically conductive film formed on a portion of an outer peripheral 
surface of the aforementioned substrate and connected to the earth pattern 
on the bottom surface, wherein at least a portion of an outer periphery of 
the cap is led downwardly toward the electrically conductive film so that 
the portion led downwardly and the electrically conductive film are 
connected together. 
With the structure described above, because the cap is fitted over the 
dielectric layer with a space therebetween, the electric fields from the 
first and second strips concentrate in the direction toward the substrate. 
In this conjunction, as the substrate, there can be used such one which 
has previously been fired independently at a high temperature. Thus, the 
dielectric loss can be minimized, as a result of which the unloaded Q of 
the resonator formed by the first and second strip lines can be made 
extremely high, whereby the filter characteristics can be protected 
against degradation.

BEST MODES FOR CARRYING OUT THE INVENTION 
In the following, exemplary embodiments of the present invention will be 
described by reference to the drawings. 
Embodiment 1 
FIGS. 1 and 2 are perspective views showing a filter according to the first 
embodiment of the invention, as viewed from top and bottom sides, 
respectively. The top surface of the filter is covered with a metal cap 1 
while the bottom surface and both of opposite sides are covered with an 
earth pattern 2. Further, input/output terminals 3 are provided at 
portions of the bottom surface and the side surfaces which are not 
provided with the earth pattern. Now, referring to an exploded perspective 
view of FIG. 3, an internal structure of the filter will be described. In 
FIG. 3, a numeral 4 denotes a substrate having a dielectric constant of 
"100", which substrate is formed by firing, for example, porcelain of 
titanium-oxides series at a high temperature of 1300.degree. to 
1400.degree. C. On the bottom surface and opposite side surfaces of the 
substrate 4 are provided with the earth pattern 2 with the input/output 
terminals 3 being provided at the other opposite sides, wherein first and 
second strip lines 5 and 6 and a third strip line 7 are provided on the 
top surface of the substrate. The first and second strip lines 5 and 6 
have respective first ends connected to the earth pattern 2 via the third 
strip line 7, while the other ends of the first and second strip lines 5 
and 6 are opened, whereby essentially quarter-wave length resonators are 
realized. By disposing these resonators in parallel with one another and 
coupling them through electromagnetic fields, there is implemented a comb 
line filter. First and second capacitor patterns 9 and 10 are provided on 
the surface of a first dielectric layer 8 which has a dielectric constant 
of "10" and is laminated over the surface of the substrate 4. The first 
and second capacitor patterns 9 and 10 are disposed in opposition to the 
first and second strip lines 5 and 6, respectively, with the first 
dielectric layer 8 being interposed therebetween, to thereby constitute 
capacitors, respectively, wherein the outer peripheral ends of the 
capacitor patterns are connected to the input/output terminals 3, 
respectively. A second dielectric layer 11 is laminated over the top 
surface of the first dielectric layer 8 for protecting the first and 
second capacitor patterns 9 and 10. The metal cap 1 is mounted on the top 
surface of the three-layer laminated structure comprising the substrate 4 
and the first and second dielectric layers 8 and 11, whereby a filter is 
completed. Parenthetically, the metal cap 1 is manufactured by forming a 
oxygen-free copper sheet of 0.2 mm in thickness and having both surfaces 
plated with silver in a thickness of about 5 .mu.m into a box-like 
structure having an open bottom with offset portions being provided at the 
side surfaces. The top ends of the offset portions bear against the 
surface of the second dielectric layer 11 for assuring an appropriate 
height for the cap while lower offset portions are bulged outwardly to 
cover the side surfaces of the substrate 4. The lower offset portions are 
soldered to the earth pattern 2 on the side surfaces of the substrate 4 to 
thereby fixedly secure the metal cap 1 while forming a shield for the 
exterior. Further formed in the side surfaces of the metal cap 1 are 
notches la for preventing the cap 1 from contacting the first and second 
capacitor patterns 9 and 10 upon mounting of the cap 1. In the structure 
described above, because the substrate 4 has been fired at a high 
temperature of 1300.degree. to 1400.degree. C. as mentioned above, the 
substrate is in the sintered state of high density which gives rise to 
only an extremely small dielectric loss. Thus, the resonator can enjoy 
extremely high unloaded Q. 
Next, description will be directed to a method of manufacturing the filter 
by referring to FIG. 4. At first, the substrate 4 of a large size fired at 
a high temperature of 1300.degree. to 1400.degree. C. is prepared, whereon 
the earth pattern 2 and a plurality of input/output terminals 3 are 
printed on a bottom surface (not shown) of the substrate 4 by using an 
electrically conductive paste containing silver powder as a main 
component, and fired at a temperature of 850.degree. to 900.degree. C. 
Subsequently, the first to third strip lines 5, 6 and 7 are printed each 
in a plurality on the top surface of the substrate 4 by using the 
electrically conductive paste mentioned above and fired at a temperature 
of 850.degree. to 900.degree. C. In succession, the first dielectric layer 
8 is printed by using a dielectric paste prepared by mixing a dielectric 
powder of barium titanate series and glass of silicon oxide-lead series 
and fired at a temperature of 850.degree. to 900.degree. C. On the surface 
of the first dielectric layer 8, the first and second capacitor patterns 9 
and 10 are printed each in a plurality and fired, as in the case of the 
strip lines 5 to 7. Additionally, the second dielectric layer 11 is 
printed and fired, as in the case of the first dielectric layer 8. A 
laminated structure formed in this manner is cut along broken lines shown 
in the drawing into individual pieces. Thereafter, on the side surfaces of 
each piece resulting from the cutting, the earth pattern 2 and the 
input/output terminals 3 are printed, as shown in FIG. 3, by using the 
aforementioned electrically conductive paste and fired as described 
previously. In that case, the third strip line 7 and the first and second 
capacitor patterns 9 and 10 are connected to the earth pattern 2 and the 
input/output terminals 3, respectively. Subsequently, the metal cap 1 is 
fitted above on the top surface of the interim product and soldered to the 
earth pattern 2 at the side surfaces, whereby the filter shown in FIGS. 1 
and 2 is realized. Owing to the manufacturing method described above, 
there can be obtained the resonator having high unloaded Q by using the 
substrate 4 fired at a high temperature of 1300.degree. to 1400.degree. C. 
and exhibiting a very low dielectric loss. Because the other constituents 
are fired at a temperature of 850.degree. to 900.degree. C., there arises 
no possibility of the earth pattern 2, the input/output terminals 3, the 
strip lines 5 to 7 and the capacitor patterns 9 and 10 being burned away. 
FIG. 5 is a plan view showing the first strip line 5, the second strip line 
6 and the third strip line 7. The first and second strip lines 5 and 6 are 
structurally adapted to be connected to the earth pattern 2 by way of the 
third strip line 7. With this structure, the third strip line 7 is cut 
upon fragmentation into the individual pieces, as shown in FIG. 4, and may 
undergo dislocation more or less. However, since the first strip line 5 
and the second strip line 6 undergo no change in the length, the resonance 
frequency, the degree of coupling and others are less susceptible to 
dispersion whereby the filters enjoying the stable or uniform 
characteristics can be obtained. It is further noted that the first and 
second strip lines 5 and 6 are bent with the widths thereof being 
increased at junctions X with the third strip line 7. By virtue of this 
configuration, concentration of a resonant current to the junction X can 
be mitigated, whereby the unloaded Q of the resonator can be enhanced. 
Besides, the blurring of the patterns due to the printing can be 
suppressed, which contributes to the availability of the resonance 
frequency stabilized highly. 
FIG. 6 is a sectional view taken along a line B--B in FIG. 5, wherein the 
first and second strip lines 5 and 6 are shown representatively by the 
first strip line 5. When the first and second strip lines 5 and 6 are 
formed through a conventional printing process, thickness of both ends of 
the strip line as viewed in the widthwise direction unavoidably tends to 
decrease, involving excessive thinness. In that case, the resonance 
current characteristically concentrates to both end portions, as a result 
of which the electrical conduction characteristic is degraded and incurs 
deterioration in the unloaded Q of the resonator. For this reason, the 
thickness of the end portions as viewed in the widthwise direction of the 
strip line should preferably be made greater than the thickness of the 
intermediate portion, as shown in FIG. 6. To this end, a mask, for 
example, having patterns corresponding to only the first and second strip 
lines 5 and 6 is formed on the substrate 4 and then thick films are 
deposited inside of the patterns by printing. Thereafter, the mask is 
burned out. Thus, there can be obtained a strip line having such a form in 
cross-section as illustrated in FIG. 6. 
By virtue of the features described above, the strip line resonator 
employed in the filter according to the instant embodiment could enjoy 
unloaded Q of extremely high value not smaller than "200". 
Next, description will be made of operation of this filter. FIG. 7 is an 
equivalent circuit diagram of the filter now under consideration. Each of 
the first and second strip lines 5 and 6 constitutes a resonator 
substantially of quarter wavelength and can be replaced by a parallel 
resonance circuitry of L and C. In the figure, M represents 
electromagnetic-field coupling between the two resonators, wherein the 
frequency band width of a signal passing through the filter is determined 
by the degree of this coupling. A symbol Ci represents capacitors which 
are formed by the first and second capacitor patterns 9 and 10 and which 
serve for matching input impedance of the filter to an external circuit 
and at the same time bears a role to cut DC components of the signal 
supplied from the external circuit. Next, description will turn to the 
passing characteristic of the filter. FIG. 8A a sectional view of the 
filter shown in FIG. 3 taken along a line A--A, while FIG. 8B shows a 
characteristic diagram illustrating changes in the filter passing 
characteristic as a function of change in the height (hereinafter referred 
to simply as H) from the top surface of the substrate 4 to the top surface 
of the metal cap 1. As can be seen in FIG. 8B, the filter characteristic 
is such that the band width decreases as H becomes smaller. The reason for 
this will be explained below by reference to FIG. 9 which is a view for 
illustrating change of an even-mode propagation velocity ratio 
(hereinafter simply represented by Ve), an odd-mode propagation velocity 
ratio (hereinafter simply represented by Vo) and a fractional band of the 
filter. As can be seen from FIG. 9, Ve and Vo are equal to each other when 
H is 1.2 mm. When H exceeds this value, then Ve&lt;Vo and the fractional 
band-width increases, while when H is smaller than the above value, then 
Ve&gt;Vo and the fractional bandwidth decreases. This shows that because the 
internal electric field distribution varies in dependence on H to thereby 
bring about corresponding change in the relation between Ve and Vo, the 
degree of coupling M between the resonators is caused to change. More 
specifically, as the degree of coupling M becomes large, the fractional 
band width increases and vice versa. 
In general, for a high frequency filter for the mobile communication, 
extremely narrow band characteristic such that the fractional band width 
is not greater than 4% is required. With the structure described above, 
such characteristic can not be realized unless Ve.gtoreq.Vo. To this end, 
the height H of the metal cap 1 must be smaller than a height at which Ve 
equals Vo. In the case of the instant embodiment, the above-mentioned 
height H was selected to be 1.0 mm, whereby there could be realized the 
narrow band filter characteristic that the fractional band width is 3.7%, 
which is suited for the mobile communication. 
When the filter of such narrow band is implemented by employing the 
resonators exhibiting small unloaded Q, insertion loss in the pass band 
will increase significantly. In contrast, with the structure according to 
the instant embodiment, there can be made available the resonators whose 
unloaded Q is not smaller than "200", whereby the resultant filter could 
enjoy high performance such that the insertion loss is not greater than 1 
dB. 
Embodiment 2 
Next, description will be made of a second embodiment of the present 
invention. FIG. 10 is an exploded perspective view of a filter according 
to the second embodiment of the present invention and FIG. 11 is a 
characteristic diagram illustrating the passing characteristic of this 
filter. In FIG. 10, a metal cap 1, an earth pattern 2, input/output 
terminals 3, a substrate 4, a third strip line 7, a first dielectric layer 
8, first and second capacitor patterns 9 and 10, and a second dielectric 
layer 11 are implemented in structures similar to those described 
hereinbefore by reference to FIG. 3. Difference from the arrangement shown 
in FIG. 3 is seen in that there are employed first and second strip lines 
12 and 13 each having a high impedance portion of a narrow width at one 
end and a low impedance portion of a large width at the other end, wherein 
the one end of high impedance is connected to the earth pattern 2 via the 
third strip line 7 with the other end of low impedance being opened, to 
thereby realize a resonator. With this arrangement, inductance increases 
in the high-impedance portion in a relative sense while in the 
low-impedance portion, capacity increases. Thus, the length of the 
resonator can be shortened when compared with that having a uniform strip 
line width. Further, as shown in FIG. 11, by virtue of the passing 
characteristic of the filter implemented in the aforementioned structure, 
an attenuation pole can make appearance at a lower frequency in the pass 
band in dependence on the inter-resonator coupling state. Thus, the filter 
is suited particularly to applications where magnitude of attenuation at a 
low frequency in the band is required to be increased. 
Embodiment 3 
Next, description will be directed to a third embodiment of the present 
invention. FIG. 12 is an exploded perspective view of a filter according 
to the third embodiment of the invention. In FIG. 12, an earth pattern 2, 
input/output terminals 3, a substrate 4, first and second strip lines 5 
and 6, a third strip line 7, a first dielectric layer 8 and first and 
second capacitor patterns 9 and 10 are implemented similarly to those 
shown in FIG. 3. Difference from the arrangement shown in FIG. 3 is seen 
in that a shield pattern 15 is provided on the top surface of the second 
dielectric layer 14, wherein the earth pattern 2 formed on the outer 
peripheral surfaces and the shield pattern 15 are connected to each other, 
to thereby allow the metal cap 1 to be spared. Further, the method of 
manufacturing this filter differs from that of the first embodiment in 
that in succession to lamination of the second dielectric layer 14, the 
shield pattern 15 is formed on the top surface of the second dielectric 
layer 14 by printing, which is then followed by cutting into individual 
pieces, and thereafter the earth pattern 2 and the input/output terminals 
3 are provided by printing on the surfaces resulting from the cutting. By 
virtue of the arrangement described above, all the steps except for the 
cutting can be realized by printing processes, whereby significant 
reduction in the manufacturing cost can be achieved. Additionally, the 
second dielectric layer 14 is so implemented as to have a dielectric 
constant of "5" which is sufficiently smaller than that of the substrate 4 
so that the electric fields from the first and second strip lines 5 and 6 
are concentrated to the substrate 4 susceptible to the least dielectric 
loss, whereby no-loaded Q of the strip-line resonator is made high. In the 
structure described above, by setting the distance between the shield 
pattern 15 and the substrate 4 to be not greater than the distance at 
which Ve becomes equal to Vo, narrow-band characteristics of the filter 
can be enjoyed as in the case of the first embodiment. Furthermore, by 
implementing the first and second strip lines 5 and 6 such that high 
impedance portions of narrow width are formed at first end portions 
thereof with low impedance portions of large width being formed at the 
other end portions, respectively, the length of the resonator can be 
shortened while the attenuation pole can make appearance at a lower 
frequency side of the band, as in the case of the second embodiment. 
Parenthetically, it should be mentioned that in the first, second and third 
embodiments described above, the frequency adjustment is performed by 
trimming the earth pattern 2 provided at the outer peripheral surface of 
the substrate 4. The earth pattern on the outer peripheral surface is 
formed for the purpose of connecting the metal cap 1 or the shield pattern 
15 to the earth pattern 2 on the bottom surface of the substrate 4. By 
positively making use of the earth pattern 2, the frequency adjustment can 
be realized. More specifically, by trimming the earth pattern 2 at one end 
of both of the first and second strip lines 5, 6, 12 and 13 (i.e., at the 
side of the third strip line 7), inductance increases in this region, 
whereby the resonance frequency can be lowered. On the contrary, by 
trimming the earth pattern 2 at the other end, the open-end capacity 
between that other end and the earth pattern 2 can be decreased, whereby 
the resonance frequency can be increased. Besides, when the other end 
portion is trimmed, the earth pattern 2 in this region functions as 
inductance, whereby an LC series resonance circuit can be formed in 
cooperation with the open-end capacity. As a result of this, an 
attenuation pole newly makes appearance at the resonance frequency of the 
LC resonance circuit, ensuring thus excellent attenuation characteristic. 
INDUSTRIAL APPLICABILITY 
As is apparent from the foregoing, there has been provided according to the 
present invention a filter which includes a substrate having first and 
second strip lines formed on a top surface and mutually coupled through an 
electromagnetic field and an earth pattern on a bottom surface, 
respectively, a dielectric layer laminated on the top surface of the 
substrate and having capacitor patterns formed on a top surface thereof in 
opposition to the first and second strip lines, and a cap fitted from the 
above of the dielectric layer and having an electrically conductive layer 
formed at least on one of top and bottom surfaces thereof, an electrically 
conductive film formed on a portion of an outer peripheral surface of the 
substrate and connected to the earth pattern formed on the bottom surface 
of the substrate, wherein at least a part of an outer peripheral portion 
of the cap is led downwardly toward the electrically conductive film so 
that the portion led downwardly and the electrically conductive film are 
connected together. 
With the structure described above, a space is provided above the 
dielectric layer and covered with the cap. In consequence, electric fields 
from the first and second strip lines are concentrated in the direction 
toward the substrate. However, since the substrate can previously be 
prepared by firing it at a high temperature in the independent state, it 
is possible to decrease the dielectric loss. As a result of this, 
no-loaded Q of the resonators formed by the first and second strip lines 
can be made extremely high, to thereby prevent the filter characteristic 
from degradation.