Noise filter and noise filter array

A noise filter includes a plurality of LC parallel resonant circuits having a plurality of coils which are connected in series and electrically connected to external electrodes at both ends thereof, and capacitors which are connected in parallel to the coils, respectively, and are disposed inside an insulator while being sequentially connected in tandem to signal wires. Resonance frequencies of the respective LC parallel resonant circuits are preferably different from each other. Further, a shield electrode is disposed between the coils, and the shield electrode also defines a capacitance-forming electrode for preventing magnetic coupling between the two coils.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a noise filter for effectively removing noise flowing in a signal wire disposed on a circuit board, and a noise filter array.

2. Description of the Related Art

Depending on the communication system of a portable telephone, for example, a single portable telephone may use a plurality of communication bands. In order to prevent degradation of reception sensitivity in each of the communication bands, it is necessary to effectively remove noise in each frequency band.

Known examples of a noise filter used for such noise removal include a choke coil, a ferrite bead, and a ladder-type LC filter.

When the choke coil mentioned above is used as the noise filter, noise countermeasures can be easily implemented because noise can be removed by simply connecting the choke coil to each signal wire. However, the choke coil can only remove noise at a specific frequency because the band for which noise removal can be performed is relatively narrow, which disadvantageously makes it difficult to remove noises in a plurality of frequency bands at the same time.

Further, when a ferrite bead is used, as in the case of the choke coil, noise countermeasures can be easily implemented because noise can be removed by simply connecting the ferrite bead to each signal wire. However, since a ferrite bead removes noise even in a low frequency band, it exerts a large influence on the signal waveform such as by attenuating a necessary signal. Further, since high attenuation cannot be attained, it may be impossible to achieve a satisfactory noise removal effect.

Further, the ladder type LC filter mentioned above comes in various types, such as a T type, a π type, or an L type. Although any one of the above-described types of ladder type LC filter can provide wide-band noise removal characteristics through appropriate setting of the inductance and capacitance, since it is necessary to ground an external electrode connected to a capacitor, it is essential to form a grounding electrode pattern on a circuit board to which the ladder type LC filter is mounted. This disadvantageously limits the freedom of wiring layout on the circuit board.

Further, while a plurality of signal wires are formed on a circuit board involving high-density mounting, depending on the component layout, it may be difficult to form grounding electrode patterns having a sufficient line width together with these signal wires. As a result, due to the influence of a parasitic inductance in the grounding electrode patterns, the frequency characteristics of the ladder type LC filter change, which disadvantageously makes it impossible to remove noise in a satisfactory manner.

On the other hand, in the related art, there has been proposed a noise filter constructed as follows (see, for example, Japanese Unexamined Patent Application Publication No. 5-267059). That is, the noise filter includes a filter element composed of one trap circuit formed by the inductance of a coil, which is composed of a plurality of coil conductors laminated in a spiral fashion within a dielectric, and a floating capacitance between the coil conductors. On either side of this element, a filter element composed of one trap circuit formed by the inductance of a coil, which is composed of a plurality of coil conductors laminated in a spiral fashion within a magnetic material, and a floating capacitance between the coil conductors, is arranged, and these filter elements are integrated with each other to thereby form the noise filter.

According to this noise filter, the resonance frequency of the trap circuit constituting each of the filter elements is set to correspond with each of a plurality of communication bands, thereby making it possible to remove noise in each of the communication bands.

However, in the noise filter described in Japanese Unexamined Patent Application Publication No. 5-267059, since not only the resonance frequency on the high frequency side but also that on the low frequency side is dependent on the floating capacitance generated between the coil conductors, it is not always easy to perform noise removal in an appropriate and satisfactory manner for each frequency band.

That is, in an LC parallel resonant circuit, the resonance frequency is dependent on the value of the LC product; the larger the LC product, the smaller the resonance frequency. Here, the setting of the resonance frequency on the high frequency side can be readily realized by adjusting the floating capacitance because the LC product may be set to be small. On the other hand, for the setting of the resonance frequency on the low frequency side, the LC product must be set to be relatively large. In this case, since problems such as distortion of the signal waveform occur when the value of the inductance L is set too large, there is naturally a limit as to how large the value of the inductance L can be set. Therefore, in order to compensate for the shortage of the inductance L, it is necessary to obtain a relatively large floating capacitance by reducing the inter-layer distance between the coil conductors or by changing the insulation material.

However, when the inter-layer distance between the coil conductors is reduced as described above, this causes deterioration in characteristics or reliability. Further, in the case where the insulation material is changed, there is a problem in that the number of manufacturing man-hours increases due to the occurrence of delamination depending on the characteristics of the material or due to an increase in the kinds of sheets to be used. In the case of Japanese Unexamined Patent Application Publication No. 5-267059, in particular, because it is necessary to fire the dielectric and the magnetic material at the same time for integration, there is a problem in that not only is the reliability in terms of strength low due to cracks, peels, or the like that are liable to occur during the manufacturing process, but also an increase in cost is caused due to the necessity of setting and managing the optimum manufacturing conditions with high precision.

Further, in the related art, there has been also proposed a construction in which a plurality of coils each composed of a plurality of coil conductors laminated in a spiral fashion within a single dielectric are formed at the same time to thereby form a plurality of trap circuits.

However, as in the case of Japanese Unexamined Patent Application Publication No. 5-267059 described above, also in the case of the noise filter of this construction, it is difficult to set each trap circuit to a desired frequency in correspondence with a plurality of communication bands, and further, there maybe cases where the coils tend to readily magnetically couple with each other. Thus, a plurality of trap circuits cannot be formed or high attenuation cannot be attained at the resonance frequency of each trap circuit, which disadvantageously makes it impossible to perform noise removal in an appropriate and satisfactory manner for each frequency band.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention provide a noise filter which makes it possible to easily and reliably set a resonance frequency in each of a plurality of frequency bands and which is capable of efficiently removing noise in each of the plurality of frequency bands, and also, a noise filter which makes it possible to attain a high attenuation at each resonance frequency by reliably preventing magnetic coupling between the coils, and a noise filter array including such a noise filter.

According to a preferred embodiment of the present invention, a noise filter for removing noise flowing in a signal wire located on a circuit board includes a pair of external electrodes that are connected to the signal wire and disposed on an outside of an insulator, and on an inside of the insulator, a plurality of coils are connected in series and have both ends thereof electrically connected to the external electrodes, respectively, and a capacitor is connected in parallel to at least one of the plurality of coils, the coils each being defined by a plurality of coil conductors which are laminated through the insulator and connected to each other in a spiral configuration through a via hole, and the capacitor is defined by a shield electrode and a capacitance-forming electrode arranged so as to be opposed to each other through the insulator, the shield electrode being located between upstream and downstream coils and commonly electrically connected to both the upstream and downstream coils, the capacitance-forming electrode being electrically connected to one of the pair of external electrodes.

According to another preferred embodiment of the present invention, a noise filter for removing noise flowing in a signal wire located on a circuit board includes a pair of external electrodes that are connected to the signal wire and that are located on an outside of an insulator, and on an inside of the insulator, a plurality of coils are connected in series and have both ends thereof electrically connected to the external electrodes, respectively, and a capacitor is connected in parallel to at least one of the plurality of coils, the coils are each defined by a plurality of coil conductors which are laminated through the insulator and connected to each other in a spiral configuration through a via hole, and the capacitor is defined by the coil conductors and a capacitance-forming electrode arranged so as to be opposed to each other through the insulator, the capacitance-forming electrode being electrically connected to one of the pair of external electrodes.

In this preferred embodiment of the present invention, a shield electrode is preferably disposed between upstream and downstream coils so as to be substantially perpendicular to a coil axis direction.

Further, according to another preferred embodiment of the present invention, a noise filter for removing noise flowing in a signal wire located on a circuit board includes a pair of external electrodes that are connected to the signal wire and are located on an outside of an insulator, and on an inside of the insulator, a plurality of coils are connected in series and have both ends thereof electrically connected to the external electrodes, respectively, and a capacitor is connected in parallel to at least one of the plurality of coils, the coils are each defined by a plurality of coil conductors which are laminated through the insulator and connected to each other in a spiral configuration through a via hole, and the capacitor is defined by the pair of external electrodes and a shield electrode arranged so as to be opposed to each other through the insulator, the shield electrode being located between upstream and downstream coils and electrically connected to both the upstream and downstream coils.

It is preferred that the shield electrode is arranged so as to have a surface area that is equal to or larger than about ½ of a surface area of a bore of at least one of the upstream and downstream coils.

It is also preferable that a plurality of LC parallel resonant circuits having different respective resonance frequencies are defined by the coils and the capacitor which is individually connected in parallel to each of the coils.

It is also preferred that an LC parallel resonant circuit on a low frequency side is defined by the coil, the capacitor connected in parallel to the coil, and a floating capacitor generated due to the presence of the coil, and an LC parallel resonant circuit on a high frequency side is defined by the coil and a floating capacitor generated due to the presence of the coil.

According to another preferred embodiment of the present invention, a noise filter array includes a plurality of the noise filters according to any of the above-described preferred embodiments that are integrated together while being arranged in an array individually in correspondence with a plurality of signal wires located on a circuit board.

In such a construction, connecting points between the coils provided to each of the signal wires are preferably commonly connected together in an ungrounded state via a noise dispersing capacitor.

In the noise filter according to a preferred embodiment of the present invention, the plurality of LC parallel resonant circuits are disposed within the insulator while being sequentially connected in series to the signal wire. Accordingly, by setting the resonance frequencies of the respective LC parallel resonant circuits to be different from each other, it is possible to effectively remove noise for each of the plurality of frequency bands.

Accordingly, by using the noise filter according to various preferred embodiments of the present invention, noise countermeasures for portable telephones, for example, can be effectively implemented.

Further, the coils used are preferably of a laminated type, and the capacitor is defined by the shield electrode being located between the upstream and downstream coils and commonly electrically connected to both the coils, and one of the two external electrodes so as to be opposed to each other through the insulator. Accordingly, the coil and capacitor for defining each LC parallel resonant circuit can have a relatively simple construction. Further, a desired resonance frequency can be readily set by adjusting the capacitance of the capacitor, varying the numbers of turns of the coil conductors, or varying the distance between the coil conductors.

Further, since the LC parallel resonant circuits each including the coil and capacitor are disposed within a single insulator, it is possible to provide a highly reliable noise filter array that is free from structural flaws with relatively little fear of cracks or peels occurring during the manufacture thereof.

Further, the plurality of LC parallel resonant circuits can be formed to have a relatively simple construction also in the case where, as in various preferred embodiments of the present invention, a laminated type coil is preferably used as the coil, and the capacitor is constructed by arranging the coil conductor and the capacitance-forming electrode, which is electrically connected to one of the two external electrodes, so as to be opposed to each other through the insulator. Further, a desired resonance frequency can be readily set by adjusting the capacitance of the capacitor, varying the numbers of turns of the coil conductors, or varying the distance between the coil conductors. Accordingly, noise can be effectively removed for each frequency band.

Further, when the shield electrode is disposed between the upstream and downstream coils so as to be substantially perpendicular to the coil axis direction, magnetic coupling between the upstream and downstream coils can be reliably prevented, whereby the setting of the resonance frequency for each LC parallel resonant circuit can be performed with reliability.

Further, the plurality of LC parallel resonant circuits can be formed to have a relatively simple construction also in the case where, a laminated type coil is used as the coil, and the capacitor is constructed by arranging one of the two external electrodes and the shield electrode, which is located between the upstream and downstream coils and commonly electrically connected to both the coils, so as to be opposed to each other through the insulator. Further, a desired resonance frequency can be readily set by adjusting the capacitance of the capacitor, varying the numbers of turns of the coil conductors, or varying the distance between the coil conductors. Therefore, noise can be effectively removed for each frequency band.

When the shield electrode is set so as to have a surface area that is equal to or larger than about ½ of the bore of at least one of the upstream and downstream coils, the magnetic coupling between the upstream and downstream coils can be prevented even more reliably.

Accordingly, the trap attenuation in each of the plurality of frequency bands can be very large without varying the resonance frequencies of the respective LC parallel resonant circuits. As a result, noise can be removed even more effectively.

When the plurality of LC parallel resonant circuit shaving respective resonance frequencies that differ from each other are defined by the coils and the capacitor individually connected in parallel to each of the coils, the resonance frequency of each of the LC parallel resonant circuits can be reliably and readily adjusted or controlled to a desired frequency by appropriately setting the inductance of the coil and the capacitance of the capacitor. Accordingly, noise removal can be performed in a satisfactory manner for each frequency band.

Also in the case where the LC parallel resonant circuit on the low frequency side is defined by the coil, the capacitor connected in parallel to the coil, and the floating capacitor generated due to the presence of the coil, and the LC parallel resonant circuit on the high frequency side is defined by the coil and the floating capacitor generated due to the presence of the coil, noise removal can be performed in a satisfactory manner for each frequency band despite the even more simplified construction. That is, with respect to the LC parallel resonant circuit on the low frequency side, a somewhat large LC product can be set by the coil and the capacitor, whereby noise on the low frequency side can be removed in a satisfactory manner. Further, with respect to the LC resonant circuit on the high frequency side, since the LC product may be set smaller than that on the low frequency side, noise on the high frequency side can be removed in a satisfactory manner by adjusting the coil and the floating capacitor generated due to the coil.

Further, in the noise filter array according to a preferred embodiment of the present invention, a plurality of the noise filters according to any of the preferred embodiments of the present invention described above are integrated together while being arranged in an array individually in correspondence with the plurality of signal wires disposed on the circuit board. Accordingly, noise in each of the plurality of signal wires can be removed by a single component (an array noise filter). Accordingly, it is not necessary to provide noise filters individually in correspondence with the respective signal wires, whereby in addition to a reduction in the number of components as compared with the prior art, it is possible to achieve an improvement in the efficiency of mounting of components and a reduction in the mounting surface area on the circuit board.

When the connecting points between the coils provided for each of the signal wires are commonly connected together in an ungrounded state via the noise dispersing capacitor, a high attenuation can be attained as compared with the case where no noise dispersing capacitor is provided, and further, the noise filter array obtained has a sharp cut off characteristic, thereby making it possible to suppress the influence exerted on the signal waveform. Further, since it is not necessary to form the grounding electrode pattern on the circuit board, it is possible to enhance the freedom of wiring layout on the circuit board, and to obviate the need to provide a large-area grounding electrode pattern in the inner portion of the circuit board. It thus becomes possible to achieve a reduction in the cost of the circuit board.

It is known that capacitive coupling between a plurality of signal wires causes problems such as cross talk. Further, it is known that generally, the signal frequency is not higher than several tens MHz and the noise frequency is within the GHz band. It is thus important to set the value of the noise dispersing capacitor (particularly, to use a capacitor having a small capacitance value) such that the influence of cross talk does not appear in the waveform. By setting the value of the noise dispersing capacitor to an appropriate value, it is possible to disperse only noise current to another signal wire.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Now, characteristic features of the present invention will be described in detail with reference to preferred embodiments thereof.

First Preferred Embodiment

FIG. 1is a plan view showing a noise filter array according to a first preferred embodiment of the present invention as mounted on a circuit board,FIG. 2is a sectional view taken along the line A-A ofFIG. 1,FIG. 3is an equivalent circuit diagram of the noise filter array according to the first preferred embodiment of the present invention, andFIG. 4is an exploded perspective view showing a method of manufacturing the noise filter array according to the first preferred embodiment 1 of the present invention.

As shown inFIGS. 1 to 3, the noise filter according to first preferred embodiment serves to remove noise flowing in a plurality of (in the present preferred embodiment, four) signal wires2located on a circuit board1. Four noise filters3are each preferably integral with a respective one of the signal wires2.

That is, this noise filter array includes a substantially rectangular insulator4formed by laminating and then integrally firing insulating sheets such as ceramic green sheets. Further, on both end sides (outer left and right portions) of the insulator4, external electrodes6,7for signal input/output are formed individually in correspondence with the respective signal wires2. The external electrodes6,7are electrically connected by soldering or the like to left and right electrode patterns2a,2bconstituting the respective signal wires2, respectively.

Further, inside the insulator4, two upstream and downstream LC parallel resonant circuits8,9are arranged so as to be connected in tandem in correspondence with the respective signal wires2. The two LC parallel resonant circuits8,9constitute the noise filter3with respect to each of the signal wires2.

As will be described later, the respective resonance frequencies of the LC parallel resonant circuits8,9are preferably different from each other so that noise can be effectively removed in each of a plurality of frequency bands.

Here, the upstream LC parallel resonant circuit8includes an input-side coil11and an input-side capacitor12connected in parallel with the input-side coil11, and the downstream LC parallel resonant circuit9includes an output-side coil13and an output-side capacitor14connected in parallel with the output-side coil13. In the present preferred embodiment, the input-side coil11on the upper side and the output coil13on the lower side will hereinafter be also referred to as the “upstream and downstream coils11,13”. The upstream and downstream coils11,13correspond to the “upstream and downstream coils” recited in the claims.

The input-side coil11is preferably formed as a spiral coil by sequentially connecting together a plurality of coil conductors16, which are laminated inside the insulator4, through a via hole17. Likewise, the output-side coil13is preferably formed as a spiral coil by sequentially connecting together a plurality of coil conductors18, which are laminated inside the insulator4, through a via hole19. In this preferred embodiment, the number of turns is set to be different between the input-side coil11and the output-side coil13so that the resonance points of the respective LC parallel resonant circuits8,9are different from each other.

Further, first ends of the input-side coil11and output coil13are connected to each other in series through a via hole20, and second ends of the input coil11and output coil13are connected to the external electrodes6,7on the input side and the output side, respectively.

Further, a shield electrode23is arranged between the upstream and downstream coils (the input coil11and the output coil13) so as to be substantially perpendicular to the coil axis direction. Two upper and lower capacitance-forming electrodes24,25are arranged so as to be opposed to the shield electrode23through the insulator4. The shield electrode23and the upper capacitance-forming electrode24constitute the input-side capacitor12, and the shield electrode23and the lower capacitance-forming electrode25constitute the output-side capacitor14.

Further, the shield electrode23is electrically connected to the via hole20that provides serial connection between the input-side coil11on the upper side and the output-side coil13on the lower side, and is embedded in the insulator4so as to allow no external connection. Further, the shield electrode23preferably has a surface area that is large enough to cover the bore of the upstream and downstream coils11,13.

That is, the shield electrode23can function as one electrode for forming the capacitance of each of the capacitors12,14as well as an electromagnetic shield for preventing electromagnetic coupling between the upstream and downstream coils11,13.

From the viewpoint of preventing the electromagnetic coupling between the upstream and downstream coils11,13, it is preferable that the surface area of the shield electrode23is equal to or larger than about ½ of the surface area of the bore of at least one of the upstream and downstream coils11,13.

Further, first end portions of the respective capacitance-forming electrodes24,25are led out to the outer side portions of the insulator4to be electrically connected to the external electrodes6,7, respectively. Further, by previously adjusting the surface area or the distance over which the shield electrode23and each of the upper and lower capacitance-forming electrodes24,25are opposed to each other in order to thereby vary the capacitances of the input-side capacitor12and output-side capacitor14, the resonance points of the respective LC parallel resonant circuits8,9are adjusted so that a noise having a frequency to be removed can be removed in a satisfactory manner. The resonance points can be also adjusted by adjusting the inductance of each of the individual coils11,13.

Next, a method of manufacturing the noise filter array according to the first preferred embodiment of the present invention will be described.

To manufacture the noise filter array according to the first preferred embodiment, as shown in, for example,FIG. 4, a predetermined number of input-side-coil-forming insulating sheets31, output-side-coil-forming insulating sheets32, capacitor-forming insulating sheets33,34,35, and interconnection insulating sheets (not shown), which are interposed between the respective insulating sheets31,32,33to35as required, are prepared. A ceramic green sheet such as a dielectric or the like is preferably used as each of these insulating sheets.

Further, four coil conductors16,18are formed in the coil-forming insulating sheets31,32in order to form the coils11,13in correspondence with the four signal wires2, respectively. Further, the shapes of the respective coil conductors16,18are preferably different between the insulating sheets31,32so that they are formed in a spiral configuration with respect to the laminating direction of the insulating sheets31,32, respectively. Further, the directions of turns of the respective coil conductors16,18are the same with respect to the direction in which signals flow.

On the other hand, of the capacitor-forming insulating sheets33,34,35, the capacitance-forming electrodes24,25are formed in the upper and lower insulating sheets33,35, respectively, and the shield electrode23is formed in the intermediate insulating sheet34. A total of four shield electrodes23and four capacitance-forming electrodes24,25are formed in parallel respectively in correspondence with the four signal wires2. Further, of the insulating sheets31to35, the via hole20or the like is formed in predetermined insulating sheets so as to provide electrical connection between the upper and lower sheets.

A material such as Ag—Pd or Ag is preferably used for each of the coil conductors16,18, the shield electrode23, and the capacitance-forming electrodes24,25.

After laminating predetermined numbers of the output-side-coil-forming insulating sheets32, capacitor-forming insulating sheets33to35, and input-side-coil-forming insulating sheets31, and, as required, interposing the interconnection insulating sheets (not shown) between the respective insulating sheets31to35, the laminate of these insulating sheets is integrally fired. Thereafter, on both side portions (outer left and right portions) of the insulator4thus obtained, the external electrodes6,7are formed in correspondence with the respective signal wires2.

Thus, the noise filter array according to the first preferred embodiment having the construction shown inFIG. 2and the equivalent circuit shown inFIG. 3is obtained. In the noise filter array according to the first preferred embodiment, the respective coil conductors16,18are sequentially connected together through the via holes17,19,20to thereby form the spiral input-side coil11and the output-side coil13, respectively. Further, the one end sides of the respective coils11,13are connected to the external electrodes6,7, and the other end sides thereof are connected to each other in series through the via hole20and are also commonly connected to the shield electrode23. Further, the capacitance-forming electrodes24,25are opposed to the shield electrode23through the insulator4(insulator layer4a), and first ends of the respective capacitance-forming electrodes24,25are connected to the external electrodes6,7, thereby forming the input-side capacitor12and the output-side capacitor14, respectively. Accordingly, a construction is realized in which the input-side capacitor12and the output-side capacitor14are connected in parallel to the input-side coil11and the output-side coil13, respectively.

When using the noise filter array according to the first preferred embodiment, in order to enable effective removal of noise in each of a plurality of frequency bands, the resonance point of each of the upstream and downstream LC parallel resonant circuits8,9is preciously set to a resonance frequency at which noise included in each frequency band is to be removed, whereby noise in two communication bands in the vicinity of 800 MHz and in the vicinity of 2 GHz, which are required as noise countermeasures for potable telephones, for example, can be effectively removed.

Further, since the shield electrode23is interposed between the input-side coil11and the output-side coil13, magnetic coupling between the upstream and downstream coils11,13can be reliably cut off. Accordingly, the resonance frequencies of the respective LC parallel resonant circuits8,9do not vary, and the trap attenuation in each of the plurality of frequency bands can be made large. For example, a high attenuation of about 20 dB or more can be secured for a resonance frequency on the high frequency side.

Further, in the noise filter array according to the first preferred embodiment, a plurality of noise filters3are integrally disposed within a single component and can together remove noises in the respective signal wires2. Accordingly, it is not necessary to provide a noise filter individually for each signal wire2, thereby making it possible to reduce the number of components. Further, since the plurality of LC parallel resonant circuits8,9are disposed within the single insulator4, it is possible to provide a highly reliable noise filter array that is free from structural flaws with relatively little fear of cracks or peels occurring during the manufacture thereof.

With respect to the noise filter array according to the first preferred embodiment of the present invention, the following evaluation experiment was carried out in order to examine the filter characteristics thereof.

Evaluation Experiment

With respect to one of the noise filters3constituting the noise filter array according to a preferred embodiment of the present invention, the frequency-dependent characteristic (hereinafter, referred to as the “IL characteristic”) of the insertion loss (IL) thereof was examined. Here, the inductance L1of the input-side coil11of the upstream LC parallel resonant circuit8, the capacitance C1of the input-side capacitor12, the inductance L2of the output-side coil13of the downstream LC parallel resonant circuit9, and the capacitance C2of the output-side capacitor14were set to approximately 24 nH, 1.2 pF, 18 nH, and 0.4 pF, respectively.FIG. 5shows the results.

As shown inFIG. 5, it was confirmed that the noise filter3has a resonance frequency in each of two communication bands in the vicinity of 800 Hz and in the vicinity of the 2 GHz, which are required as noise countermeasures for portable telephones, and thus can effectively remove noise included in each of the communication bands.

Second Preferred Embodiment

FIG. 6is a sectional view of a noise filter array according to a second preferred embodiment of the present invention. InFIG. 6, the portions that are denoted by the same reference numerals as those ofFIGS. 1 to 4indicate portions that are the same as or equivalent to those of the noise filter array according to the first preferred embodiment.

The noise filter array according to the second preferred embodiment preferably has the same equivalent circuit as that shown inFIG. 3.

Note that, however, that in the noise filter array according to second preferred embodiment, the input-side capacitor12and the output-side capacitor13are formed preferably by arranging the capacitance-forming electrodes24,25so as to be opposed to a portion of the coil conductors16,18forming the input-side coil11and the output-side coil13through the insulator4(insulator layers4a), respectively.

That is, the input-side capacitor12is formed preferably by arranging the capacitance-forming electrodes24so as to be opposed to a portion of the coil conductors16on the output side forming the input-side coil11through the insulator4(insulator layer4a), and the output-side capacitor14is formed preferably by arranging the capacitance-forming electrode25so as to be opposed to a portion of the coil conductors18on the input side forming the output-side coil13through the insulator4(insulator layer4a). Further, first ends of the respective capacitance-forming electrodes24,25are led out to the both outer side portions of the insulator4to be electrically connected to the external electrodes6,7, respectively. Thus, two LC parallel resonant circuits8,9are provided, in which the input-side capacitor12and the output-side capacitor14are connected in parallel to the input-side coil11and the output-side coil13, respectively.

Further, by adjusting the surface area or the distance over which the capacitance-forming electrodes24,25are opposed to the respective coil conductors16,18to thereby vary the capacitances of the input-side capacitor12and output-side capacitor14, the resonance points of the respective LC parallel resonant circuits8,9are adjusted to resonance frequencies at which noise is to be removed. The resonance points can be also adjusted by adjusting the inductance of each of the input-side coils11and output-side coil13.

Further, in the second preferred embodiment, as in the case of the first preferred embodiment, the shield electrode23is preferably provided between the input-side coil11on the upper side and the output-side coil13on the lower side, and is electrically connected to the via hole20that provides serial connection between the input-side coil11on the upper side and the output-side coil13on the lower side.

When, as described above, the shield electrode23is provided between the input-side coil11on the upper side and the output-side coil13on the lower side, electromagnetic coupling between the upper and lower coils11,13is prevented even in cases where the component size is small and the coils11,13are in close proximity to each other, thereby making it possible to secure high attenuation. However, depending on the component size, there may be cases where a sufficient distance can be secured between the upper and lower coils11,13. In such cases, it is possible to omit the shield electrode23because the magnetic coupling between the upper and lower coils11,13becomes extremely small.

Further, while in the second preferred embodiment, the shield electrode23is electrically connected to the via hole20that provides electrical connection between the upper and lower coils11,13, in preventing the magnetic coupling between the upper and lower coils11,13, the shield electrode23may be electrically separated from the via hole20.

Otherwise, the construction and effects of the second preferred embodiment are the same as those of the first preferred embodiment, so detailed description is omitted here to avoid repetition.

Third Preferred Embodiment

FIG. 7is a sectional view of a noise filter array according to third preferred embodiment of the present invention. InFIG. 7, the portions that are denoted by the same reference numerals as those ofFIGS. 1 to 4indicate portions that are the same as or equivalent to those of the noise filter array according to the first preferred embodiment.

The noise filter array according to third preferred embodiment has the same equivalent circuit as that shown inFIG. 3.

However, in the noise filter array according to third preferred embodiment, the input-side capacitor12and the output-side capacitor14preferably include external electrodes6,7for signal input/output, which are respectively formed on both end sides (outer left and right portions) of the insulator4in correspondence with the respective signal wires2, and the shield electrodes23that are commonly connected to the upstream and downstream coils11,13through the via hole20.

That is, the input-side capacitor12preferably includes the input-side external electrode6, and the shield electrode23opposed to the input-side external electrode6through the insulator4(4b). Further, the output-side capacitor14preferably includes the output-side external electrode7, and the shield electrode23opposed to the output-side external electrode7through the insulator4(4b). The respective coils11,13are thus connected in parallel to the capacitors12,14, thereby defining the respective LC parallel resonant circuits8,9.

Further, in the noise filter array according to the third preferred embodiment, by adjusting the distance over which each of the external electrodes6,7and the shield electrode23are opposed to each other to thereby vary the capacitances of the input-side capacitor12and output-side capacitor14, the resonance points of the respective LC parallel resonant circuits8,9are adjusted to resonance frequencies at which noise is to be removed. The resonance points can be also adjusted by adjusting the inductance of each of the input-side coils11and output-side coil13.

Otherwise, the construction and effects of third preferred embodiment are the same as those of first preferred embodiment, so detailed description is omitted here.

Fourth Preferred Embodiment

FIG. 8is a sectional view of a noise filter array according to the fourth preferred embodiment of the present invention,FIG. 9is an equivalent circuit diagram thereof, andFIG. 10is an exploded perspective view showing a manufacturing method thereof.

InFIGS. 8 to 10, the portions that are denoted by the same reference numerals as those ofFIGS. 1 to 4indicate portions that are the same as or equivalent to those of the noise filter array according to first preferred embodiment.

The noise filter array according to the fourth preferred embodiment has, in addition to the construction according to the first preferred embodiment described with reference toFIGS. 1 to 4, a construction in which the connecting points between the respective coils11,13of the upstream and downstream LC parallel resonant circuits8,9, which constitute the noise filter3provided for each signal wire2, are commonly connected together in an ungrounded state via noise dispersing capacitors38.

That is, in the noise filter array, the input-side capacitor12and the output-side capacitor14are preferably formed by arranging the capacitance-forming electrodes24,25so as to be opposed to the shield electrode23provided inside the insulator4, respectively. Further, noise dispersing electrodes36are arranged above and below the shield electrode23so as to be opposed to the shield electrode23through the insulator4(insulator layers4a). The noise dispersing capacitor38is preferably defined by the shield electrode23and each noise dispersing electrode36.

Further, in the noise filter array according to fourth preferred embodiment, while the shield electrode23and the capacitance-forming electrodes24,25are provided for respective signal wires2so as to extend along the signal wires2, the upper and lower noise dispersing electrodes36are formed continuously along the direction that is substantially perpendicular to the respective signal wires2(the direction that is substantially perpendicular to the plane ofFIG. 8) so as to cross the respective electrodes23,24,25. Further, the noise dispersing electrodes36are embedded in the insulator4so as to allow no external connection. That is, in addition to defining one capacitance-forming electrode of each noise dispersing capacitor38, each noise dispersing electrode36also defines the electrode for commonly connecting the noise dispersing capacitors38to each other in an ungrounded state.

Next, a method of manufacturing the noise filter array according to the fourth preferred embodiment will be described with reference toFIG. 10.

Although the manufacturing method for the noise filter array according to the fourth preferred embodiment is basically the same as that in the first preferred embodiment, since it is necessary to form the noise dispersing capacitors38simultaneously with the formation of the upstream and downstream LC parallel resonant circuits8,9for each of the four signal wires2, the noise dispersing electrode36is formed simultaneously in each of the insulating sheets33,35in which the respective capacitance-forming electrodes24,25are formed. Here, the respective noise dispersing electrodes36extend in a direction crossing the capacitance-forming electrodes24,25(the direction along the longitudinal direction of the insulating sheets33,35).

Further, after laminating predetermined numbers of the output-side-coil-forming insulating sheets32, capacitor-forming insulating sheets33to35, and input-side-coil-forming insulating sheets31, and, as required, interposing the interconnection insulating sheets (not shown) between the respective insulating sheets31to35, the laminate of these insulating sheets is integrally fired. Thereafter, on both side portions (outer left and right portions) of the insulator4thus obtained, the external electrodes6,7for signal input/output are formed in correspondence with the respective signal wires2.

The noise filter array according to the fourth preferred embodiment is thus obtained, which has a construction in which, as shown inFIG. 8, the two upstream and downstream LC parallel resonant circuits8,9are formed within the insulator4for each of the signal wires2, and the connecting points between the respective coils11,13of the LC parallel resonant circuits8,9are commonly connected together in an ungrounded state via the noise dispersing capacitors38, and which has the equivalent circuit as shown inFIG. 9.

In the noise filter array constructed as described above, a noise current flowing in one signal wire2is reduced due to the loss in the LC parallel resonant circuits8,9of each signal wire2, and further dispersed to another signal wire2via the noise dispersing capacitor38. Therefore, when the above-described noise filter array is used, noise in each frequency band can be even more effectively removed than in the first preferred embodiment. Further, since the noise filter has a sharp cut off characteristic, the influence on the signal waveform can be suppressed to be small.

Further, the electrode pattern for grounding, which is required in the prior art, becomes unnecessary, whereby an improvement can be achieved in terms of the freedom of the wiring layout of the circuit board1(FIG. 1). Since it becomes possible to use a circuit board1having a simple construction, it is possible to achieve a reduction in cost.

Otherwise, the construction and effects of the fourth preferred embodiment are the same as those of the first preferred embodiment, so detailed description is omitted here to avoid repetition.

While in the fourth preferred embodiment described above, the connecting points between the respective coils11,13of the upstream and downstream LC parallel resonant circuits8,9are commonly connected together in an ungrounded state via the noise dispersing capacitor38, the present invention is not limited to this construction. For example, as shown inFIG. 11, a construction is also possible in which the noise dispersing capacitors38are commonly connected together in an ungrounded state on the output sides of the downstream LC parallel resonant circuits9. Alternatively, although not shown, a construction is also possible in which, conversely, the noise dispersing capacitors38are commonly connected together in an ungrounded state on the input sides of the upstream LC parallel resonant circuits8.

With respect to the noise filter array according to the fourth preferred of the present invention, the following evaluation experiment was carried out in order to examine the filter characteristics thereof.

Evaluation Experiment 1

The IL characteristic of the noise filter array having the construction according to the fourth preferred embodiment was examined. Here, in order to prevent a difference in IL characteristic from occurring due to the influence of cross talk, as in the equivalent circuit shown inFIG. 12, measurement was carried out by connecting terminal resistors37of 50Ω to the left and right ends of three of the four noise filters3. For the purpose of comparison of characteristics, the IL characteristic was examined also with respect to the construction according to the first preferred embodiment with no noise dispersing capacitor38provided. Here, the measurement was carried out while uniformly setting the inductances of the input-side coils11of the upstream LC circuits8to about 20 nH, the capacitances of the input-side capacitors12to about 1.7 pF, the inductances of the output-side coils13of the downstream LC circuits9to about 13 nH, and the capacitances of the output-side capacitors14to about 0.4 pF.FIG. 13shows the results.

As shown inFIG. 13, in the case where the noise dispersing capacitors38are provided, large signal attenuation was attained in the two communication bands in the vicinity of 800 MHz and in the vicinity of 2 GHz, which are required as noise countermeasures for portable telephones, and between the two bands. Thus, it was confirmed that noise included in each of the communication bands can be effectively removed.

Evaluation Experiment 2

With respect to the noise filer array having the construction according to the fourth preferred embodiment, the IL characteristic in the case where the capacitance of the noise dispersing capacitor38is varied within the range of 0 pF to 15 pF was measured. In this case as well, the measurement was carried out by performing wiring connection so as to realize the equivalent circuit shown inFIG. 12.FIG. 14shows the results.

As shown inFIG. 14, it was confirmed that the larger the capacitance of the noise dispersing capacitor38is, the larger the attained signal attenuation is. However, when the capacitance becomes too large, cross talk occurs in the signal frequency band, so the influence on the signal waveform becomes large. In view of this, it is considered appropriate to set the capacitance of the noise dispersing capacitor38to 4 pF through 10 pF.

Evaluation Experiment 3

The IL characteristic was measured with respect to the case (FIG. 9) where the connecting points between the respective coils11,13of the upstream and downstream LC parallel resonant circuits8,9are commonly connected together in an ungrounded state via the noise dispersing capacitors38, and the case (FIG. 11) where the noise dispersing capacitors38are commonly connected together in an ungrounded state on the output sides of the downstream LC parallel resonant circuits9. The measurement conditions in this case were set to be the same as those for Evaluation Experiment 1.FIG. 15shows the results.FIG. 15also shows the IL characteristic in the case where no noise dispersing capacitor38is provided.

As shown inFIG. 15, in either cases of the construction shown inFIG. 9and the construction shown inFIG. 11, in comparison to the case where no noise dispersing capacitor38is provided, a large signal attenuation was attained in each of the two communication bands in the vicinity of 800 MHz and in the vicinity of 2 GHz, which are required as noise countermeasures for portable telephones. Thus, it was confirmed that noise included in each communication band can be effectively removed.

Incidentally, in each of the first through fourth preferred embodiments described above, the plurality of LC parallel resonant circuits8,9whose resonance frequencies are different from each other are formed by individually connecting the capacitors12,14in parallel to the respective coils11,13, respectively. Accordingly, the inductances of the coils11,13and the capacitances of the capacitors12,14can be readily adjusted, whereby the resonance frequency of each of the LC parallel resonant circuits8,9can be reliably set or controlled to a desired frequency required for the noise removal. Therefore, noise removal can be performed in a satisfactory manner for each of the frequency bands.

On the other hand, the above-described advantages can be also accomplished in a satisfactory manner by the constructions according to the fifth, sixth and seventh preferred embodiments described below.

That is, as described above, in an LC parallel resonant circuit, the resonance frequency is dependent on the value of the LC product. As the LC product becomes larger, the resonance frequency becomes smaller toward the low frequency side. Further, provided that the value of the LC product is the same, the larger the inductance L, the larger the attenuation becomes, and the larger the ratio of the capacitance C, the narrower the attenuation band becomes. Here, the setting of the resonance frequency on the high frequency side can be readily realized by adjusting the floating capacitance because the LC product may be small. Moreover, a wide attenuation band can be secured because a small floating capacitance suffices. On the other hand, for the setting of the resonance frequency on the low frequency side, the LC product must be set to be relatively large. In this case, since problems such as distortion of the signal waveform occur when the value of the inductance L is set too large, there is naturally a limit as to how large the value of the inductance L can be set. Further, when, in order to compensate for the limitations on the inductance L, the inter-layer distance between the coil conductors is reduced to set a large floating capacitance or the insulation material is changed, problems such as the loss of reliability due to degradation in characteristics or an increase in cost due to an increase in manufacturing man-hours occur.

In view of this, in the fifth to seventh preferred embodiments described below, with respect to noise on the low frequency side, an LC parallel resonant circuit capable of providing a somewhat large LC product is defined by the combination of a coil and a capacitor connected in parallel with the coil and, further, with respect to noise on the high frequency side, an LC parallel resonant circuit having a required LC product is defined by a coil and a floating capacitance generated between coil conductors (coil conductor layers) for forming the coil. Accordingly, a requisite noise removable action can be secured for each frequency band by means of a construction that is simpler than those of Embodiments 1 to 4. In the following, a more detailed description will be given in this regard by way of Embodiments 5 to 7.

Fifth Preferred Embodiment

FIG. 16is a sectional view of a noise filter array according to the fifth preferred embodiment of the present invention,FIG. 17is an equivalent circuit diagram of the noise filter array according to the fifth preferred embodiment of the present invention, andFIG. 18is an exploded perspective view showing a manufacturing method of the noise filter array according to the fifth preferred embodiment of the present invention. InFIGS. 16 to 18, the portions that are denoted by the same reference numerals as those ofFIGS. 1 to 4indicate portions that are the same as or equivalent to those of the noise filter array according to first preferred embodiment.

In the noise filter array according to the fifth preferred embodiment, an input-side capacitor12bis formed by arranging the capacitance-forming electrode24so as to be opposed to a portion of the shield electrode23via the insulator4(insulator layer4a).

That is, the input-side capacitor12bis formed by arranging the capacitance-forming electrode24so as to be opposed to a portion of the shield electrode23, which is arranged between the upstream and downstream coils11,13so as to be substantially perpendicular to the coil axis direction, through the insulator4(insulator layer4a). Further, one end of the capacitance-forming electrode24is led out to one end side (outer left side portion) of the insulator4to be electrically connected to the external electrode6. Accordingly, with respect to the input-side coil11, a floating capacitor12a(FIG. 17), which is naturally generated between the coil conductors (coil conductor layers)16as a result of the formation of the coil11, and the input-side capacitor12bgenerated by the capacitance-forming electrode24are both connected in parallel, thereby forming the LC parallel resonant circuit8on the low frequency side. Further, the LC parallel resonant circuit9on the high frequency side preferably includes the output-side coil13, and a floating capacitor14a(FIG. 17) naturally generated between the coil conductors (coil conductor layers)18as a result of the formation of the coil13.

Further, in the noise filter array according to the fifth preferred embodiment, by adjusting the surface area or the distance over which the capacitance-forming electrode24is opposed to each of the coil conductors16to thereby vary the capacitance of the input-side capacitor12b, the resonance point of the LC parallel resonant circuit8on the low frequency side is adjusted to a resonance frequency at which noise is to be removed. It is also possible to adjust the resonance points of the respective LC parallel resonant circuits8,9by adjusting the inductances of the input-side coil11and output-side coil13or by adjusting the capacitances of the floating capacitors12a,14a.

Further, in the fifth preferred embodiment, the shield electrode23is provided between the input-side coil11on the upper side and the output-side coil13on the lower side, and is electrically connected to the via hole20that provides serial connection between the input-side coil11on the upper side and the output-side coil13on the lower side.

As described above, according to the fifth preferred embodiment, in each noise filter3, the LC parallel resonant circuit8on the upstream side is constructed so as to have a requisite LC product through the combination of the input-side coil11, the floating capacitor12a, and the input-side capacitor12b, noise on the low frequency side can be effectively removed. That is, since a relatively large LC product can be set by the input-side coil11and the input-side capacitor12b, noise on the low frequency side can be effectively removed while avoiding such problems that the signal waveform is distorted as the inductance L is set to an excessively large value, or a characteristic degradation occurs or an increase in cost is caused by an increase in manufacturing man-hours because a large floating capacitor is set.

Further, the LC parallel resonant circuit9on the downstream side preferably includes the coil13and the floating capacitor14anaturally generated as a result of the formation of the coil13, whereby noise on the high frequency side can be effectively removed. That is, the setting of the resonance frequency on the high frequency side can be readily realized by adjusting the capacitance of the floating capacitor14abecause a small LC product suffices. Further, the ratio of the floating capacitance may be small, which proves advantageous because the attenuation band of noise on the high frequency side is not narrowed. For example, the communication bands for portable telephones are 875 MHz to 885 MHz on the low frequency side and 2110 MHz to 2170 MHz on the high frequency side, so the communication band on the high frequency side is wider. According to the construction of the fifth preferred embodiment, noise on the high frequency side can be effectively removed.

Therefore, according to the fifth preferred embodiment, a requisite noise removable action can be secured for each frequency band even through the construction of each noise filter3is simpler than those of the first through fourth preferred embodiments.

Otherwise, the construction and effects of the fifth preferred embodiment are the same as those of the first preferred embodiment, so detailed description is omitted here to avoid repetition.

Next, a method of manufacturing the noise filter array according to the fifth preferred embodiment will be described. Since the manufacturing method according to the fifth preferred embodiment is basically the same as that of the first preferred embodiment, it will be described here briefly.

In this case, four coil conductors16,18are disposed in the coil-forming insulating sheets31,32in order to provide the coils11,13in correspondence with the four signal wires2, respectively. Further, of the capacitor-forming insulating sheets33,34, the capacitance-forming electrode24is disposed in the insulating sheet33on the upper side, and the shield electrode23is disposed in the insulating sheet34on the lower side. Further, a total of four shield electrodes23and four capacitance-forming electrodes24are arranged in parallel in correspondence with the four signal wires2. Further, of the insulating sheets31to34, the via hole20or the like is formed in predetermined insulating sheets so as to provide electrical connection between the upper and lower sheets.

Further, after laminating predetermined numbers of the output-side-coil-forming insulating sheets32, capacitor-forming insulating sheets33,34, and input-side-coil-forming insulating sheets31, and, as required, interposing the interconnection insulating sheets (not shown) between the respective insulating sheets31to34, the laminate of these insulating sheets is integrally fired.

Thereafter, on both side portions (outer left and right portions) of the insulator4thus obtained, the external electrodes6,7are formed in correspondence with the respective signal wires2. Thus, the noise filter array according to the fifth preferred embodiment having the construction as shown inFIG. 16and having and equivalent circuit as shown inFIG. 17is obtained.

Sixth Preferred Embodiment

FIG. 19is a sectional view of a noise filter array according to the sixth preferred embodiment of the present invention. InFIG. 19, the portions that are denoted by the same reference numerals as those ofFIGS. 1 to 4indicate portions that are the same as or equivalent to those of the noise filter array according to the first preferred embodiment.

The noise filter array according to the sixth preferred embodiment preferably has the same equivalent circuit as that shown inFIG. 17.

However, in the noise filter array according to the sixth preferred embodiment, the input-side capacitor12bis preferably formed by arranging the capacitance-forming electrodes24so as to be opposed to a portion of the coil conductors16forming the input-side coil11through the insulator4(insulator layers4a).

That is, the input-side capacitor12bis formed by arranging the capacitance-forming electrodes24so as to be opposed to a portion of the output side of the coil conductors16forming the input-side coil11through the insulator4(insulator layers4a). Further, one end of each of the capacitance-forming electrodes24is led out to one end side (outer left side portion) of the insulator4to be electrically connected to the external electrode6.

Accordingly, with respect to the input-side coil11, the floating capacitor12a(seeFIG. 17), which is naturally generated between the coil conductors (coil conductor layers)16as a result of the formation of the coil11, and the input-side capacitor12bgenerated by the capacitance-forming electrodes24are both connected in parallel, thereby forming the LC parallel resonant circuit8on the low frequency side. Further, the LC parallel resonant circuit9on the high frequency side includes the input-side coil13, and the floating capacitor14a(seeFIG. 17) naturally generated between the coil conductors (coil conductor layers)18as a result of the formation of the coil13.

Further, in the noise filter array according to the sixth preferred embodiment, by adjusting the surface area or the distance over which the capacitance-forming electrode24is opposed to each of the coil conductors16to thereby vary the capacitance of the input-side capacitor12b, the resonance point of the LC parallel resonant circuit8on the low frequency side is adjusted to a resonance frequency at which noise is to be removed. It is also possible to adjust the resonance points of the respective LC parallel resonant circuits8,9by adjusting the inductances of the input-side coil11and output-side coil13or by adjusting the capacitances of the floating capacitors12a,14a.

Further, in the sixth preferred embodiment, as in the fifth preferred embodiment, the shield electrode23is preferably provided between the input-side coil11on the upper side and the output-side coil13on the lower side, and is electrically connected to the via hole20that provides serial connection between the input-side coil11on the upper side and the output-side coil13on the lower side.

Otherwise, the construction and effects of the sixth preferred embodiment are the same as those of first preferred embodiment, so detailed description is omitted here to avoid repetition.

Seventh Preferred Embodiment

FIG. 20is a sectional view of a noise filter array according to a seventh preferred embodiment of the present invention. In FIG.20, the portions that are denoted by the same reference numerals as those ofFIGS. 1 to 4indicate portions that are the same as or equivalent to those of the noise filter array according to the first preferred embodiment.

The noise filter array according to the seventh preferred embodiment has the same equivalent circuit as that shown inFIG. 17.

However, in the noise filter array according to the seventh preferred embodiment, the input-side capacitor12bpreferably includes the external electrode6for signal input/output, which is formed on one end side (outer left side portion) of the insulator4in correspondence with each of the signal wires2, and the shield electrodes23located between the upstream and downstream coils11,13, the shield electrodes23being substantially perpendicular to the coil axis direction and electrically connected to the coils11,13through via hole20.

That is, the input-side capacitor12bpreferably includes the input-side external electrode6, and the shield electrodes23opposed to the input-side external electrode6through the insulator4(insulator layers4a). Accordingly, with respect to the input-side coil11, the floating capacitor12a(seeFIG. 17), which is naturally generated between the coil conductors (coil conductor layers)16as a result of the formation of the coil11, and the input-side capacitor12bgenerated by the shield electrodes23are both connected in parallel, thereby forming the LC parallel resonant circuit8on the low frequency side. Further, the LC parallel resonant circuit9on the high frequency side preferably includes the output-side coil13, and the floating capacitor14a(seeFIG. 17) naturally generated between the coil conductors (coil conductor layers)18following the formation of the coil13.

Further, in the noise filter array according to the seventh preferred embodiment, by adjusting the distance over which the respective external electrodes6and the shield electrodes23are opposed to each other or the like to thereby vary the capacitance of the input-side capacitor12b, the resonance point of the LC parallel resonant circuit8on the low frequency side is adjusted to a resonance frequency at which noise is to be removed. It is also possible to adjust the resonance points of the respective LC parallel resonant circuits8,9by adjusting the inductances of the input-side coil11and output-side coil13or by adjusting the capacitances of the floating capacitors12a,14a.

Otherwise, the construction and effects of the seventh preferred embodiment are the same as those of the fifth preferred embodiment, so detailed description is omitted here to avoid repetition.

While in each of various preferred embodiments described above, the description is directed to the case where the two upstream and downstream LC parallel resonant circuits8,9are provided with respect to the respective signal wires2, the present invention is not limited to this. It is also possible to adopt a construction in which three or more LC parallel resonant circuits are connected in tandem with respect to the respective signal wires2. In that case, noise removing characteristics over an even wider band can be attained by setting the inductance L and the capacitance C as appropriate for each LC parallel resonant circuit so that an appropriate resonance frequency is obtained.

Further, while in each of the above-described various preferred embodiments, the description is directed to the case where the four noise filters3are combined in correspondence with the four signal wires2located on the circuit board1and integrated into a noise filter array, the number of the noise filters3is not particularly limited. The present invention is also applicable to cases where a single noise filter3is provided. Further, while in the fourth preferred embodiment as well the description is directed to the case where the four noise filters3are combined in correspondence with the four signal wires2and integrated into a noise filter array, in this case, too, the number of signal wires2or the number of noise filters3is not particularly limited.

In other respects as well, the present invention is not restricted to the preferred embodiments of the present invention described above but can be subject to various applications and modifications within the scope of the present invention.

According to the present invention, it is possible to provide a noise filter which makes it possible to easily and reliably set a resonance frequency in each of a plurality of frequency bands and which is capable of efficiently removing noise in each of the plurality of frequency bands, and also, a noise filter which makes it possible to attain a high attenuation at each resonance frequency by reliably preventing magnetic coupling between the coils, and a noise filter array including such a noise filter.

The noise filter and the noise filter array according to various preferred embodiments of the present invention can be suitably used for applications such as removal of noise in portable telephones, and further can be used for a wide variety of other applications (for example, applications such as removal of noise in other high-frequency circuits).