RF signal blocking device

An RF signal blocking device includes a capacitor unit and an inductor unit defining an LC resonance circuit. The capacitor unit and inductor unit are set such that a cutoff frequency in the LC resonance circuit after disconnection by fusing of a second inductor including a fuse wire, among first and second inductors of the inductor unit, fluctuates within a prescribed frequency band. The RF signal blocking device reliably blocks an RF signal in a prescribed frequency band after an abnormality of fusing in the second inductor including a fuse wire occurs due to an input of an RF signal having an RF power value greater than or equal to a predetermined level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an RF signal blocking device having a function of blocking an RF signal in a prescribed frequency band after an RF signal having an RF power value greater than or equal to a predetermined level is inputted.

2. Description of the Related Art

As illustrated in one example of a coil with a fuse function shown inFIG. 7, a coil500with a fuse function has been known which is configured such that a solenoid coil503is enclosed in a cylindrical glass tube505provided with conductive caps504at opposite ends. See, for example, Japanese Patent Laying-Open No. 9-330824. The solenoid coil503is including a fuse line502wound around an outer peripheral surface of a cylindrical ferrite core501. Coil500with a fuse function has both a normal fuse function for protection from a short circuit current and an overload and a coil function which is useful for suppression of an inrush current and noise reduction, thus one element can achieve two functions. Therefore, a space for disposing coil500with a fuse function is reduced, so that high-density packaging of electronic equipment can be readily performed, enabling miniaturization of a device.

In recent years, communication mobile terminals such as mobile phones and mobile information terminals with a higher frequency have been developed. In a case where coil500with a fuse function described above is used for communication mobile terminals with higher frequency, the following problem may arise. In other words, even in a case where fuse line502is fused due to a flow of an overcurrent to coil500with a fuse function, when a high-frequency signal (RF signal) is provided, the fused portion of fuse line502may possibly be coupled by capacitive coupling, magnetic field coupling, or both to thereby transmit an RF signal through the fused portion of fuse line502.

SUMMARY OF THE INVENTION

In view of the problems described above, preferred embodiments of the present invention provide an RF signal blocking device with a new configuration having a function capable of reliably blocking an RF signal in a prescribed frequency band after an RF signal having an RF power value greater than or equal to a predetermined level is inputted.

An RF signal blocking device according to a preferred embodiment of the present invention is an RF signal blocking device having a function of blocking an RF signal in a prescribed frequency band after an RF signal having an RF power value greater than or equal to a predetermined level is inputted, and the signal blocking device includes an LC resonance circuit including a capacitor unit that includes at least one capacitor and an inductor unit that includes a first inductor and a second inductor connected in parallel, and having a resonant frequency higher than the prescribed frequency band. The second inductor includes a fuse wire which is fused due to an input of an RF signal having an RF power value greater than or equal to the predetermined level, and a resonant frequency of the LC resonance circuit after fusing of the second inductor is set in the prescribed frequency band.

According to a preferred embodiment of the present invention configured as described above, an LC resonant circuit including a capacitor unit that includes at least one capacitor and an inductor unit that includes a first inductor and a second inductor connected in parallel and having a resonant frequency higher than a prescribed frequency band allows a signal in a prescribed frequency band to pass through in a normal time. On the other hand, the second inductor of the inductor unit includes a fuse wire which is fused due to an input of an RF signal having an RF power value greater than or equal to a predetermined level, thus the second inductor including a fuse wire is fused when an RF signal having an RF power value greater than or equal to the predetermined level is inputted.

Since the inductor unit is defined by connecting the first inductor and the second inductor in parallel, inductance (L) of this parallel circuit is relatively small. However, when the second inductor including a fuse wire is disconnected by fusing, the number of inductors connected in parallel is reduced, so that inductance (L) of the inductor unit increases. Thus, since a resonant frequency f (cutoff frequency) of the LC resonance circuit is determined by
f=1/(2π(LC)1/2)
based on capacitance (C) of the capacitor unit and inductance of the inductor unit, an increase in the inductance causes the resonant frequency in the LC resonance circuit to fluctuate to be low, so that an RF signal with the fluctuated resonant frequency is blocked.

Thus, by setting the capacitor unit and the inductor unit such that the resonant frequency in the LC resonance circuit after the disconnection of the second inductor including a fuse wire by fusing fluctuates within the prescribed frequency band, an RF signal blocking device is provided which involves a new configuration having a function capable of reliably blocking an RF signal in a prescribed frequency band after an abnormality of fusing in the second inductor including a fuse wire occurs due to an input of an RF signal having an RF power value greater than or equal to a predetermined level.

Further, it is preferable that a magnetic body arranged close to the fuse wire is further included.

With such a configuration, since a magnetic body exhibits a large loss in a high-frequency band, and an RF signal inputted to a fuse wire is lost as heat by the magnetic body, the magnetic body arranged close to the fuse wire generates heat to effectively heat the fuse wire in the case where an RF signal having an RF power value greater than or equal to a predetermined level is inputted to the fuse wire, so that the fuse wire is fused efficiently. Further, it is more preferable that the fuse wire is provided at the magnetic body.

Further, it is preferable that the capacitor unit and the inductor unit constitute a parallel circuit, and an input terminal connected to one connection point of the parallel circuit and an output terminal connected to the other connection point are provided.

With such a configuration, the LC resonance circuit including the capacitor unit and the inductor unit connected in parallel has a characteristic that impedance becomes maximum at the resonant frequency. By connecting the input terminal to one connection point of the parallel circuit including the capacitor unit and the inductor unit connected in parallel and connecting the output terminal to the other connection point, the second inductor including the fuse wire is disconnected by fusing when an RF signal having an RF power value greater than or equal to a predetermined level is inputted to the LC resonance circuit, and the resonant frequency of the LC resonance circuit fluctuates within the prescribed frequency band. After that, the RF signal in the prescribed frequency band inputted to the input terminal is reflected by the LC resonance circuit, and therefore, the RF signal in the prescribed frequency band is reliably blocked.

Further, it may also be preferable that the capacitor unit and the inductor unit constitute a series circuit, and an input terminal and an output terminal connected to one open end of the series circuit are provided, and the other open end is grounded.

With such a configuration, the LC resonance circuit including the capacitor unit and the inductor unit connected in series has a characteristic such that impedance becomes minimum at the resonant frequency. By connecting the input terminal and the output terminal to one open end of the series circuit including the capacitor unit and the inductor unit connected in series and grounding the other connection point, the second inductor including a fuse wire is disconnected by fusing when the RF signal having an RF power value greater than or equal to the predetermined level is inputted to the LC resonance circuit, and the resonance frequency of the LC resonance circuit fluctuates within the prescribed frequency band. After that, the RF signal in the prescribed frequency band inputted to the input terminal flows to the ground through the LC resonance circuit, and therefore, the RF signal in the prescribed frequency band is reliably blocked.

Further, it is preferable that the LC resonance circuit is included in a chip component.

With such a configuration, the LC resonance circuit preferably is formed integrally to be a single chip component, so that the RF signal blocking device is compact. Therefore, the RF signal blocking device is easily and readily mountable to a wiring substrate or the like.

According to various preferred embodiments of the present invention, the capacitor unit and the inductor unit of the LC resonance circuit are configured such that the resonant frequency of the LC resonance circuit after disconnection by fusing of the second inductor including the fuse wire, among the first and second inductors of the inductor unit, fluctuates within the prescribed frequency band. Accordingly, an RF signal blocking device is provided which involves a new configuration having a function capable of reliably blocking an RF signal in a prescribed frequency band after an abnormality of fusing in the second inductor including a fuse wire occurs due to an input of an RF signal having an RF power value greater than or equal to a predetermined level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Preferred Embodiment

A first preferred embodiment of an RF signal blocking device of the present invention will be described with reference toFIGS. 1 to 4.FIG. 1represents an RF signal blocking device according to the first preferred embodiment of the present invention.FIG. 2represents an internal configuration of the RF signal blocking device shown inFIG. 1.FIGS. 3A,3B,4A and4B are explanatory diagrams representing functions of the RF signal blocking device according toFIG. 1, whereFIGS. 3A and 4Arepresent an equivalent circuit, andFIGS. 3B and 4Brepresent a frequency characteristic. For simplification of description,FIGS. 1 to 4Bshow only a basic configuration according to a preferred embodiment of the present invention, and other configuration details are not illustrated. Further,FIGS. 3A and 3Brepresent the RF signal blocking device in a normal state, andFIGS. 4A and 4Brepresent a state where an abnormality has occurred in the RF signal blocking device.

As shown inFIG. 1, an RF signal blocking device1preferably is configured as a chip component and provided therein with an LC resonance circuit4shown inFIG. 3A, and includes a multilayer substrate2, which is preferably formed by baking a laminate constituted of dielectric ceramic layers and a magnetic ceramic layer, and external electrodes3a,3blocated at opposite ends of multilayer substrate2. RF signal blocking device1blocks an RF signal in a prescribed frequency band after an RF signal having an RF power value greater than or equal to a predetermined level is inputted. Further, external electrodes3a,3bare respectively provided as an input terminal PI and an output terminal PO of LC resonance circuit4.

Multilayer substrate2is constituted by a lamination of insulator layers20to27, and29and a magnetic layer28. A first inductor22a(inductor L1) is provided on insulator layer22, and a second inductor28a(inductor L2) is provided on magnetic layer28. A first capacitor electrode24aand a second capacitor electrode25aare respectively provided on insulator layers24,25. A capacitor unit5(capacitor C1) includes first capacitor electrode24a, insulator layer24, and second capacitor electrode25a. In the present preferred embodiment, insulator layers20to27, and29preferably are constituted by alumina-based glass ceramic, and magnetic layer28is constituted by magnetic ceramic containing ferrite as a main component.

Specifically, insulator layer22is provided with a meander-shaped first inductor22adefining inductor L1. One end of first inductor22ais connected to external electrode3a(input terminal PI), and the other end is connected to external electrode3b(output terminal PO).

Insulator layer24is provided with first capacitor electrode24adefining one electrode of capacitor C1 and is connected to external electrode3b(output terminal PO). Further, insulator layer25is provided with second capacitor electrode25adefining the other electrode of capacitor C1 and is connected to external electrode3a(input terminal PI).

Magnetic layer28is provided with second inductor28adefining a coil L2. Second inductor28aincludes a fuse wire having a thickness and a length configured to fuse due to an input of an RF signal having an RF power value greater than or equal to a predetermined level. One end of second inductor28ais connected to external electrode3a(input terminal PI), and the other end is connected to external electrode3b(output terminal PO). The term “fuse” indicates that a wiring pattern including a fuse wire constituting second inductor28ais melted by heat and disconnected, for example.

Next, one example of a method for manufacturing RF signal blocking device1shown inFIG. 1will be described schematically. RF signal blocking device1of the present preferred embodiment is manufactured preferably by firstly forming multilayer substrate2having ceramic as a base material by baking and thereafter forming external electrodes3a,3bat opposite ends of the formed multilayer substrate2.

A ceramic green sheet defining each of the layers20to29constituting multilayer substrate2is obtained by using a film forming device to form a sheet of slurry of mixed powders including alumina and glass mixed together with an organic binder and solvent, and is configured so that baking at a temperature lower than about 1000° C. can be applied, in other words, low-temperature baking can be applied, for example.

Specifically, insulator ceramic green sheets constituting insulator layers20-27and29are prepared preferably by using a sheet forming machine to form a sheet of ceramic slurry made of mixed powders of alumina and glass, an organic binder, a solvent, and the like. Further, a magnetic ceramic green sheet constituting magnetic layer28is prepared by using a sheet forming machine to form a sheet of ceramic slurry made of ferrite material powders, an organic binder, a solvent, and the like.

Next, a conductor paste containing conductive powders of Ag, Cu, and the like is used to form inductor patterns (first inductor22a, second inductor28a) and capacitor electrode patterns (first capacitor electrode24a, second capacitor electrode25a) so as to be arranged vertically and horizontally in plural on one ceramic green sheet by screen printing or the like. In other words, first inductor pattern22ais formed on insulator ceramic green sheet22, and second inductor pattern28ais formed on magnetic ceramic green sheet28. Further, first capacitor electrode24ais formed on insulator ceramic green sheet24, and second capacitor electrode25ais formed on insulator ceramic green sheet25. It should be noted that an insulator ceramic green sheet forming insulator layer27laminated on magnetic layer28and arranged close to second inductor28a(inductor L2) may be formed as a magnetic layer with the use of a magnetic ceramic green sheet.

Then, the insulator ceramic green sheets and the magnetic ceramic green sheet are laminated and crimped to fabricate a mother laminate. Subsequently, the mother laminate is press-cut by a cutting blade or cut by a dicer to be divided into substrates (multilayer substrate2).

Next, the insulator ceramic green sheets, the magnetic ceramic green sheet, the inductor patterns, and the capacitor electrode patterns are baked integrally to form multilayer substrate2. Then, a conductive paste is applied on a surface of multilayer substrate2, and a plating process is applied on the surface to form external electrodes3a,3b, so that production of RF signal blocking device1is completed.

It should be noted that external electrodes3a,3bmay be formed by dividing a via conductor, which is formed so as to extend across division lines dividing the substrates, from the mother laminate. The via conductor is formed by forming a via hole at a predetermined position on the ceramic green sheet preferably using a laser and thereafter supplying a conductive paste so as to cover an inner surface of the via hole or fill the inner portion.

Further, the division into the substrates may be performed after baking the mother laminate. In the case of mounting a mounted portion on multilayer substrate2, post-baking division capable of performing division after mounting the mounted portion is preferable to improve the handling ability at the time of mounting.

As shown inFIGS. 3A and 3B, in the present preferred embodiment, a parallel circuit is provided in which capacitor unit5constituted of capacitor C1 is connected in parallel with inductor unit6constituted of two inductors L1, L2 connected in parallel. Input terminal PI (external electrode3a) is connected to one connection point of the parallel circuit having capacitor unit5and inductor unit6connected in parallel, and output terminal PO (external electrode3b) is connected to the other connection point, so that LC resonance circuit4is formed which blocks an RF signal in a frequency band f1 higher than a prescribed frequency band f0.

Next, a frequency characteristic of RF signal blocking device1will be described. A resonant frequency f of LC resonance circuit4can be expressed as follows.
f=1/(2π(L·C)1/2)

Therefore, resonant frequency f of LC resonance circuit4becomes smaller as inductance L or capacitance C increases.

However, as shown inFIG. 4A, when an RF signal having an RF power value greater than or equal to a predetermined level is inputted from input terminal PI, second inductor28aincluding a fuse wire, among inductors L1, L2 connected in series to define inductor unit6, is fused and disconnected, so that inductance L of inductor unit6increases.

Thus, as described above, since resonance frequency f of LC resonance circuit4fluctuates to be lower when inductance L increases, the frequency characteristic of RF signal blocking device1fluctuates so as to block an RF signal in a prescribed frequency band as shown inFIG. 4B. In the present preferred embodiment, capacitor unit5and inductor unit6are set such that the resonance frequency of LC resonance circuit4constituted by capacitor C1 and inductor L1 becomes f0.

In an output circuit of mobile communication terminals such as mobile phones or mobile information terminals, RF signal blocking device1is inserted between a coupler connected to an antenna and an output of a power amplifier that amplifies the RF signal. Frequency band f0 at which an RF signal is blocked in the case where an RF signal having an RF power value greater than or equal to a predetermined level is set to the same frequency band of a transmission signal of communication mobile terminals. With such a configuration, when an RF signal having an RF power value greater than or equal to a predetermined level is outputted from the power amplifier, second inductor28aof inductor L2 is fused and disconnected, so that the frequency band of a cutoff frequency of LC resonance circuit4fluctuates to f0. Accordingly, the RF signal in frequency band f0 outputted from the power amplifier is blocked after the frequency band of the cutoff frequency fluctuates, and therefore, damage to a coupler and an antenna due to the RF signal in frequency band f0 having an RF power value greater than or equal to the predetermined level outputted from the power amplifier is prevented.

As described above, according to the present preferred embodiment, LC resonance circuit4including capacitor unit5provided with capacitor C1 and inductor unit6including two inductors L1, L2 connected in parallel, and blocking an RF signal in frequency band f1 higher than prescribed frequency band f0 allows a signal in prescribed frequency band f0 to pass through in a normal time. On the other hand, second inductor28aof inductor L2 of inductor unit6includes a fuse wire which is fused due to an input of an RF signal having an RF power value greater than or equal to a predetermined level, thus second inductor28a(inductor L2) including a fuse wire is fused when an RF signal having an RF power value greater than or equal to a predetermined level is inputted to LC resonance circuit4.

Since inductor unit6is formed preferably by connecting two inductors L1, L2 in parallel, the number of inductors connected in parallel is reduced when second inductor28a(inductor L2) including the fuse wire is disconnected by fusing. Hence, inductance L of inductor unit6increases, and the increase of inductance L causes the frequency band of the cutoff frequency (resonant frequency) in LC resonance circuit4to fluctuate to be low from f1 to f0. Accordingly the RF signal in fluctuated frequency band f0 of the cutoff frequency is blocked.

Thus, by setting capacitor unit5and inductor unit6such that the frequency band of the cutoff frequency in LC resonance circuit4after the disconnection of second inductor28a(inductor L2) including a fuse wire by fusing fluctuates within prescribed frequency band f0, RF signal blocking device1is provided which involves a new configuration having a function capable of reliably blocking an RF signal in prescribed frequency band f0 after an abnormality of fusing in second inductor28a(inductor L2) including a fuse wire occurs due to an input of an RF signal having an RF power value greater than or equal to a predetermined level.

Further, the ferrite (magnetic body) of magnetic layer exhibits a large loss in a high-frequency band, and an RF signal inputted to second inductor28ais lost as heat by ferrite. Thus, ferrite arranged close to second inductor28agenerates heat to effectively heat the wiring pattern of second inductor28ain the case where an RF signal having an RF power value greater than or equal to a predetermined level is inputted to second inductor28a, so that second inductor28ais fused efficiently.

Further, LC resonance circuit4including capacitor unit5and inductor unit6connected in parallel has a characteristic that the impedance becomes maximum at resonant frequency f. Thus, by connecting input terminal PI to one connection point of the parallel circuit including capacitor unit5and inductor unit6connected in parallel and connecting output terminal PO to the other connection point, inductor L2 including second inductor28ais disconnected by fusing when an RF signal having an RF power value greater than or equal to a predetermined level is inputted to LC resonance circuit4, and the cutoff frequency of LC resonance circuit4fluctuates within prescribed frequency band f0. After that, the RF signal in prescribed frequency band f0 inputted to input terminal PI is reflected by LC resonance circuit4, and therefore, the RF signal in prescribed frequency band f0 is reliably blocked.

Further, by configuring LC resonance circuit4to be a chip component, RF signal blocking device1is compact, so that RF signal blocking device1is easily and readily mounted to a wiring substrate or the like.

Second Preferred Embodiment

A second preferred embodiment of the RF signal blocking device according to the present invention will be described with reference toFIGS. 5A,5B,6A and6B.FIGS. 5A,5B,6A and6B are diagrams illustrating the RF signal blocking device of the second preferred embodiment according to the present invention, whereFIGS. 5A and 6Aillustrate an equivalent circuit, andFIGS. 5B and 6Billustrate a frequency characteristic.FIGS. 5A and 5Brepresent a normal state of the RF signal blocking device, andFIGS. 6A and 6Brepresent a state where an abnormality has occurred in the RF signal blocking device.

The second preferred embodiment is different from the first preferred embodiment in that, as shown inFIG. 5A, capacitor unit5and inductor unit6are connected in series to define an LC resonance circuit4a. Input terminal PI and output terminal PO are connected to one open end of the series circuit including capacitor unit5and inductor unit6connected directly, and the other open end is grounded, so that the RF signal in frequency band f1 higher than prescribed frequency band f0 is blocked as shown inFIG. 5B. Other configuration features and operation are preferably the same as or similar to those of the first preferred embodiment. Therefore, description of the configuration and operation will be omitted by allotting the same reference numerals.

With such a configuration, LC resonance circuit4aincluding capacitor unit5and inductor unit6connected in series has a characteristic that impedance becomes minimum at the resonant frequency. Thus, by connecting input terminal PI and output terminal PO to one open end of capacitor unit5and inductor unit6connected in series and grounding the other connection point, second inductor28a(inductor L2) including a fuse wire is disconnected by fusing as shown inFIG. 6A, when an RF signal having an RF power value greater than or equal to the predetermined level is inputted to LC resonance circuit4a. After the cutoff frequency of LC resonance circuit4afluctuates within the prescribed frequency band f0 as shown inFIG. 6B, the RF signal in prescribed frequency band f0 inputted to input terminal PI flows to the ground through LC resonance circuit4a. Therefore, the RF signal in prescribed frequency band f0 is reliably blocked.

Further, since an RF signal having an RF power value greater than or equal to the predetermined level flows to the ground through LC resonator4a, damage to other circuit elements due to the RF signal is prevented.

The present invention is not limited to the preferred embodiments described above, and various modifications other than those described above can be applied as long as the modification does not depart from the spirit of the present invention. For example, the number of capacitors defining the capacitor unit5and the number of inductors connected in parallel to define inductor unit6are not limited to the examples described above. For example, the number of capacitors and the number of inductors may be set as needed depending on a circuit configuration of the external device to which the RF signal blocking device is provided.

Further, although magnet layer28provided with second inductor28apreferably includes ferrite in the preferred embodiments described above, an insulator layer provided with second inductor28aby a normal ceramic dielectric may be provided. Further, the configuration of arranging ferrite (magnetic body) close to inductor L2 is not limited to the example described above. Ferrite may be arranged close to inductor L2 in any form, for example, by arranging ferrite partially in contact with a central portion of second inductor28aof inductor L2, as long as second inductor28ais capable of being heated by ferrite generating heat due to loss of the RF signal.

Further, the configuration of the RF signal blocking device is not limited to a chip component. For example, any configuration of the RF signal blocking device may be adopted, for example, by providing the RF signal blocking device with an internal wiring pattern of a resin multilayer substrate to which a plurality of circuit elements are mounted.

Further, the inductor (wiring) pattern of the inductor may be a lumped constant line such as a coil or a distributed constant line such as a strip line.

Preferred embodiments of the present invention can be applied to various devices that use an RF signal.