Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
2001-04-03
2003-03-18
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
Reexamination Certificate
active
06534708
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a signal transmission cable comprising a conductor portion for transmission of an electrical signal and an insulator sheath covering the conductor portion and, in particular, to such a signal transmission cable having a noise absorber suppressing noise leaking out of and invading into the cable.
In order to transmit electrical signals such as communication signals between electronic devices and between electronic apparatus, use is made of signal transmission cables such as communication cables. A typical one of the transmission cables usually comprises a conductor portion for transmission of signals therethrough and an outer insulator sheath surrounding the conductor portion. A coaxial type of the signal transmission cables comprises a central conductor portion for transmission of signals therethrough, an outer conductor portion to be grounded, an insulator layer interposed and insulating between the central conductor portion and the outer conductor portion, and an outer insulator sheath surrounding the outer conductor portion. It is well known as the so called electromagnetic interference (EMI) that high frequency electrical noise is generated from active electronic elements, high frequency circuit components, and high frequency electronic apparatus, flows through the signal transmission cable and is radiated from the cable. On the contrary, electrical noise invades through the signal transmission cable to those active electronic elements, high frequency circuit components, and high frequency electronic apparatus.
It is well known in the art that a cylindrical ferrite core is attached onto an electric power code to an electronic apparatus, for example, computer so as to suppress a high frequency noise from flowing into, or from, the computer through the electric power code. The ferrite core absorbs the high frequency noise current flowing through the power code. The ferrite core used has a large volume in comparison with electronic apparatus which have rapidly been small-sized with electronic circuit components disposed at a high density.
It is also well known in the art that a concentrated constant circuit such as a decoupling capacitor is assembled in a power circuit line in the electronic apparatus so as to suppress undesired radiation from the power line.
It is also another problem that a high frequency noise is often caused or induced from a semiconductor or an integrated circuit device of a high speed operation type such as a random access memory (RAM), a read only memory (ROM), a microprocessor (MPU), a central processing unit (CPU), or an image processor arithmetic logic unit (IPALU) because an electric signal flows in a high speed circuit therein with rapid change in current and voltage value.
In addition, electronic elements and cables are disposed with a high density in a small-sized electronic apparatus. Therefore, those elements and lines are very close to each other and thereby affected to each other to cause EMI.
In order to suppress the high frequency noise from those semiconductor devices and the EMI within the small-sized electronic apparatus, the conventional ferrite core cannot be used because it has a relatively large volume.
On the other hand, use of the concentrated constant circuit cannot sufficiently suppress the high frequency noise caused in the circuit using electronic elements of the high speed operation type because the noise has an increased frequency so that the circuit line actually acts as a distributed constant circuit.
Japanese Unexamined Patent Publication (JP-A) H11-185542 discloses a cable with a thin-film magnetic shield. The cable is generally used as an interface cable for connecting OA (office automation) apparatus such as a personal computer, game apparatus, and communication equipment to one another and as an internal wiring cable for connection of various components in the apparatus.
A first conventional cable with a thin-film magnetic shield is disclosed in the above-mentioned Japanese publication and comprises a plurality of signal conductors as a conductor portion arranged at the center for transmission of signals, an insulating tape wrapped around the conductor portion, a laminated tape wrapped around the insulating tape, and an insulator covering the laminated tape. The laminated tape comprises a laminate of a metal leaf or foil having high conductivity and at least one high-permeability thin film made of a material having high permeability.
With this structure, radiation noise is effectively shielded. Specifically, since the metal leaf (typically, copper leaf) having high conductivity is surrounded by the high-permeability thin film, the radiation noise surviving through the metal leaf without being absorbed thereby can be shielded by the high-permeability thin film. Thus, the radiation noise is first shielded by the metal leaf, and then by the high-permeability thin film arranged therearound. As a consequence, the above-mentioned cable is improved in shielding effect over a wide range, easy in handling, and smart in appearance because the diameter of the cable need not substantially be increased.
A second conventional cable with a thin-film magnetic shield is also disclosed in the above-mentioned publication. This cable is similar in structure to the first conventional cable mentioned above except that the insulating tape is provided with slits. With this structure, the cable as a whole is prevented from occurrence of an antenna effect and the influence of eddy current of the high-permeability thin film is suppressed. Therefore, it is possible to suppress the radiation noise over a wide frequency band.
However, the high frequency current or the high frequency radiation noise contains a harmonic component. In this event, a signal path exhibits the behavior as a distributed constant circuit. Therefore, the conventional countermeasure against the noise is not effective because such countermeasure assumes the lumped constant circuit.
In the above-mentioned publication, the high-permeability thin film is typically a magnetic thin film formed by rolling a permalloy (Fe—Ni alloy). Such magnetic thin film as the high-permeability thin film has following problems. Specifically, the frequency characteristic (“f” characteristic) of the magnetic characteristic thereof is inferior particularly at the high frequency. In addition, the electric characteristic is degraded.
Alternatively, use may be made of the high-permeability thin film made of a Co-based amorphous material, for example, Co—Fe alloy. In this case, however, the frequency characteristic of the magnetic characteristic thereof is inferior particularly at the high frequency, like in the above-mentioned case. Furthermore, although the Co-based amorphous material can be manufactured in a laboratory, the cost is high. Accordingly, this material can not practically be used in the industry.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a signal transmission cable capable of efficiently suppressing only a high frequency noise.
It is another object of this invention to provide a signal transmission cable capable of achieving the above-mentioned effect without requiring any additional space.
This invention is applicable to a signal transmission cable comprising a conductor portion for transmitting an electric signal therethrough and an insulator sheath covering said conductor portion. A typical example of the signal transmission cable is a coaxial cable further comprising an outer conductor portion around said conductor portion and an inner insulator layer disposed between said conductor portion and said outer conductor portion, said outer conductor portion being directly covered with said insulating sheath. According to this invention, the signal transmission cable is provided with a high loss magnetic film formed on at least one area of said insulator sheath and covering at least a part of an outer surface of said sheath. The high loss magnetic film has the maximum complex permeability &mgr;&Prim
Awakura Yoshio
Ono Hiroshi
Bradley N Ruben, PC
Mayo III William H.
NEC TOKIN Corporation
Reichard Dean A.
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