Patent Publication Number: US-2013248222-A1

Title: Coaxial cable and production method for a coaxial cable

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2012-070192, filed on Mar. 26, 2012; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a coaxial cable and a production method for a coaxial cable. 
     BACKGROUND 
     A conventional imaging apparatus includes a head separated imaging apparatus. The head separated imaging apparatus includes a head unit and a main unit that are separated from each other. The head unit includes an image sensor (e.g., charge coupled device (CCD) image sensor or complementary metal oxide semiconductor (CMOS) image sensor). The main unit processes an image signal sent from the head unit. The head unit and the main unit are connected to each other via a camera cable in the head separated imaging apparatus. The camera cable houses a plurality of coaxial cables. Data is sent and received between the head unit and the main unit via the coaxial cables. Further, various coaxial cables have been conventionally proposed for such data sending and reception. 
     By the way, in recent years, downsizing of the head unit of the head separated imaging apparatus is desired. Also, arrangement intervals of connection terminals of the head unit are reduced. Therefore, it has been difficult to connect a center conductor of each of the coaxial cables housed in the camera cable to each of the connection terminals of the head unit. In view of this, it is assumed that the thickness of an insulating layer that coats the center conductor is reduced, to thereby thin the coaxial cable. However, there is a fear that thinning the insulating layer may make it difficult to maintain characteristic impedance (e.g., 75Ω) required for the coaxial cable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of an imaging apparatus according to an embodiment. 
         FIG. 2  is an overhead view of a head unit and a camera cable according to the embodiment. 
         FIG. 3A  is a side view of one of coaxial cables according to the embodiment. 
         FIG. 3B  is a front view of the coaxial cable according to the embodiment. 
         FIG. 4  is a diagram for explaining characteristic impedance of the coaxial cable. 
         FIG. 5A  is a view (plan view) showing connections of the coaxial cables according to the embodiment. 
         FIG. 5B  is a view (side view) showing the connections of the coaxial cables according to the embodiment. 
         FIG. 6  shows an example of conventional coaxial cables. 
         FIGS. 7A and 7B  show a different example of the conventional coaxial cables. 
         FIG. 8A to 8C  are front views of coaxial cables according to modified examples of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A coaxial cable according to an embodiment includes: a center conductor; an insulating layer that coats the center conductor; an outer conductor that coats the insulating layer; and an insulating coating film that coats the outer conductor. The insulating layer has, at least one end portion thereof, an outer diameter smaller than at other portions thereof. 
     Hereinafter, an embodiment will be described with reference to the drawings. 
     Embodiment 
       FIG. 1  is a configuration diagram of an imaging apparatus  100  according to an embodiment (hereinafter, referred to as imaging apparatus  100 ). The imaging apparatus  100  is, for example, an endoscopic apparatus. The imaging apparatus  100  includes a head unit  200 , a camera control unit (CCU)  300 , and a camera cable  400  that connects the head unit  200  and the CCU  300  to each other. 
     The head unit  200  includes an image sensor  210 , a tape automated bonding (TAB) device  220  (wiring board), a circuit board  230 , a base  240 , and a casing  250 . The image sensor  210  is, for example, a solid-state image sensor such as a complementary metal oxide semiconductor (CMOS) image sensor or a charge coupled device (CCD) image sensor. 
     The TAB device  220  is one that has a circuit formed on a heat-resistant film by etching. The TAB device  220  is connected to the image sensor  210  via a bump or a bonding pad. 
     A driver circuit for the image sensor  210  (e.g., circuit for amplifying output) is mounted on the circuit board  230 . The circuit board  230  is connected to terminals of the TAB device  220  and to a wiring of the camera cable  400 . 
     The base  240  is provided with the image sensor  210 , the TAB device  220 , and the circuit board  230 . The casing  250  houses the base  240  provided with the TAB device  220  on which the image sensor  210  is mounted. 
     The CCU  300  includes an interface (IF) circuit  301 , a memory  302 , a processor  303 , a driver  304 , a controller  305 , and a power-supply circuit  306 . 
     The IF circuit  301  is an interface for sending and receiving a control signal and data to/from the head unit  200 . The memory  302  is a non-volatile memory, for example, an electrically erasable programmable read-only memory (EEPROM). The memory  302  stores setting data (operation mode) and correction data for the head unit  200 . 
     The processor  303  is a processor for processing an image. The processor  303  performs various corrections (e.g., noise correction, white balance correction, and γ correction) on an image signal sent from the head unit  200 . The processor  303  outputs the image signal after the corrections to an external display apparatus  500  (e.g., cathode ray tube (CRT) monitor or liquid-crystal monitor). 
     The driver  304  is a driver circuit for the image sensor  210 . The driver  304  changes a drive system or a frame rate of the image sensor  210  according to control by the controller  305 . Further, the driver  304  outputs a pulse signal (e.g., vertical synchronous pulse signal or horizontal synchronous pulse signal (transfer pulse signal, reset gate pulse signal)) to the image sensor  210 . 
     The controller  305  reads out the correction data and the setting data from the memory  302 . The controller  305  controls the processor  303  and the driver  304  based on the read-out correction data and setting data. 
     The power-supply circuit  306  is connected to an external power supply. The power-supply circuit  306  converts an electric power from the external power supply into a predetermined voltage and supplies it to circuit components (IF circuit  301 , memory  302 , processor  303 , driver  304 , and controller  305 ) of the CCU  300 . Further, the electric power from the power-supply circuit  306  is also supplied to the head unit  200  via the camera cable  400 . 
     (Configuration in Head Unit  200 ) 
       FIG. 2  is an overhead view of the head unit  200  and the camera cable  400 . It should be noted that, since the base  240  hides the circuit board  230 , the circuit board  230  is not shown in  FIG. 2 . Further, the casing  250  of the head unit  200  is not shown in  FIG. 2 . As shown in  FIG. 2 , the image sensor  210  is provided to an end surface  240 A of the base  240  of the head unit  200  in such a state that the image sensor  210  is mounted on the TAB device  220 . The TAB device  220  on which the image sensor  210  is mounted is fixed on the base  240  in such a state that the TAB device  220  is folded along a top surface  240 B and a bottom surface  240 C of the base. 
     The TAB device  220  is provided with a plurality of terminals  221  for connecting to a plurality of cables  410  housed in the camera cable  400 . Some of the terminals  221  are connected to not the cables  410  but terminals of the circuit board  230  (not shown). 
     The camera cable  400  houses the plurality of cables  410  for, e.g., data signal (image signal) transmission, synchronous signal (vertical synchronous pulse signal and horizontal synchronous pulse signal) transmission, bias voltage application, electric power supply, and ground (GND). The cables  410  for data transmission and synchronous signal transmission out of the cables  410  housed in the camera cable  400  are coaxial cables (hereinafter, referred to as coaxial cables  410 ). 
       FIG. 3A  is a side view of one of the coaxial cables  410 .  FIG. 3B  is a front view of the coaxial cable  410 . As shown in  FIGS. 3A and 3B , the coaxial cable  410  includes a center conductor  411 , an insulating layer  412  that coats the center conductor  411 , an outer conductor  413  that coats the insulating layer  412 , and an insulating coating film  414  that coats the outer conductor  413 . 
     The center conductor  411  is constituted of a twisted wire material obtained by twisting a plurality of conductive wire (e.g., solid wire made of Cu-2 mass % Ag alloy wire) materials together. The center conductor  411  transmits a data signal (image signal), a synchronous signal, and the like. It should be noted that it is also possible to use not the twisted wire material but a single conductive wire for the center conductor  411 . 
     The insulating layer  412  includes a first insulating layer  412 A and a second insulating layer  412 B. The first insulating layer  412 A and the second insulating layer  412 B are each constituted of a dielectric insulating material (e.g., Teflon (registered trademark) or polyester). The first insulating layer  412 A coats an outer periphery of the center conductor  411 . The second insulating layer  412 B, in turn, coats an outer periphery of the first insulating layer  412 A. It should be noted that a material having a higher heat resistance than the second insulating layer  412 B is preferably used for the first insulating layer  412 A. It is for the purpose of preventing fusion of the first insulating layer  412 A and the second insulating layer  412 B when the outer periphery of the first insulating layer  412 A is coated with the second insulating layer  412 B. 
     The outer conductor  413  is a braided shield obtained by braiding thin metal wires (e.g., copper wires) called braided wires or a served shield obtained by winding the thin metal wires transversely. It should be noted that a metal foil may be used for the outer conductor  413  in order to carry out accurate measurement or suppress attenuation at a frequency equal to or higher than an ultra high frequency. The outer conductor  413  is connected (grounded) to ground (GND) on the CCU  300  side. 
     The insulating coating film  414  is constituted of an insulating material (e.g., polyethylene). The insulating coating film  414  coats an outer periphery of the outer conductor  413 . The insulating coating film  414  insulates the coaxial cable  410  and functions also as a protective coating film. 
       FIG. 4  is a diagram for explaining characteristic impedance of the coaxial cable  410 . As shown in  FIG. 4 , provided that a diameter (outer diameter) of the center conductor  411  is denoted by d, a diameter (outer diameter) of the insulating layer  412  is denoted by D, and a relative permittivity of the insulating layer  412  is denoted by ∈, the characteristic impedance Z 0  (Ω) of the coaxial cable  410  is expressed by Expression (1) below. 
     
       
         
           
             
               
                 
                   
                     Z 
                     0 
                   
                   = 
                   
                     
                       60 
                       
                         ɛ 
                       
                     
                      
                     
                       Log 
                       ɛ 
                     
                      
                     
                       D 
                       d 
                     
                      
                     3 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     By the way, in recent years, the head unit  200  that connects to the camera cable  400  has become smaller. Therefore, as mentioned above, there is a need for thinning each of the coaxial cables  410  housed in the camera cable  400 . However, the characteristic impedance Z 0  of the coaxial cable is generally set to 50Ω or 75Ω. In other words, the characteristic impedance of the coaxial cable  410  cannot be changed. 
     Therefore, reducing the diameter d of the center conductor  411  or reducing the thickness of the insulating layer  412  while using a material having a high relative permittivity ∈ (e.g., porous polyethylene) as the material of the insulating layer  412  is assumed from the above Expression (1) in order to thin the coaxial cables  410 . However, the diameter d of the center conductor  411  is already reduced sufficiently and it is very difficult to further reduce it in terms of electric resistance and strength. In addition, in the case where the material having a high relative permittivity ∈ (e.g., porous polyethylene) is used as the material of the insulating layer  412 , problems are caused in terms of strength and flexibility. 
       FIGS. 5A and 5B  are views showing connections of the center conductors  411  of the coaxial cables  410  to the terminals  221  of the TAB device  220 .  FIG. 5A  is a plan view and  FIG. 5B  is a side view. It should be noted that  FIG. 5  each show, for the sake of description, a case where the three coaxial cables are connected to the terminals  221  of the TAB device  220 . 
     As shown in  FIG. 5 , each of the coaxial cables  410  in this embodiment is configured to be easily connected to each of the terminals  221  of the TAB device  220  by stripping the second insulating layer  412 B at an end portion of the coaxial cable  410  on the head unit  200  side, to thereby reduce a diameter (outer diameter) at the end portion of the coaxial cable  410 . The center conductor  411  of each of the coaxial cables  410  is electrically connected to each of the terminals  221  of the TAB device  220  with a solder P. Alternatively, another method (e.g., silver (Ag) paste) may be used to electrically connect the center conductor  411  of each of the coaxial cables  410  to each of the terminals  221  of the TAB device  220 . 
     It should be noted that the first insulating layer  412 A constituting the insulating layer  412  preferably has a diameter (outer diameter) D 1  equal to or smaller (shorter) than each of arrangement intervals W of the terminals  221  in order to easily connect the center conductors  411  of the coaxial cables  410  to the terminals  221  of the TAB device  220 . Further, a stripping length of the second insulating layer  412 B of the insulating layer  412  is preferably set to about 5 mm for the purpose of suppressing fluctuations of characteristic impedance of the coaxial cable  410 . 
     An example in which coaxial cables  410 A housed in a conventional camera cable  400 A are connected to terminals  221  of a TAB device  220  is shown in  FIG. 6 . An insulating layer  412  at an end portion of each of the conventional coaxial cables  410 A on a head unit  200  side is not thinned. Thus, the insulating layer  412  has a diameter (outer diameter) D 2  larger (longer) than each of arrangement intervals W of the terminals  221  of the TAB device  220 . 
     Therefore, even if connection of the terminal  221  of the TAB device  220  is tried in such a state that the coaxial cables  410 A are arranged in parallel, the position of a center axis of the coaxial cable  410 A and the position of the terminal  221  of the TAB device  220  are misaligned with each other. Therefore, it is difficult to easily connect the center conductors  411  of the coaxial cables  410 A to the terminals  221  of the TAB device  220 . In addition, since a width (D 2 ×3) of the arranged coaxial cables  410 A is larger than a width Z of the base  240 , the coaxial cables  410 A arranged at the both ends protrude from the base  240 . Therefore, it becomes difficult to downsize the head unit  200 . 
     Another example in which coaxial cables  410 A housed in a conventional camera cable  400 A are connected to terminals  221  of a TAB device  220  is shown in  FIGS. 7A and 7B .  FIG. 7A  is a plan view and  FIG. 7B  is a side view. As shown in  FIGS. 7A and 7B , when the coaxial cables  410 A are arranged to be deviated from each other in upper and lower directions such that the coaxial cables  410 A arranged at both ends do not protrude from the base  240 , the entire thickness is increased in turn. Consequently, it becomes difficult to downsize the head unit  200 . 
     Also regarding the soldering, it is necessary to connect the center conductor  411  of the upper coaxial cable  410  (arranged in middle) after connections of the center conductors  411  of the lower coaxial cables  410 A (arranged at both ends). Therefore, it is difficult to easily connect the center conductors  411  of the coaxial cables  410 A to the terminals  221  of the TAB device  220 . 
     As described above, it has been difficult to downsize the head unit  200  in the conventional coaxial cables  410 A. However, as shown in  FIG. 5 , the diameter (outer diameter) at the end portion of the coaxial cable  410  is reduced by stripping the second insulating layer  412 B at the end portion of the coaxial cable  410  on the head unit  200  side in the coaxial cables  410  according to this embodiment. Therefore, it is possible to easily connect each of the coaxial cables  410  to each of the terminals  221  of the TAB device  220  of the downsized head unit  200 . Further, since the second insulating layer  4128  at the end portion of the coaxial cable  410  is stripped by only about 5 mm, no problems of characteristic impedance are caused regarding a signal having a band frequency of 1 GHz or less. 
     (Production of Camera Cable  400 ) 
     Next, a production method for the camera cable  400  will be described with reference to  FIGS. 3A and 3B . First, a polyester to be the first insulating layer  412 A is welded to an outer periphery of the twisted wire material obtained by twisting a plurality of conductive wire (e.g., solid wire made of Cu-2 mass % Ag alloy wire) materials together, the twisted wire material being to be the center conductor  411 , so that the center conductor  411  is coated with the first insulating layer  412 A. At this time, it should be noted that the diameter (outer diameter) D 1  of the first insulating layer  412 A is set to be smaller than each of the arrangement intervals W of the terminals  221  of the TAB device  220 . 
     Subsequently, a polyester to be the second insulating layer  412 B is welded to the outer periphery of the first insulating layer  412 A, so that the first insulating layer  412 A is coated with the second insulating layer  412 B. It should be noted that a material having a higher heat resistance than the second insulating layer  412 B is preferably used for the first insulating layer  412 A. 
     Subsequently, an outer periphery of the insulating layer  412  constituted of the first insulating layer  412 A and the second insulating layer  412 B is coated with a braided shield obtained by braiding thin metal wires (e.g., copper wires) called braided wires or a served shield obtained by winding the thin metal wires transversely, the braided shield or the served shield being to be the outer conductor  413 . It should be noted that the outer periphery of the insulating layer  412  may be coated with a metal foil as the outer conductor  413  instead of the braided shield or the like in order to carry out accurate measurement or suppress attenuation at a frequency equal to or higher than an ultra high frequency. The outer conductor  413  is connected (grounded) to ground (GND) on the CCU  300  side. 
     Subsequently, an outer periphery of the outer conductor  413  is coated with the insulating material (e.g., Teflon (registered trademark) or polyethylene) to be the insulating coating film  414 . In this manner, the coaxial cables  410  are prepared. 
     Subsequently, the insulating coating film  414 , the outer conductor  413 , and the second insulating layer  412 B of each of the coaxial cables  410  are, on one end side of the coaxial cable  410 , stripped by approximately 5 to 7 mm from the end portion thereof by the use of a tool such as a stripper, so that the first insulating layer  412 A is exposed. Subsequently, the exposed first insulating layer  412 A is stripped by approximately 1 to 2 mm from the end portion thereof by the use of the tool such as the stripper, so that the center conductor  411  is exposed. 
     As described above, the diameter (outer diameter) at the end portion of each of the coaxial cables  410  according to this embodiment is reduced by stripping the second insulating layer  412 B at the end portion of the coaxial cable  410  on the head unit  200  side. Therefore, it is possible to easily connect each of the coaxial cables  410  to each of the terminals  221  of the TAB device  220  of the downsized head unit  200 . Further, since the second insulating layer  412 B at the end portion of the coaxial cable  410  is stripped by only about 5 mm, no problems of characteristic impedance are caused regarding a signal having a band frequency of 1 GHz or less. 
     It should be noted that, when the diameter (outer diameter) at the end portion of the coaxial cable  410  is reduced, the second insulating layer  412 B at both end portions of the coaxial cable  410  may be stripped rather than stripping the second insulating layer  412 B at only one end portion of the coaxial cable  410  on the connection side (one side) to the head unit  200 . 
     Modified Examples of Embodiment 
       FIGS. 8A to 8C  show modified examples of the embodiment. As shown in  FIG. 8A , an outer periphery of a first insulating layer  412 A may be coated with a fusion preventing tape T (e.g., heat-resistant tape) before coating of a second insulating layer  412 B in order to prevent fusion of the first insulating layer  412 A and the second insulating layer  412 B. Alternatively, as shown in  FIG. 8B , fusion preventing powder F may be applied to an outer periphery of a first insulating layer  412 A before coating of a second insulating layer  412 B in order to prevent fusion of the first insulating layer  412 A and the second insulating layer  412 B. 
     Alternatively, an outer periphery of a first insulating layer  412 A may be coated with a second insulating layer  412 B in such a state that a wire material C may be placed along a longitudinal direction of the first insulating layer  412 A. In this case, when the second insulating layer  412 B is stripped, it is possible to easily strip the second insulating layer  412 B by pulling the wire material C. It should be noted that a material (e.g., iron wire) having enough strength to tear the second insulating layer  412 B that coats the outer periphery of the first insulating layer  412 A is used as the wire material C. 
     In addition, enameled wires may be used for the center conductor  411  and the first insulating layer  412 A. In this case, since the first insulating layer  412 A becomes very thin, pulling of the coaxial cables  410  and connection to the TAB device  220  (and/or the circuit board  230 ) become easy. Further, since the end portions of the coaxial cables  410  are further thinned, it is possible to further downsize the head unit  200 . 
     Other Embodiment 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.