Abstract:
In a coaxial cable having a signal conductor and a shield conductor between which a first dielectric member is placed, a second dielectric member covers the shield conductor and said first dielectric member. The second dielectric member has a dielectric constant equal to that of the first dielectric member.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a coaxial cable.  
           [0002]    In a digital visual interface, for example, use is made of a transmission minimized differential signaling (TMDS) circuit based on one of digital flat panel standards. The TMDS circuit includes as a transmission cable a single twin-nax cable or two coaxial cables each of which comprises a signal conductor with a drain conductor added thereto.  
           [0003]    Traditionally, various types of coaxial cables have been developed and widely used. For example, Japanese Unexamined Patent Publication (JP-A) No. H06-275142 discloses a coaxial cable which comprises a center conductor, a first insulating layer covering the center conductor, a special conductor covering the first insulating layer, and a second insulating layer covering the special conductor. Japanese Unexamined Patent Publication (JP-A) No. S60-101808 discloses a coaxial cable assembly or unit comprising a plurality of coaxial cables each of which is equivalent in structure to that mentioned above and which are collectively accommodated in a tubular member comprising an inner insulator, an outer insulator, and a shield conductor interposed therebetween.  
           [0004]    In such a coaxial cable, it is general that an inner one of the insulators is made of a material having a relatively low dielectric constant while an outer one of the insulators is made of a material having a relatively high dielectric constant. Therefore, if the coaxial cable is used in the TMDS circuit, the signal conductor and the drain conductor are not coincident in transmission characteristic with each other.  
           [0005]    In a conventional twin-nax cable, the drain conductor comprises a stranded wire which is not subjected to dielectric treatment. Therefore, the signal conductor and the drain conductor are different from each other in inductance, impedance, conductance, and capacitance which dominate or determine EMI characteristics of a high-frequency transmission path. This may results in deterioration in high-frequency transmission path characteristics.  
           [0006]    Furthermore, frequency response is also different between the signal conductor and the drain conductor. Therefore, in the strict sense, the drain conductor and the signal conductor will be different in transmission speed from each other.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of this invention to provide a coaxial cable excellent in transmission characteristic.  
           [0008]    It is another object of this invention to provide a coaxial cable capable of achieving excellent high-frequency transmission path characteristics.  
           [0009]    Other objects of the present invention will become clear as the description proceeds.  
           [0010]    According to an aspect of the present invention, there is provided a coaxial cable which comprises a signal conductor, a first dielectric member covering the signal conductor, a shield conductor disposed around the first dielectric member, and a second dielectric member covering the shield conductor and the first dielectric member and having a dielectric constant equal to that of the first dielectric member.  
           [0011]    According to another aspect of the present invention, there is provided a coaxial cable which comprises a signal conductor, a first dielectric member covering the signal conductor, a shield conductor disposed around the first dielectric member, a second dielectric member covering the shield conductor and the first dielectric member and having a dielectric constant equal to that of the first dielectric member, a drain conductor extending in parallel to the signal conductor, a third dielectric member covering the drain conductor and having a dielectric constant equal to that of the first dielectric member, an additional conductor extending in parallel to the signal conductor and the drain conductor, and an additional dielectric member covering the additional conductor and having a dielectric constant equal to that of the first dielectric member, the shield conductor covering the first, the third, and the additional dielectric members.  
           [0012]    According to still aspect of the present invention, there is provided a coaxial cable which comprises a signal conductor, a first dielectric member covering the signal conductor, a shield conductor disposed around the first dielectric member, a second dielectric member covering the shield conductor and the first dielectric member and having a dielectric constant equal to that of the first dielectric member, a drain conductor interposed between the first dielectric member and the shield conductor, a third dielectric member interposed between the shield conductor and the drain conductor and having a dielectric constant equal to that of the first dielectric member, an additional signal conductor extending in parallel to the signal conductor, an additional dielectric member covering the additional signal conductor and having a dielectric constant equal to that of the first dielectric member, and an additional drain conductor covering the additional dielectric member, the additional dielectric member being interposed also between the additional drain conductor and the shield conductor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    [0013]FIG. 1 is a perspective view of an end portion of a coaxial cable according to a first embodiment of this invention;  
         [0014]    [0014]FIG. 2 is a block diagram of a TMDS circuit in which a driver and a receiver are connected through a cable harness using the coaxial cable illustrated in FIG. 1;  
         [0015]    [0015]FIG. 3 is an equivalent electric circuit of the cable harness used in FIG. 2;  
         [0016]    [0016]FIG. 4 is a perspective view of an end portion of a coaxial cable according to a second embodiment of this invention;  
         [0017]    [0017]FIG. 5 is a cross sectional view of the coaxial cable illustrated in FIG. 4;  
         [0018]    [0018]FIG. 6 is a perspective view of an end portion of a coaxial cable according to a third embodiment of this invention;  
         [0019]    [0019]FIG. 7 is a cross sectional view of the coaxial cable illustrated in FIG. 6; and  
         [0020]    [0020]FIG. 8 is an equivalent electric circuit of the coaxial cable illustrated in FIGS. 6 and 7. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Referring to FIG. 1, description will be made of a coaxial cable according to a first embodiment of this invention.  
         [0022]    The coaxial cable depicted at  10  comprises a center conductor  11  extending along a center axis, an inner dielectric member  12  covering an outer peripheral surface of the center conductor  11 , a mesh-like outer conductor  13  disposed on an outer peripheral surface of the inner dielectric member  12 , an outer dielectric member  14  covering an outer peripheral surface of the outer conductor  13 , a shield conductor  15  covering the outer dielectric member  14 , and an insulating sheath  16  covering an outer peripheral surface of the shield conductor  15 . The inner dielectric member  12 , the outer dielectric member  14 , and the insulating sheath  16  are made of a same dielectric material and have relatively low dielectric constants equal to one another. The shield conductor  15  comprises an aluminum tape wound along an inner surface of the insulating sheath  16 .  
         [0023]    When the coaxial cable  10  is used, the center conductor  11  serves as a signal conductor for signal transmission. The outer conductor  13  serves as a drain conductor for a signal return path. The inner dielectric member  12  forms a first dielectric member. The insulating sheath  16  forms a second dielectric member. The outer dielectric member  14  forms a third dielectric member.  
         [0024]    Referring to FIG. 2, the description will be directed to a TMDS circuit based on one of digital flat panel standards known in the art.  
         [0025]    The TMDS circuit of FIG. 2 comprises a driver  17 , a receiver  18 , and a cable harness  19  connecting the driver  17  and the receiver  18 . When the TMDS circuit is operated, first and second signal currents  11  and  12  flows from the receiver  18  to the driver  17 . On the other hand, a return current Ir flows from the driver  17  to the receiver  18 .  
         [0026]    The cable harness  19  comprises two coaxial cables  10  illustrated in FIG. 1. The TMDS circuit having the above-mentioned structure is represented by an equivalent electric circuit illustrated in FIG. 3. Each of the coaxial cables  10  is operable so that a common mode noise is removed in a high-frequency region of 1 GHz or more.  
         [0027]    Referring to FIGS. 4 and 5, the description will be made of a coaxial cable according to a second embodiment of this invention.  
         [0028]    The coaxial cable depicted by  20  comprises two center conductors  21  extending in parallel to each other in a longitudinal direction of the cable, two inner dielectric members  22  covering outer peripheral surfaces of the center conductors  21 , respectively, mesh-like outer conductors  23  disposed on outer peripheral surfaces of the inner dielectric members  22 , respectively, an outer dielectric member  24  collectively covering outer peripheral surfaces of the outer conductors  23 , a shield conductor  25  covering an outer peripheral surface of the outer dielectric member  24 , and an insulating sheath  26  covering an outer peripheral surface of the shield conductor  25 . The inner dielectric members  22 , the outer dielectric member  24 , and the insulating sheath  26  are made of a same dielectric material and have relatively low dielectric constants equal to one another. The shield conductor  25  comprises an aluminum tape wound along an inner surface of the insulating sheath  26 .  
         [0029]    When the coaxial cable  20  is used, each of the center conductors  21  serves as a signal conductor for signal transmission. Each of the outer conductors  23  serves as a drain conductor for a signal return path. Each of the inner dielectric members  22  serves as a first dielectric member. The insulating sheath  26  forms a second dielectric member. The outer dielectric member  24  forms a third dielectric member.  
         [0030]    The coaxial cable illustrated in FIGS. 4 and 5 has the two center conductors  21  and the two outer conductors  23  as described above. Therefore, a cable harness connecting a driver and a receiver of a TMDS circuit can be implemented by a single coaxial cable  20 .  
         [0031]    Next referring to FIGS. 6 and 7, description will be made of a coaxial cable according to a third embodiment of this invention.  
         [0032]    The coaxial cable depicted at  30  comprises two signal conductors  31 , two first dielectric members  32  covering outer peripheral surfaces of the signal conductors  31 , respectively, a drain conductor  33 , a third dielectric member  34  covering an outer peripheral surface of the drain conductor  33 , a shield conductor  35  collectively covering the signal conductors  31  and the drain conductor  33  through the first and the third dielectric members  32  and  34 , and an insulating sheath  36  as a second dielectric member covering an outer peripheral surface of the shield conductor  35 . Herein, two cables  41  each comprising the signal conductor  31  covered with the first dielectric member  32  and one cable  42  comprising the drain conductor  33  covered with the third dielectric member  34  are braided in three strands. The drain conductor  33  is greater in diameter than the signal conductors  31 . Therefore, transmission stress can be reduced.  
         [0033]    The first dielectric member  32 , the third dielectric member  34 , and the insulating sheath  36  are made of a same dielectric material and have relatively low dielectric constants equal to one another. The shield conductor  35  is made of an aluminum Mylar foil shield known in the art. The insulating sheath  36  is made of a polyaluminum tape known in the art. A combination of the shield conductor  35  and the insulating sheath  36  may be made by collectively and helically winding a braided shield tape and a polyvinylchloride tape known in the art. In this event, use can be made as the braided shield tape of that is produced from a tin-plated soft annealed copper wire in the manner known in the art.  
         [0034]    The coaxial cable  30  illustrated in FIGS. 6 and 7 can be represented by an equivalent electric circuit illustrated in FIG. 8. The coaxial cable  30  is operable so that a common mode noise is removed in a high-frequency region of 1 GHz or more. The drain conductor  33  is greater in diameter than the signal conductor so that transmission stress can be reduced. As a consequence, transmission characteristics uniform in all of inductance, impedance, conductance, and capacitance are achieved and an attenuation factor is reduced.