Abstract:
A twinax cable is described. The twinax cable has at least one twinax wire pair with a first shield tape wrapped around it and then surrounded by a second shield tape wrapped around the twinax wire pair and the first shield tape. The shield tapes are wrapped such that the metallic sides of the tape face and make contact with each other.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/420,833, filed Dec. 8, 2010 and entitled “TWINAX CABLE DESIGN FOR IMPROVED ELECTRICAL PERFORMANCE”, the subject matter of which is incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to twinax cables with improved electrical performance. 
       BACKGROUND OF THE INVENTION 
       [0003]    The network communications industry has selected the use of twinax wire-pair cable assemblies for the transport of high data rate information. One of the challenges of a twinax wire-pair is that the shield tape that surrounds the twinax wire-pair can interact with the wire-pair in a destructive fashion, producing large resonant spikes (in its transfer function response) that can restrict its intended bandwidth and not support the intended application. 
         [0004]    For applications requiring a high data rate with low latency performance such as in Storage Area Networks (e.g., Fiber Channel over Ethernet) and High Performance Computing (e.g., Infiniband), the copper media selected for connectivity must have a very high bandwidth capacity, such as with twinax cable, to support the un-modulated baseband signal. To obtain low latency while having low power dissipation, baseband digital communication is typically used instead of a complex modulation scheme requiring sophisticated coding techniques. A drawback is media analog bandwidth. For example, in order to support 10 Gbps (Gigabit/sec) data communication; the media must support at least 5 GHz (typically around 7 GHz) of analog bandwidth. The twinax cable industry has demonstrated that twinax cable can support this data rate and even beyond these data rates (see references to single data rate Infiniband and double data rate Infiniband). In order to achieve these bandwidths, the cable design is refined to address performance parameters in this high frequency range. 
         [0005]    For example, to minimize insertion loss, the copper wire surface must be exceptionally smooth (to reduce surface roughness loss), it should be plated with silver (to enhance the smoothness via plating and reduce corrosion; silver also has a conductivity higher than copper), and the wire dielectric must have very low loss (i.e., low dielectric loss or loss tangent). The shield tape plays a critical role in the electro-magnetic wave characteristic that the wire-pair forms. Additionally, there are a number of cable applications that utilize twinax wire-pairs. SFP+ cable utilizes two twinax wire-pairs in its construction (called a one-lane media (where a lane refers to a transmit pair and a receive pair) and also called CX1 standing for “copper times 1 lane”). 40 Gbps Ethernet cable utilizes 8 twinax wire-pairs (CX4) and 100 Gbps Ethernet cable utilizes 20 such twinax wire-pairs (CX10) in its cable construction. 
         [0006]      FIGS. 1A-1B  illustrate the twinax cable construction for a typical SFP+ CX1 application.  FIG. 1A  shows a perspective view of a SFP+ cable  100  and  1 B shows a cross-sectional view of the cable  100 . Each of the two twinax wire-pairs  102  is shielded with a shield tape  104  that is either spirally or longitudinally wrapped about it. A shield tape  104  generally includes a thin sheet of aluminum metal (approximately 1 mil thick) laminated upon an insulating substrate (e.g., polyethylene plastic). (Spiral wrapping is generally preferred due to its superior mechanical properties.) For electrical properties like return loss, it is important to keep the shield in close proximity to the wire-pair  102  as well as keeping the wire-pair  102  together, especially during bending and normal handling of the wire-pair  102 . A disadvantage of the spiral wrapped shield is that a resonance can occur. A longitudinally wrapped shield over the twinax wire-pair is preferred from an electrical (differential mode propagation) standpoint because this resonance does not occur but a cable with a longitudinally wrapped shield has poorer mechanical properties, such as separation of pairs during bending resulting in a change of impedance and higher return loss. From a common mode propagation viewpoint, the longitudinally wrapped twinax cable has a lower attenuation which can be problematic due to mode conversion within the cable or within the connector. In the choice between longitudinal or spiral wrap of the shield tape one must consider multiple cabling parameters. The twinax cable assembly  100  (including two of these shielded twinax wire-pairs) is then further shielded with both a shield tape  106  and a wire braid  108 . The shields have two different purposes, one is for a signal ground and the other is for a safety ground. The twinax wire-pair shield  104  is used for the signal ground and the outer shield  106 ,  108  is used for the safety ground. The twinax wire-pair shield tape  104  wraps about a drain wire  110  which should, but doesn&#39;t always, have good conductivity to the shield tape  104 . The drain wire  110  is used to bridge the signal ground from the cable into the connector signal ground plane. 
         [0007]      FIG. 2A  illustrates a wire-pair  102  of a twinax cable whose shield  104  is spirally wrapped.  FIG. 2B  shows a cross-sectional view of the SFP+ CX1 cable  100 . The main purpose of the drain wire  110  is to connect the shield ground of the cable to the signal ground of the connector. Since an appreciable amount of the signal is contained (coupled to and propagated within) on the shield  104 , how the shield  104  terminates onto the connector is critical to the signal integrity, hence the importance of the high frequency low impedance connection between the drain wire  110  and the shield  104 .  FIG. 2C  illustrates the drain wire  110  to shield  104  contact problems that can occur in a twinax cable  100 . When a high frequency low impedance connection is not made (lower views in  FIG. 2C ), the return loss is increased which can lower the performance of the cable  100 . 
         [0008]      FIGS. 3A-3C  describes the electro-magnetic (EM) field lines that form in a twinax cable wire-pair  102  as well as how the current within the shield  104  travels. In  FIG. 3A , the shield  104  is spirally wrapped tightly around the wire-pair  102  and drain wire  110 . The shield tape  104  is tightly wrapped around the wire-pair  102  and the drain wire  110  in order to consistently keep the wire-pair  102  together in order to maintain the characteristic impedance (and get good return loss performance) and provide a low resistive contact between the drain wire  110  and the shield  104 .  FIGS. 3B and 3C  show the electric and magnetic field lines within the twinax cable wire-pair  102 . The problem is that the shield current does not exclusively follow the EM wave that is traveling longitudinally down the cable. Portions of the energy follow the shield tape  104  in a spiral path and portions will follow the EM field longitudinally (through capacitive coupling within the overlap region). The portion of the signal that follows a spiral path creates a resonance at a frequency corresponding to the overlap periodicity. For example, if the distance between the overlaps is of length 7 mm (and is periodic) the resonant frequency has a fundamental frequency of 28.6 GHz and has second and third harmonics of 14.3 and 7.15 GHz. The resonance that occurs near 7.15 GHz dramatically increases the attenuation of a twinax cable in the vicinity of this resonant frequency. The frequency range where the attenuation is generally effected is from a frequency of 5 times lower frequency than the resonance (hence 1.4 GHz and higher). 
         [0009]      FIG. 4  shows that for a longitudinally applied shield ( FIG. 4A ), the current in the shield travels in the longitudinal path along with the EM wave formed between the conductors. This results in an insertion loss transfer function that performs well as shown in  FIG. 4C , although it also includes the aforementioned mechanical and common mode problems. The spirally wrapped shield shown in  FIG. 4B  can perform poorly as shown in  FIG. 4D , with a resonance peak frequency that is proportional to the shield tape overlap to overlap distances. 
         [0010]    What is needed in the art is a twinax cable including spirally wrapped shields around respective wire-pairs with an improved attenuation spectrum, and possibly other improved electrical and mechanical characteristics which may increase the useable length of the cable in a number of applications. 
       SUMMARY OF THE INVENTION 
       [0011]    A twinax cable is described. The twinax cable has at least one twinax wire pair with a first shield tape wrapped around it and then surrounded by a second shield tape wrapped around the twinax wire pair and the first shield tape. The shield tapes are wrapped such that the metallic sides of the tape face and make contact with each other. 
         [0012]    In some embodiments the two shield tapes may be wrapped in alternate directions. The second shield tape may be the same size, larger, or smaller than the first shield tape. 
         [0013]    In some embodiments the wire pair can include a drain wire. The drain wire may be located to the side of the wire pair or may be spirally wrapped around the wire pair. In some embodiments the drain wire may even be stranded wire. 
         [0014]    In some embodiments, a second twinax wire pair may be included and wrapped with two shield tapes in a similar manner as the first wire pair. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0015]      FIG. 1A  is a perspective view of a two-pair twinax cable. 
           [0016]      FIG. 1B  is a cross-sectional view of the twinax cable of  FIG. 1A . 
           [0017]      FIG. 2A  is a longitudinal cross-sectional view of a twinax wire-pair with a spirally wrapped shield. 
           [0018]      FIG. 2B  is a transverse cross-sectional view of a typical SFP+ cable with twinax wire-pairs. 
           [0019]      FIG. 2C  are transverse cross-sectional views of twinax wire-pairs illustrating examples of problems in twinax cables that can cause poor wire to shield contact. 
           [0020]      FIG. 3A  is a perspective view of a spirally wrapped twinax wire-pair demonstrating how current flows in the wire shield. 
           [0021]      FIG. 3B  is a transverse axial cross-sectional view of a twinax wire-pair illustrating the electric field between the two wires. 
           [0022]      FIG. 3C  is a transverse axial cross-sectional view of a twinax wire-pair illustrating the magnetic field between the two wires. 
           [0023]      FIGS. 4A-D  are schematic views and respective insertion loss graphs demonstrating the difference in wire-pair insertion loss between a longitudinaly applied shield and a spirally wrapped shield. 
           [0024]      FIGS. 5A-C  illustrate longitudinal cross-sections of spirally wrapped shields with varying longitudinal impedance. 
           [0025]      FIG. 6A  illustrate a top view of a spirally wrapped twinax wire-pair with a double shield having improved resonance performance. 
           [0026]      FIG. 6B  is a perspective view of a spirally wrapped twinax wire-pair, similar to  FIG. 6A , where the two shields have approximately equal widths. 
           [0027]      FIG. 6C  is a perspective view of a spirally wrapped twinax wire-pair where the two shields have unequal widths. 
           [0028]      FIG. 6D  is a longitudinal cross-sectional view of the wire-pair of  FIG. 6A  further illustrating how the wire-pair can be connected to a connector via crimp ferrules. 
           [0029]      FIG. 6E  is a cross-sectional view of the wire air of  FIG. 6A  further illustrating how the wire-pair can be connected to a connector via crimp ferrules. 
           [0030]      FIG. 7A  is a longitudinal cross-sectional view of a twinax cable wire-pair and connector interface. 
           [0031]      FIG. 7B  is a perspective view of an embodiment of an inner crimp ferrule. 
           [0032]      FIG. 7C  is a perspective view of an embodiment of an outer crimp ferrule. 
           [0033]      FIGS. 7D-7G  are perspective views illustrating a method of terminating a double shielded twinax pair to a PCB using a pair manage. 
           [0034]      FIG. 7H  is a transverse cross-sectional view of the twinax cable and connector interface of  FIG. 7A . 
           [0035]      FIG. 7I  shows two transverse cross-sectional views of an interface of two twinax cable wire-pairs to a single connector. 
           [0036]      FIGS. 8A-D  are transverse cross-sectional views of a twinax wire-pair illustrating various methods of installing a drain wire. 
           [0037]      FIG. 9A  is a transverse cross-sectional view of a twinax cable wire-pair. 
           [0038]      FIG. 9B  is a cross-sectional views of a twinax cable wire-pair with a spirally wrapped drain wire. 
           [0039]      FIGS. 10A-C  are a cross-sectional view of a twinax wire pair and corresponding schematic views illustrating the electro-magnetic field distribution between the wires of a twinax cable wire-pair. 
           [0040]      FIGS. 11A-D  are transverse cross-sectional views of various twinax cable wire-pair constructions, particularly illustrating variations in shield, dielectric, and drain wire configurations. 
       
    
    
       [0041]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]      FIG. 5  shows a longitudinal cross-sectional view of a portion of a shield tape&#39;s overlap region (A). The shield tape&#39;s longitudinal impedance has essentially two components; a spiral resistive/inductive component and a capacitive component that arises from the tape&#39;s overlap region. The overlap capacitance is controlled by the substrate thickness and the overlap width and length. In order to decrease the shield resonance magnitude and/or to increase the resonant frequency, some methods can be applied. In order to shift the resonant frequency up to higher frequencies, the over-lap to over-lap distance can be decreased. This is accomplished by having more wraps per unit length. To decrease the magnitude of the resonant spike, the overlap capacitance can be increased. One simple method of raising the capacitance (and hence lowering the impedance) is to decrease the tape&#39;s substrate thickness and/or increase the overlap length (as shown in  FIG. 5A ). Raising the capacitance lowers the longitudinal impedance and increases the amount of current that follows the EM wave longitudinally down the cable. Another technique is to use double metallic sided shield tape (metal on both sides of a substrate), which increases the capacitance as well. Another way of decreasing the magnitude of the shield resonance peak is to break up and/or alter the periodicity of the overlap regions (and hence reduce the in-phase constructive interference terms). In  FIG. 5B , a constant overlap to overlap periodicity is shown (where at each of the overlap regions a reflection occurs that constructively interfere with each other at the cables endpoints and form a resonant response), in  FIG. 5C  a random overlap length as a function of longitudinal length is shown (which will break up the constructively adding interference waves, thereby reducing and broadening the resonant peak) and  FIG. 5D  shows a sinusoidal varying overlap length. In C or D, the length between overlap to overlap periodicity is varied and the resonant peak is decreased accordingly. One way to implement a random or periodic overlap is to modulate (or dither) the tape application mechanism either longitudinally or tangential to the cable. Another way to implement either a random or any periodic pattern to produce a longitudinally varying overlap is to use a metallic shield tape whose width has the random width or periodic width. 
         [0043]      FIGS. 6A-6E  illustrate a proposed technique for reducing the shield resonance as well as improving the connection from the cable to the connector. If the shield tape overlap region can be effectively “short-circuited”, the magnitude of the resonance is reduced. This particular technique includes double wrapping shield tapes  104   a ,  104   b  around the wire-pair  102  (shown in  FIGS. 6A and 6B ). The two tapes  104   a ,  104   b  can include a thin sheet of aluminum metal or other conductive material laminated upon an insulating substrate and have their metallic faces (crosshatched surfaces in  FIGS. 6B and 6C ) towards each other, which may eliminate the need for a drain wire. In this technique, the shield wraps  104   a ,  104   b  can have the same widths as shown in  FIGS. 6A-6B , or different widths ( FIG. 6C ), and wound in alternative directions which improves the conductivity between the shield tapes  104   a ,  104   b  and hence minimizes the shield resonant magnitude. The longitudinal cross-sectional view shown in  FIG. 6D , describes how the two shield tapes  104   a ,  104   b  connect/conduct between them, as well as how the cable can be connected to a connector via crimp ferrules  122 ,  124 . The cross-sectional front view is shown in  FIG. 6E . Although  FIGS. 6B and 6C  show a drain wire  105  which typically may be used as a signal ground or reference and is typically electrically conductive with tapes  104   a ,  104   b , drain wire  105  may not be needed when using a pair manager as described below. 
         [0044]      FIG. 7A  shows a longitudinal cross sectional view of a twinax cable wire-pair  102  and a proposed connector interface to it. In  FIG. 7A , an outer crimp ferrule  122  (an example of which is shown in more detail in  FIG. 7C ) is used to create a force acting on the outer shield  104   b  and to the inner crimp/support ferrule  124  (an example of which is shown in more detail in  FIG. 7B ) so that the outer shield  104   b  and the inner crimp/support ferrule  124  make a very low (360°) electrical resistive connection. The other end of the inner crimp/support  124  ferrule is used to make contact to the connector&#39;s circuit board  120  and keeping the impedance of the twinax cable wire-pair  102  as close to 100 ohm as possible. A “pair-manager” (at least ferrule  124 , but can also include ferrule  122 ) is shown in  FIG. 7A  with at least a purpose of maintaining the characteristic impedance of the cable as it enters into the connector and to assist in mechanically aligning the twinax cable wire-pair  102  to the circuit board  120 . 
         [0045]    An example of the connector and circuit board  120  can be a SFP+ connector as shown in U.S. patent application Ser. No. 12/487,778 (Patel et al.), filed Jun. 19, 2009, incorporated by reference as if fully set forth herein. However, the present invention is not limited to such an application an can be used with other connector and cable types such as InfiniBand, QSFP, HDMI, CFP, CXP and other applications. 
         [0046]      FIGS. 7D-7G  are perspective views illustrating a method of terminating a double shielded twinax pair according to the present invention, particularly showing: unwrapping the outer shield  104   b  ( FIG. 7D , outer ferrule  122  has already been slid over the outer shield  104   b  and back onto the twinax pair out of view); connecting the twinax pair to a PCB  120  using an inner ferrule  124  ( FIG. 7E ); rewrapping the outer shield  104   b  over the inner ferrule  124  ( FIG. 7F ); and sliding the outer ferrule  122  over the outer shield  104   b  and inner ferrule  124  where it can be crimped or otherwise bonded to the shield  104   b  and/or ferrule  124 . Outer ferrule  122  can be crimped to electrically bond inner ferrule  124  to the conductive portions of shields  104   a ,  104   b . Alternatively, inner ferrule  124  can be electrically bonded to the conductive portions of shields  104   a ,  104   b  by soldering, welding (ultrasonic or otherwise), using a conductive adhesive, or by other techniques, which may eliminate the need for outer ferrule  122 . The pair manager and twinax cable according to the present invention provides a low impedance signal return path. 
         [0047]    Inner ferrule  124  is typically connected to a ground pad, or in other words a signal return, on PCB  120 . One advantage of the pair manager according to the present invention is that it can provide broadband improvement in differential return loss for the cable assembly due to more contact with the shield tapes  104   a ,  104   b , in addition to providing termination to PCB  120  signal ground or reference, for the shield tapes  104   a ,  104   b  which tapes provide improved insertion loss characteristics for the cable/assembly according to the present invention. 
         [0048]    In  FIG. 7H , a cross-sectional side view is shown for a single twinax cable wire-pair. In  FIG. 7I , two options are shown for connecting two wire-pairs  102  to the connector. 
         [0049]    A cross-sectional view of a standard twinax cable wire par  102  is again shown in  FIG. 8A , and some proposed methods to minimize the shield resonance by moving the drain wire  110  to the side as shown in  FIG. 8B , or through the use of stranded wire as the drain wire  110  located to the side as shown in  FIG. 8C  or through the use of two stranded wires located to both sides as shown in  FIG. 8D . Locating the drain wire  110  to the side increases the force on the wire and shield to improve the conductivity between them. Using stranded wire should improve the conductivity between the drain wire  110  and the shield  104  because the wire can deform to the shape that surrounds it making a better (lower resistance) contact. 
         [0050]      FIGS. 9A and 9B  show a way of ensuring a better method of contacting the shield to the drain wire. Here a cross sectional view of a twinax cable wire-pair  102  is again shown in  FIG. 9A  and a proposed method to minimize the shield resonance by spirally wrapping the drain wire  110  around the wire-pair  FIG. 9B . Spiral wrapping the drain wire  110  improves the contact force that the shield tape  104  makes on the drain wire  110  and hence lowers the contact resistance. 
         [0051]      FIG. 10A  is a cross-sectional view of a twinax cable wire pair  102  and the electro-magnetic field distribution is shown in  FIGS. 10B and 10C . Due to the boundary conditions at the dielectric air interface, the electric field is distorted (see B). This distortion that occurs with the electric field and not with the magnetic field causes an increase of loss to occur within the cable. This is best understood by use of the diagram in  FIG. 100 . The guided energy within the electro-magnetic field is traveling longitudinally along the twinax cable as defined by Maxwell&#39;s equation. Here the EM field must be transverse so as to obtain a minimum loss wave guide. If the electric field distorts, some of the energy is lost due to the fields not being entirely transverse (this energy is converted to different propagating modes). 
         [0052]      FIGS. 11A-11D  illustrate cross-sectional views of various twinax cable wire-pair constructions. In  FIG. 11A , a typical twinax cable wire-pair  102  where each wire is independently formed and is held together in close proximity by the shield tape  104  that surrounds it is shown. The problems with the embodiment of  FIG. 11A  include return loss (where the wires may move apart during cable flexures), and attenuation (where some of the energy within the electro-magnetic field is lost at the dielectric air boundary). Both the return loss and the attenuation can be improved through the use of an additional drain wire  110  as shown in  FIG. 11B  or either bonding the two wires together as shown in  FIG. 11C  or co-extruding the wires together as shown in  FIG. 11D . In the embodiments of  FIGS. 11B-D , the amount of energy lost due to electric field discontinuities is minimized. In the embodiments of  FIGS. 11C and 11D , the return loss should be improved as long as the manufacturing process keeps the wire-pair&#39;s distance constant. Another technique is to begin with circular shaped wires with dielectric and squeeze two wires together which deforms the dielectric into more of a “D” shape. 
         [0053]    Another technique of differential signal transmission with improved insertion loss is to utilize two 50Ω coax cables. Two signals (who have the same magnitude but 180° phase difference between them) are launched simultaneously into two different coax cables. In this technique, there is a reduction of insertion loss but maintaining a low skew between the two signals can be difficult due to variation in the two different coax cables. 
         [0054]    For a SFP+ application, for example, the present invention can be used in a communication system that includes equipment such as switches, servers, and/or an active patch panel as is described in U.S. patent application Ser. No. 12/726,412 (Nordin et al.), filed Mar. 18, 2010, titled “Active Patch Panel,” incorporated by reference as if fully set forth herein. The twinax cable can be used in a cable assembly according to the aforementioned Patel et al. &#39;778 patent application, for example, to interconnect various equipment. The present invention can also be used in communication systems of other applications, such as InfiniBand, QSFP, HDMI, CFP, CXP and other applications employing twinax cable. 
         [0055]    While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing without departing from the spirit and scope of the invention as described.