Abstract:
A data cable for high-speed data transmission at signal frequencies of &gt;10 GHz includes at least one core pair which is surrounded by a film-shaped pair shield having an inner shielding film and an outer shielding film which, are in electrical contact with one another and in which the inner shielding film is wound around the core pair. By virtue of this measure, an undesired resonance effect is avoided which, in previously wound pair shields, has not allowed use for relatively high signal frequencies. At the same time, an undesired common-mode signal, which would occur in the case of a longitudinally folded shielding film, is thus suppressed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation application, under 35 U.S.C. §120, of copending International Application PCT/EP2015/059078, filed Apr. 27, 2015, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2014 207 879.2, filed Apr. 25, 2014; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The invention relates to a data cable for high-speed data transmissions having at least one core pair including two cores which are surrounded by a film-like or film-shaped pair shield. 
         [0004]    A data cable of that type is known, for example, from European Patent Application EP 2 112 669 A2, corresponding to U.S. Patent Application US 2009/0260847. 
         [0005]    In the field of data transmission, for example in computer networks, data cables are used for data transmission, in which a plurality of data lines are typically combined in a common cable sheath. For high-speed data transmissions, shielded core pairs are used as data lines, wherein the two cores are specifically routed in parallel or, alternatively, are twisted together. Each core includes an independent conductor, for example a solid conductor wire or a stranded wire, each of which is surrounded by insulation. The core pair of a respective data line is surrounded by a (pair) shielding. The data cable is typically formed of a plurality of shielded core pairs of that type, which form a conductive core and are surrounded by a common outer shielding and by a common cable sheath. Data cables of that type are used for high-speed data links, and are constructed for data transmission rates in excess of 5 Gbit/s, specifically at frequencies exceeding 14 GHz. The outer shielding is significant with respect to both EMV and EMI properties, and carries no signals. Conversely, the respective pair shield dictates both the symmetry and the signal properties of a respective core pair. 
         [0006]    Data cables of that type are typically “symmetrical data lines,” in which the signal is transmitted through one core, and the inverted signal is transmitted through the other core. The differential signal component between those two signals is evaluated, in such a way that external effects, which impact upon both signals, are eliminated. 
         [0007]    In many cases, data cables of that type are prefitted to connectors. In applications for high-speed transmissions, connectors are frequently configured as “small form pluggable” connectors, or “SFP” connectors for short. A number of variants in execution are available for that purpose, including “SFP−”, “SFP+” or “CXP-QSFP” connectors. Those connectors are provided with special connector housings, which are known, for example, from International Publication WO 2011 072 869 A1, corresponding to U.S. Pat. No. 8,444,430 or International Publication WO 2011 089 003 A1, corresponding to U.S. Pat. No. 8,556,646. Alternatively, a direct “back plane” connection or connector is also possible. 
         [0008]    In their interior, connector housings of that type incorporate a printed circuit board or card, which is partially provided with integrated electronics. On the reverse side of the connector, the respective data cable is to be connected to that card. To that end, the individual cores of the data cable are soldered or welded to the card. The opposite end of the card is typically configured as a connecting tab with connecting contacts, which is plugged into a mating connector. Cards of that type are also described as “paddle cards”. 
         [0009]    In that configuration, the pair shielding of a respective core pair, as is known, for example, from European Patent Application EP 2 112 669 A2, corresponding to U.S. Patent Application US 2009/0260847, is configured as a longitudinally folded shielding film. The shielding is consequently folded around the core pair in a longitudinal direction of the cable, wherein the two ends overlap in a longitudinally-oriented overlap zone. The shielding film used for shielding purposes is a multi-layer shielding film, formed of at least one conductive (metal) layer and an insulating layer. An aluminum layer is customarily employed as the conductive layer, and a PET film as the insulating layer. The PET film is configured as a substrate, to which a metal coating is applied for the formation of the conductive layer. 
         [0010]    In addition to the longitudinally folded shielding of parallel pairs, the option is available, in principle, for the helical winding of a shielding film of that type around the core pair. However, at higher signal frequencies, in excess of approximately 15 GHz, for structural reasons, any such braiding of the core pair with a shielding film is not possible without further measures, on the grounds of resonance effects. At those high frequencies, the shielding film is therefore applied as a longitudinally folded film. 
         [0011]    A longitudinally applied film of that type, however, is associated with unwanted and negative secondary effects. Longitudinally folded shielding does not provide adequate damping of the “common mode signal,” also described as the in-phase signal, of the type associated with the application of a braided shielding film. 
         [0012]    The generation of the common mode signal or in-phase signal in symmetrical lines of that type with parallel pairs is known, in principle. Moreover, the damping of the unwanted common mode signal is handicapped, in that the common mode signal component is generally propagated more rapidly than the differential signal component, which is of practical value. The absent or severely reduced damping of that common mode signal, in comparison with braided core pairs, therefore results in the impairment of “skew” or of “mode conversion performance.” 
         [0013]    In high-speed data connections of that type, the objective is generally an increase in transmission capacity. Data transmission rates, and consequently the frequency range of data cables of that type are constantly increasing, with an associated increase in problems associated with common mode signal components. 
       SUMMARY OF THE INVENTION 
       [0014]    It is accordingly an object of the invention to provide a data cable, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known cables of this general type and which achieves improved data transmission in a high-speed data link of this type, at high signal frequencies in excess of 10 GHz. 
         [0015]    With the foregoing and other objects in view there is provided, in accordance with the invention, a data cable configured for high-speed data transmissions, comprising at least one, and preferably a plurality of core pairs of two longitudinally-extending cores, in which each core pair is surrounded by a respective film-like or film-shaped pair shield. The pair shield has a first inner shielding film and a second outer shielding film and the inner shielding film is wound around the core pair. The two shielding films are in mutual electrical contact. 
         [0016]    The consideration informing this construction is the combination of the benefits of a helically-wound pair shielding with those of a longitudinally folded pair shielding. This construction employs the knowledge that resonance effects associated with a helically-wound pair shielding at high signal frequencies are generated by the circumstance whereby, in a conventionally wound pair shielding, which is customarily multi-layered, the two conductive layers of the wound shielding are mutually insulated in the overlap zone, thereby forming a capacitor. Simultaneously, the helical winding forms a coil in such a way that, overall, an oscillating circuit with a predefined resonant frequency is constituted, which cannot be displaced to a higher frequency band by structural measures associated with a conventional construction. 
         [0017]    Due to the configuration of pair shielding in two layers, which are electrically interconnected, the formation of an oscillating circuit of this type can be reliably suppressed on the grounds that, as a result of the electrical connection, no coil-type winding is present, and the coil is thus virtually short-circuited. The resonant frequency is the root of (1/(L*C)). Since the inductance is also reduced, at least to a significant degree, the resonant frequency can easily be set to values in excess of 15 GHz. Conversely, this resonant or critical frequency in conventional metal film braidings, depending upon geometry, is subject to an upper limit on the order of 15 GHz. Accordingly, the basic concept of a longitudinally folded pair shielding can be adopted, at least with respect to its functional result. At the same time, winding, preferably with overlapping, permits the reliable suppression of the disadvantage of a longitudinally folded pair shielding, namely, the high common mode signal. Overall, therefore, the pair shielding described herein, which is constituted of the two shielding films, permits the achievement of effective shielding, with no disruptive secondary effects. Resonance effects, and the correspondingly high damping of the signal, together with the inadequate damping of the common mode signal, specifically in case of the overlapping of the inner shielding film, are effectively prevented. In comparison with a longitudinally folded film, this construction is distinguished by simpler construction, superior symmetry and enhanced (bending) flexibility. 
         [0018]    The cores in each respective core pair are thus specifically configured in a mutually parallel configuration, and are consequently not twisted. 
         [0019]    The inner shielding film is appropriately wound around the core pair in an overlapping configuration. The desired damping of the common mode signal is reliably and advantageously achieved by overlapping. 
         [0020]    According to a first variant, only a small overlap is provided. The overlap is preferably on the order of less than 20%, specifically less than 10%, and more specifically less than 5% of the width of the inner shielding film. This figure lies, for example, within the range of 1% to 5%. The width of the shielding film is typically on the order of 4 to 6 mm. The width of the overlap zone of the inner shielding film therefore ranges from 0 to a maximum of 0.6 mm, and the maximum overlap is therefore specifically on the order of 10%. Preferably, it is lower than this. Investigations have shown that a small overlap of this type is still sufficient for the achievement of the desired properties. In comparison with a large overlap, this configuration is associated with a higher frequency range (&gt;20 GHz). The common mode signal is also at least partially damped. This variant provides the advantage of an exceptionally high flexibility of the data cable, together with a high degree of symmetry. 
         [0021]    According to a second variant, conversely, a comparatively large overlap is provided, within the range of 20% to 40%. In this variant, in comparison to the variant with the small overlap, a lower critical frequency is achieved. Simultaneously, however, the damping of the common mode signal component is improved, i.e. the unwanted signal component is suppressed more effectively. Investigations have also shown that the second outer shielding film permits an accurate setting of the resonant frequency, in such a way that a useful frequency band of e.g. up to exactly 20 GHz can be achieved. 
         [0022]    As an alternative to an overlapped winding, the inner shielding film can be wound around the core pair with no overlap, and specifically with no gaps, i.e. in a butt-jointed configuration. This permits the more reliable suppression and exclusion of capacitor effects. At the same time, a gap-free winding ensures the reliable provision of fully-enclosed shielding. In this case, this is ensured by the second outer shielding film, even in the event of bending. 
         [0023]    Appropriately, at least one and preferably both shielding films are configured in multiple layers, with a conductive layer and a non-conductive substrate. The two shielding films are thus specifically configured as “Al-PET” films. The outer film can, in principle, also be configured as a metal film, or as an Al-PET—Al-film, i.e. with a substrate, to which a conductive layer is applied on both sides. In the interests of effective electrical bonding, the two shielding films are configured with their conductive layers or sides in a mutually inward-facing configuration. 
         [0024]    Moreover, it is appropriately provided that the outer shielding film is likewise wound, specifically in the opposing direction to the inner shielding film. This permits the reliable achievement of effective electrical contacting and the bridging of butt joints in the inner shielding film. The pair shielding can thus be described as a double-wound helical pair shielding. 
         [0025]    According to a first variant, the outer shielding film is thus preferably wound at least in a butt-jointed configuration, and particularly with an overlap, in such a way that a closed shielding layer is formed. 
         [0026]    According to a specifically preferred further development, the outer shielding film is wound in a gapped configuration, i.e. adjoining turns of the winding are disposed with a mutual longitudinal clearance. The clearance, and thus the gap, is preferably on the order of only a few percent, for example between 1 and 10% of the width of the shielding film. This variant of execution is preferably applied in combination with the winding of the inner shielding film with a large overlap (20-40% of the width thereof). Due to this specific selection of the configuration and winding of the second shielding film, the accurate setting of the resonant frequency can be achieved. Moreover, the advantage of particularly effective common mode damping is maintained. 
         [0027]    Moreover, at least one sheath wire is preferably provided, and bonded in an electrically conductive configuration to at least one, and preferably to both shielding films. A sheath wire of this type ensures, for example, the secure electrical contacting of the pair shielding to a contact element, for example to a connector. According to a first variant, this sheath wire is disposed between the two shielding films, and is specifically oriented in parallel to the individual cores, for example in an intermeshing area. According to a second variant, the sheath wire is bonded to the exterior of the outer shielding film. Preferably, in general, two sheath wires are disposed symmetrically to a plane of symmetry of the core pair. In the case of the outer sheath wire, it is disposed on the connection axis of the two conductors in the core pair. 
         [0028]    Moreover, in an appropriate further development, a fixing film is also wound around the pair shielding of a respective core pair. Specifically, this is an adhesive film, which is adhered to the pair shielding. The shielding structure of the pair shielding is secured accordingly. The fixing film is specifically an insulating film, such that each pair shielding is provided with exterior electrical insulation, specifically e.g. in relation to a common outer shielding. 
         [0029]    In general, in a preferred configuration, the data cable has a core assembly or cable core formed of a plurality of electrically conductive components, wherein at least one, and preferably a plurality of the conductors are constituted by the core pair which is provided with the pair shielding. 
         [0030]    Appropriately, the cable core is formed exclusively of core pairs of this type. Moreover, the cable core is surrounded by a common outer shielding. This is specifically configured in a multi-layer configuration. The constituents thereof, according to preference or in combination, may be a braided shielding or shielding films, specifically metal-plated films, etc. In turn, an outer cable sheathing is customarily disposed around the outer shielding. 
         [0031]    In the configuration described herein, the data cable, and specifically the pair shielding, are appropriately constructed for the exceptionally effective contact bonding of the pair shielding to a printed circuit board in a typical connector (small form pluggable SFP+, SFP28, QSFP28, etc.) for high-speed data transmission (or “paddle card”). German Patent Application DE 10 2013 225 794 A1, corresponding to U.S. Patent Application US 2016/0294122, describes a preferred form of contact bonding of this type. In the assembled state, the data cable is therefore connected to a connector of this type. 
         [0032]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0033]    Although the invention is illustrated and described herein as embodied in a data cable, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0034]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0035]      FIG. 1  is a diagrammatic, cross-sectional view of a core pair, fitted with a pair shielding; 
           [0036]      FIG. 2  is a side-elevational view of the core pair represented in  FIG. 1 ; 
           [0037]      FIG. 3  is an enlarged, longitudinal-sectional view of the pair shielding in an overlap zone; 
           [0038]      FIG. 4  is an enlarged, cross-sectional view of a data cable according to a first embodiment variant; 
           [0039]      FIG. 5  is a cross-sectional view of a data cable according to a second embodiment variant; and 
           [0040]      FIG. 6  is a diagram in which insertion damping I is plotted against frequency f in GHz for different pair shieldings in a symmetrical core pair. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    Referring now in detail to the figures of the drawings, in which components of equivalent function are identified by the same reference numbers, and first, particularly, to  FIG. 1  thereof, there is seen a core pair  4  for use in a high-speed data cable  2  (see  FIGS. 4 and 5 ), with a pair shielding  6 . The core pair  4  in this case includes two cores  8 , each of which in turn includes a central conductor  10 , which is surrounded by insulation  12 . The conductor  10  is customarily a solid conductor. Alternatively, stranded wires can also be used. 
         [0042]    The two cores  8  are preferably configured in a mutually parallel configuration, and are consequently not twisted together. 
         [0043]    The core pair  4  as a whole is surrounded by a multi-layered pair shielding, which includes an inner shielding film or foil  14  and an outer shielding film or foil  16 . Specifically, these two shielding films  14 ,  16  form a closed configuration of the pair shielding  6 . Finally, the pair shielding  6  is enclosed and is specifically wound within a fixing film or foil  18 , which is specifically configured as an adhesive film. The fixing film  18  is formed of plastic, and is electrically non-conductive, and thus electrically insulating. 
         [0044]    Additionally,  FIG. 1  includes an exemplary representation of an optional sheath wire  20 , which is preferably disposed in an intermeshing zone of the two cores  8 . The sheath wire  20  is moreover specifically disposed between the two shielding films  14 ,  16 . Alternatively, two sheath wires  20  are preferably externally bonded to the outer shielding film  16 , as represented e.g. in  FIG. 5 . The two sheath wires  20  are disposed on an imaginary plane of symmetry or connecting line of the two conductors  10 . In the event of the external positioning of the at least one sheath wire  20 , the latter is therefore specifically held between the outer shielding film  16  and the fixing film  18 . 
         [0045]    The core pair  4 , together with the pair shielding  6  and the fixing film  18  and, where applicable, the sheath wires  20 , are also described hereinafter as a shielded pair  30 . 
         [0046]    The two shielding films  14 ,  16  are preferably each metal-coated plastic films, specifically “Al-PET” films. These films are each provided with a substrate  22 , configured as an insulating layer, to which a conductive layer  24  is applied (see in this respect specifically  FIG. 3 ). In the event of the external positioning of the sheath wires, the outer side of the outer shielding film  16  must also be configured as a conductive layer  24 . The outer shielding film  16  is then, for example, a substrate  22  with conductive layers  24  applied to both sides, or a metal film which, in principle, has conductive layers  24  on either side. 
         [0047]    The two shielding films  14 ,  16  are oriented in such a way that their respective conductive layers  24  are mutually inward-facing, and specifically are in mutual contact, in such a way that the two conductive layers  24  are bonded in an electrically conductive configuration. 
         [0048]    As can be seen in  FIG. 2 , the inner shielding film  14  is helically wound around the core pair  4 . The shielding film  14  is customarily wound with a very small pitch, i.e. in a very close-wound configuration. The smaller the pitch, the greater the displacement of the unwanted resonance effect to higher frequencies. Typically, the pitch is only a few mm, for example on the order of 2 to 6 mm, i.e. for each 360° winding, the shielding film advances by 2 to 6 mm in the longitudinal direction  28 . 
         [0049]    The inner shielding film  14  is wound with an overlap  26 , in such a way that adjoining winding sections are mutually overlapped in the longitudinal direction  28 . According to a preferred configuration, this overlap  26  is equal to approximately one third of the width B of the inner shielding film  14 . 
         [0050]    The outer shielding film  16  is also preferably wound, but in the opposite direction to the inner shielding film  14 . The outer shielding film  16  is, for example, disposed with the same pitch as the inner shielding film  14 . Alternatively, the pitch thereof differs from that of the latter and is, for example, smaller or even greater. The outer shielding film  16  can also be provided with an overlap, or can be wound in a butt-jointed configuration. 
         [0051]    In a preferred configuration, however, a gapped winding is provided, in such a way that a clearance A is formed between two adjoining winding sections. The clearance A, for example, lies within the range of 1-5% of the width B of the outer shielding film  16 . 
         [0052]    The fixing film  18  is specifically a plastic substrate film, to which an adhesive layer is applied. This film is also preferably wound in a manner not shown in  FIG. 2 . 
         [0053]    With reference to the enlarged sectional representation of the pair shielding  6  in an overlap zone shown in  FIG. 3 , it will be seen that the inner shielding film  14 , in its mutually opposing edge zones, and consequently in the overlap zone  26 , is disposed with the conductive layer  24  facing outwards. Therefore, the inner shielding film  14  is not enclosed at the edge zones. In the overlap zone  26 , the inner shielding film  14  is thus disposed in an alternating sequence of the substrate  22  and the conductive layer  24 . Accordingly, the edge zones of the conductive layer  24  of the inner shielding film  14  are separated in a mutually insulated manner in the overlap zone  26 , thereby resulting in the above-mentioned oscillating circuit with the unwanted resonance effect whereby, specifically at higher frequencies in excess of 5 GHz, unwanted damping occurs as a result of the resonance effects. Due to the additional provision of the outer shielding film  16  described herein, these unwanted effects are at least reduced. At the same time, the overlap  26  selected in the exemplary embodiment shown in  FIG. 3  damps the unwanted common mode signal. 
         [0054]    Customarily, in a data cable  2 , a plurality of conductors  30  are combined in a cable core  32 , as represented in  FIGS. 4 and 5 . In both variants, each of the conductors includes a shielded pair  30 . However, other types of conductors can also be incorporated. 
         [0055]    The two variants of the data cable  2  represented in  FIGS. 4 and 5  are mutually distinguished specifically with respect to the composition of the individual shielded pairs  30 . In the variant represented in  FIG. 4 , shielded pairs  30  of the type described with reference to  FIG. 1  are used. 
         [0056]    In the variant represented in  FIG. 5 , an alternative embodiment is employed. In this case, two sheath wires  20  are disposed externally between the outer shielding film  16  and the fixing film  18 . 
         [0057]    In both variants it is preferred that two shielded pairs  30  are firstly wound in a plastic film, as represented in the exemplary embodiment. This core area is then circumferentially enclosed by a plurality of further shielded pairs  30 . In the exemplary embodiment the further shielded pairs  30  are six in number. 
         [0058]    These shielded pairs, and consequently the cable core  32 , are preferably enclosed in a multi-layer sheathing configuration. In data cables  2  of this type, the cable core  32  is generally surrounded by a common outer shield  34 . In the exemplary embodiment, an additional inner layer of plastic film is also wound around the cable core  32 . 
         [0059]    In the exemplary embodiment, the outer shield  34  is configured in a multi-layer configuration, including a combination of film or foil shielding  36  and, for example, braided shielding  38 . Finally, this outer shield  34  is enclosed in a common cable sheath  40 . 
         [0060]      FIG. 6  shows the “insertion damping” I of various shielded pairs of different types, plotted against the frequency f of the data signal being transmitted (in GHz). Curves A and B represent conventional embodiment variants. Curve A represents a shielded pair which is only surrounded by a single-layer shielding film. Conversely, curve B represents a shielded pair which is surrounded by a longitudinally folded shielding film. 
         [0061]    Curve B also represents a characteristic trend for a winding variant in which the inner film  14  is wound with only a small overlap  26 , as described heretofore. 
         [0062]    Curve C is a characteristic curve for a variant associated, for example, with the shortest possible pitch of an Al-PET film, e.g. associated with the use of a 26 AWG (American Wire Gauge) wire. Through the use of an extremely short winding, the critical frequency can thus be displaced to a higher frequency band. 
         [0063]    Curve D is a characteristic curve for the second variant described heretofore, in which the outer shielding film  16  is preferably wound in a gapped configuration, with a small clearance A on the order, for example, of approximately 3% of the width of the shielding film  16 , as described with reference to  FIG. 2 . At the same time, the inner shielding film  14  is preferably wound with a large overlap  26  on the order, for example, of approximately 30% of its width. 
         [0064]    It will clearly be seen that, in a conventional core pair with a wound pair shielding (curve A), insertion damping shows a steep increase with effect from a signal frequency of approximately 5 GHz. Accordingly, the suitability of a data cable of that type for higher signal frequencies is still conditional. 
         [0065]    Conversely, a core pair  4  with a longitudinally folded shielding film (curve B), even at higher frequencies in excess of 5 GHz, shows a significantly smaller increase in damping, even in high-frequency ranges well in excess of 25 GHz. However, as mentioned at the outset, that is achieved at the expense of an unwanted increase in the “common mode signal.” 
         [0066]    Through the use of the special pair shielding  6  described herein, the insertion damping characteristic curve approximates more closely to the characteristic curve associated with a longitudinally folded pair shielding (curve B). A pair shielding  6  of this type, formed of the two shielding films  14 ,  16 , even at higher frequencies in excess of 10 GHz, continues to show acceptable damping, so that a data cable  2  of this type is also suitable for the transmission of high-frequency data signals. 
         [0067]    Overall, the special construction of the pair shielding  6  described herein delivers the following advantages: the resonance effect (which acts as a type of band-stop filter) is inhibited, or is at least displaced to a significantly higher frequency band. At the same time, the effective suppression of the common mode signal is achieved by the overlapping  26 . Overall, the disadvantages of a longitudinally folded pair shielding are significantly reduced while, at the same time, the unwanted resonance effect associated with spiral-wound shieldings is at least extended to a non-disturbing frequency range in excess of 10 GHz, and preferably in excess of 15 or 20 GHz. Helical winding also permits simpler manufacture. In longitudinally folded pair shieldings, the formation of films is associated with a high degree of wear. Moreover, overlaps generate asymmetry and, overall, the flexibility of pairs is reduced by longitudinal films. Moreover, there are disadvantages associated with the production of longitudinal films. Thus, a dedicated individual unit is required for each individual set of dimensions.