Flat flexible transmission cable
&#39; United States Patent 1 Balaster et al.  
 . [22] Filed:  
 [ FLAT FLEXIBLE TRANSMISSION CABLE [73] Assignee: AMP Incorporated, Harrisburg, Pa.  
 Aug. 23, 1973 [21] Appl. No.: 390,813  
 [ 1 Apr. 8, 1975 3,609,600 9/1971 Kassabgi 333/84 R Primary ExaminerAlfred E. Smith Assistant Examiner-William 1:1. Punter Attorney, Agent, or FirmRussell J. Egan, Esq.  
 [ 5 7 ABSTRACT Improved flat flexible cable having at least one shielded transmission line is disclosed. The subject cable is formed with either a single or a plurality of signal conductors on one side of a flat flexible insulator substrate and one or a plurality of ground conductors on the opposite side of the substrate, the ground conductors preferably being interconnected. Pads are formed at periodically spaced intervals along each of the signal and ground conductors so that the cable can be produced with infinite length and cut to finite lengths at the pads nearest the desired length while maintaining constant electrical transmission properties- (such as characteristic impedance, cross-talk, attenuation, etc.) throughout the length of the line and including the termination area.  
 18 Claims, 8 Drawing Figures PATENTEDAPR 8|975 SHEET 1 OF PATENTEDAPR m SHEET 2 (IF 3 PATENTEDAPR 191s 3. 876,964 9 .313953 S A El so T FLAT FLEXIBLE TRANSMISSION CABLE BACKGROUND OF THE INVENTION 1. The Field Of The Invention The present invention relates to an improvement in shielded flat flexible cable and in particular to cable which can be produced in infinite length and cut for termination at lengths approximating the desired length without degrading the electrical transmission characteristics of the lines.  
 2. Description Of The Prior Art There are several problems associated with the known methods for the production of flat flexible transmission lines. One problem is the production of cable that can be manufactured in long lengths and cut to whatever shorter length is required for the particular equipment design. The cables produced by the conventional prior art methods have been produced in finite lengths, to fit a particular application, with pads formed at the ends thereof adapted to receive appropriate connectors. The termination pads disturb the delicate electrical characteristics of the transmission line (characteristic impedance, cross-talk, attenuation, etc.) and thus make continuous lengths of such known transmission line impractical. Another problem involved in the known methods of producing flexible flat cable is to provide controlled impedance throughout the cable length with adequate electrical shielding characteristics for use in high density wire applications where the signal rise times are in the nano-second region.  
  One attempt to produce a shielded flat flexible cable is disclosed in US. Pat. No. 2,963,535. The cable includes a plurality of signal conductors 21 and two shielding conductors 22 formed on one side of a flexible plastic base material 23. The cable thus produced is sliced into thirds, stacked and laminated with an insulating cover layer 24 to form a single cable. Such a teaching has many obvious disadvantages amongst which are the additional manufacturing steps required for the slicing and laminating operations as well as possible alignment problems. Further, this teaching does not solve the problem of providing appropriate termination means so that long lengths of cable can be produced and subsequently cut to discrete lengths. Also, this teaching does not disclose how to control the characteristic impedance of the cable thus produced.  
 SUMMARY OF THE INVENTION The subject shielded flat flaxible cable includes a flexible substrate having at least one signal transmission conductor formed on one side thereof and at least one shielding or grounding conductor formed on the opposite side of the substrate. Both the signal and ground conductors are provided with a plurality of periodically spaced termination pads each adapted to receive crimp connector means and which maintain integrity of the electrical transmission properties of the cable. When additional shielding is desired for high density wiring applications, the subject cable is folded upon itself so that the ground conductors substantially enclose the signal transmission conductor. When a plurality of ground conductors are used, it is preferable to have these ground conductors interconnected to provide a common ground having a plurality of alternate paths.  
  It is therefore an object of the present invention to produce flat flexible cable having controlled electrical transmission properties of characteristic impedance,  
 cross-talk (both cable-to-cable and line-to-line), attenuation, capacitance, propagation time, etc., with adequate electrical shielding characteristics for use in high density wiring applications where signal rise times are in the nano-second region.  
  It is another object of the present invention to produce flat flexible cable having electrical shielding on at least two sides of a flat signal conductor.  
  It is still another object of the present invention to teach a method of manufacturing shielded flat flexible cable which eliminates problems related to alignment of the signal and ground conductors and problems of interconnecting ground conductors.  
  It is yet another object of the present invention to provide flat flexible cable in which at least one signal conductor is formed on one side of a flexible insulator substrate at least one ground conductor is formed on the other side of the substrate, and the substrate folded upon itself so that simple and inexpensive top and bottom shielding are provided around the signal conductors with electrical interconnection between the shielding via the fold.  
  It is a further object of the present invention to produce flat flexible transmission cable in which the characteristic impedance of the line as well as other associated electrical transmission properties can easily be controlled by the pitch and/or period of the ground conductor and signal conductor.  
  It is a still further object of the present invention to produce continuous flat flexible transmission cable which can be terminated at virtually any point along the line thereby eliminating the previously known termination window in the cable element and the need for separate art work and separate production for each individual length of cable while maintaining electrical integrity throughout the length of the line up to and possibly including the termination thereof.  
  It is still another object of the present invention to produce continuous flat flexible transmission cable in which a plurality of termination pads are formed on signal and ground conductors at regularly spaced intervals, with the termination pads decreasing the effective resistance of the line and thus reducing the attenuation while maintaining electrical integrity throughout the length of the line.  
  It is still another object of the present invention to produce continuous flat flexible transmission cable having a plurality of periodically spaced termination pads in both the signal and ground conductors, which pads serve to add series inductance along the line thus increasing the characteristic impedance over parallel conductor designs of similar dimensions. The periodic termination pads additionally provide greater effective skin conductor areas to reduce attenuation effects over the parallel conductor designs of similar dimensions.  
  The foregoing objects and other advantages will be apparent to those skilled in the art from the following detailed description taken with reference to the accompanying drawings.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a first embodiment of the subject flat flexible transmission cable before being folded to form a shielded line;  
  FIG. 2 is a plan view of the first embodiment of FIG. 1 showing the subject cable in the finished folded condition;  
  FIG. 3 is a vertical transverse section through the subject cable taken along line 3-3 of FIG. 2;  
  FIG. 4 is a plan view of a second embodiment of the subject invention showing multiple signal lines and multiple ground lines;  
  FIG. 5 is a plan view of a third embodiment of the present invention;  
  FIG. 6 is a perspective view of a fourth embodiment of the present invention;  
  FIG. 7 is a transverse vertical section taken along line 7-7 in FIG. 6 and showing the fourth embodiment cable after being folded; and  
  FIG. 8 is a transverse vertical section, similar to FIG. 7, schematically illustrating a fifth embodiment with interconnection of the top and bottom ground conductors through the odd numbered signal conductors to form a series of completely shielded transmission lines.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first embodiment of the subject flat flexible transmission cable is intended for use in high density wiring situations where it is desirable to provide controlled characteristic impedance with adequate electrical shielding and having signal rise times in the nanosecond region. In this embodiment a flat flexible dielec tric substrate 10 has a signal conductor 12 formed on one side and a ground conductor 14 formed on the opposite side. Both the signal and ground conductors can be formed by any of the well known printing and etching techniques. The cable is formed with the signal conductor laterally offset from the longitudinal axis of the cable and the ground conductor is formed with a regularly meandering pattern, in this instance a zig-zag configuration. The pattern of the ground conductor is such that it covers substantially the entire width of the cable. The cable is cut transversely at points 16, near the terminal pads 18 and 20 of the signal and ground conductors, respectively, and the portion of the cable between the cuts is folded upon itself, as shown in FIGS. 2 and 3. The folded cable is fixed in the folded condition by any appropriate means, such as by known adhesives. Terminal connector 22 and 24 are attached to the signal and ground conductor pads 18 and 20, respectively, either from opposite directions, as shown in FIG. 2, or from the same side of the transmission cable. Examples of suitable crimped on terminal connectors may be found in US. Pat. Nos. 3,395,381; 3,553,836; 3,663,922; and 3,697,925. However, soldered or any other type of terminal connector may be used. The characteristic impedance of the folded transmission line is determined by the wave length or the pitch of the ground line as well as the thickness and dielectric properties of the substrate. This foldover concept can also be utilized with a plurality of conductors, as will be discussed with reference to the fourth and fifth embodiments, since the problem of critical alignment of parallel shields is obviated.  
  The second embodiment of the subject invention is shown in FIG. 4. In this embodiment the flexible dielectric substrate 26 has a plurality of substantially parallel signal transmission conductors 28 formed on one side thereof with each of the signal conductors having formed thereon a plurality of periodically spaced pads 30 which are suitable for terminations purposes. A plurality of substantially parallel ground conductors 32 are formed on the opposite side of the substrate 26 with pads 34 formed therealong in spaced relationship. It should be noted that the number of ground conductors exceeds the number of transmission conductors by one so that the transversely outermost conductor always is a ground conductor. At least one conductor 36 interconnects all of the ground conductors 32, preferably at each pad 34. Additional ground pads 38 are formed on the laterally outside ground conductors in substantial alignment with the pads 30 of the signal conductors. As in the previous embodiment, the flexible substrate may be of any well known dielectric material, such as Mylar, and the circuits may be formed thereon by any of the well known printing and etching techniques.  
  The pattern of the signal and transmission conductors of this embodiment is formed by a plurality of interlocking ground and signal pads. The pads are alternated from between the top and bottom layers so that an insulation displacement crimp can be affected on any pad without disturbing the opposite conductor layer. Termination of each conductor is possible at spaced intervals along the cable with ground pads 38 making it possible for conventional (not matched impedance) connectors to be fixed to the cable in line. Individual ground connections can be made with ground pads 34 for matched impedance connector schemes.  
 The rectangular pads of the strip line conductors also provide a small series inductance at each interval which acts to increase the normal characteristic impedance without increasing the slot width between signal and ground conductors. The electrical characteristics of this design are comparable to known parallel conductor cables. The impedance is controlled by adjusting the signal and ground pad and conductor proximity. As mentioned before, the disclosed cable can be terminated using any of the well known flat flexible cable connectors, such as those crimp connectors mentioned in the above listed patents as well as soldered on connectors.  
  The third embodiment of the present invention is shown in FIG. 5. In this embodiment the flexible dielectric substrate 40 has a signal conductor 42 formed on one side with a plurality of termination pads 44 periodically spaced therealong. The opposite side of the substrate has a ground line formed thereon which includes a pair of parallel spaced conductors 46 provided with periodically spaced, terminal pads 48 and 50, respectively, and interconnected by a plurality of parallel spaced conductors 52. As with the previous embodiments, the signal and ground conductors can be formed on the substrate by any of the well known and previously discussed techniques. This embodiment of the subject cable has controlled electrical transmission properties throughout its length including the termination areas. This embodiment is also suitable for matched impedance connectors and can be made in either a folded configuration, similar to the first embodiment, or multi-line configuration, similar to the second embodiment.  
  The fourth embodiment, see FIGS. 6 and 7, differs from the previous embodiments in that the flexible insulator substrate 54 has a plurality of parallel spaced signal conductors 56 formed on one side and two separate ground conductors 58 and 60 formed on the opposite side. The signal conductors are transversely offset with respect to the cable but aligned with one of the ground conductors, the ground conductors being symmetrical with respect to the cable. The cable is folded and secured in the folded condition in the same manner as the first embodiment discussed above. The ground conductors in this embodiment can be interconnected by terminal connectors (not shown).  
  The fifth embodiment, see FIG. 8, has a folded flexible insulator substrate 62 having a plurality of signal conductors 64 and two ground conductors 66 and 68. Every other signal conductor is connected to both ground conductors by means 70 to form a series of transmission lines similar, in many respects, to coaxial cable. The connection of the conductors may be effected by anyone of a number of known means including spot bonding, stapling, stitching with metallic wire, etc.  
  The ground conductors 58 and 60, for ease of illustration only, have been shown in FIG. 6 with a sawtooth pattern that would have the ground conductors in phase in the folded condition of the cable. It should be understood that other patterns, both in and out of phase with one another, are also suitable for use in the subject cable. The primary requirement of the ground line pattern is that it must extend past the outer most edges of the transmission lines.  
  Experimentation with samples of cable produced according to the present invention have shown that it has improved electrical transmission characteristics. The subject cable displayed improved cross-talk characteristics, for both cable to cable and channel to channel cross-talk, while providing reduced high frequency attenuation and stable characteristic impedance. The subject cable also has many advantages over the known prior art in manufacturing since there is a reduced amount of art work required and special cable processing for separate applications is eliminated.  
  Part of the characteristic impedance of the subject cable is directly related to the thickness of the dielectric material between the signal and ground conductors. To enable high impedance within the design limit is thus a major development concern particularly for the folded embodiments. Increases slot width in the ground plane, as a means of increasing the characteristic impedance, also increases the cable to cable crosstalk. Reduced signal conductor width and thickness serves to increase the impedance while reduction of the thickness dimension will increase the high frequency attenuation. Any discontinuities in the signal or ground patterns for purposes of termination result in altered electrical characteristics which generally degrade cable performance.  
 The design requirements for the subject cable dictate geometric patterns for the signal and ground conductor which provide high characteristics impedance with minimum thickness of the dielectric material between the signal and ground conductor layers. Cross-talk and high frequency attenuation must also be kept to within acceptable limits. The pattern shown in the accompanying drawings are designed to introduce series inductance within the lines as a possible means of increasing the typical characteristic impedance. The periodic rectangular pads in each line act as small series inductances. Thus impedance is increased by adding inductive reactance rather than the usual method of decreasing the capacitance. This technique maintains the cross-talk values at approximately the same level while achieving higher characteristic impedances. The design also provides for reduced high frequency attenuation of the signal line by increasing the overall effective surface area of the conductors. Finally the pads can also serve as termination areas providing for the previously discussed continuous termination feature.  
  In order to provide flat flexible cable performance approaching that of coaxial cable, it is anticipated that the folded configuration will be required. This concept introduces a problem of electrical connection of the upper and lower ground shields. One method of interconnecting incorporates a fold over of the ground layer as illustrated in the first embodiment. The folded approach introduces problems of alignment of the signal and ground patterns on conventional parallel line (slot) cables. However, the zig-zag, general cross bar or transverse mesh patterns alleviate this problem of pattern alignment and provides for better cable to cable crosstalk characteristics than the standard slot design. The characteristic impedance of the line is controlled by adjustment of the ground pattern pitch. The characteristic impedance parameter can thus be determined within the ground pattern alone so that the alignment between the ground and signal pattern is no longer of critical importance. Moreover this approach provides a greater portion of the shielding directly over and under the signal conductors where it is most needed rather than to the sides of the signal conductors where radiation is relatively small.  
  The present invention may be subject to many variations and modifications without departing from the spirit or essential characteristics thereof. The present embodiments should therefore be considered in all respects as merely illustrative and not restrictive of the scope of the invention.  
 What is claimed is:  
  1. An improved flexible flat cable with at least one shielded transmission line comprising:  
 a flat flexible insulating substrate;  
 at least one signal conductor formed on one side of said substrate, said at least one signal conductor being transversely offset with respect to the longitudinal axis of said cable; and  
 at least one ground conductor formed on the opposite side of said substrate, said at least one ground conductor having a regular meandering configuration covering substantially the entire width of said cable;  
 each of said signal and said ground conductors having a plurality of termination pads regularly spaced along the length thereof whereby said cable may be produced in great length, cut to discrete lengths and terminated by attachment of appropriate connectors to the end most pads while maintaining continuous electrical characteristics throughout the length of the cable,  
 said cable being folded upon itself between the end most termination pads whereby said at least one signal conductor is shielded on at least two sides by electrically connected ground lines.  
  2. A flat flexible cable according to claim 1 wherein said meandering pattern is a zig-zag configuration.  
  3. A flat flexible cable according to claim 1 wherein said meandering pattern is a sinusoidal configuration.  
  4. A flat flexible cable according to claim 1 wherein said at least one ground conductor includes a pair of conductors each following the same regular meandering pattern but being out of phase with respect to each other.  
  5. A flat flexible cable according to claim 1 wherein said at least one ground conductor comprises a pair of spaced ground conductors, further comprising a plurality of conductors interconnecting said spaced conductors.  
  6. An improved flexible flat cable with at least one shielded transmission line comprising:  
 a flat flexible insulating substrate;  
 a plurality of substantially parallel, spaced conductors formed on one side of said substrate;  
 a plurality of substantially parallel, spaced ground conductors formed on the opposide side of said substrate, said plurality of ground conductors exceeding said plurality of signal conductors by one, said ground and said signal conductors being transversely offset with respect to each other whereby each said signal conductor lies between two ground conductors;  
 each of said signal and said ground conductors havin g a plurality of termination pads regularly spaced along the length thereof whereby said cable may be produced in great length, cut .to discrete lengths and terminated by attachment of appropriate connectors to the end most pads while maintaining continuous electrical characteristics throughout the length of the cable.  
  A flat flexible cable according to claim 6 further comprising:  
 conductor means interconnecting each of said plurality of ground conductors.  
  8. A flat flexible cable according to claim 6 wherein respective ones of said termination of pads of each said signal conductor are in substantial transverse alignment, and  
 respective ones of said termination pads of each said ground conductor are in substantial transverse alignment.  
  9. A flat flexible cable according to claim 8 wherein each row of transversely aligned termination pads said ground conductors are longitudinally offset from each row of transversely aligned termination pads of said signal conductors.  
  10. A flat flexible cable according to claim 9 further comprising a plurality of additiona termination pads formed on each of the laterally exterior ground donductors, said additional termination pads being transversely aligned with but spaced outwardly of the responsive rows of transversely aligned termination pads of said signal conductors.  
  11. A flat flexible cable according to claim 10 wherein connectors are attached to the end most aligned row of signal conductor termination pads and additional termination pads of said ground lines thereby providing an unmatched impedance connection for said cable.  
  12. A flat flexible cable having at least one transmission line comprising:  
 a flat flexible insulator substrate; plurality of transmission line signal conductors formed on one side of said substrate, each said signal conductors having a plurality of enlarged termination pads formed therealong at regularly spaced intervals, the respective termination pads of each signal conductor being in substantial transverse alignment across said cable,  
 a plurality of ground conductors formed on the opposite side of said substrate, each ground conductor having a plurality of enlarged termination pads formed therealong at regularly spaced intervals, the respective termination pads of each ground conductor being positioned in substantial transverse alignment across said cable and longitudinally between the termination pads of said signal conductors whereby constant specific characteristic impedance throughout the length of the cable is achieved.  
  13. A flat flexible transmission cable according to claim 12 further comprising a plurality of conductors interconnecting each of said plurality of ground conductors.  
 14. A flat flexible transmission cable according to claim 12 further comprising an additional termination pad formed on each of the transversely exterior ground conductors, said additional termination pads being in longitudinal alignment with but transversely spaced outwardly from respective rows of termination pads of said signal conductors thereby providing unmatched impedance termination for said cable.  
  15. A flat flexible transmission cable according to claim 12 wherein said cable is terminated by matched impedance connectors fixed to the end most pads of said signal and ground conductors.  
  16. A flat flexible cable according to claim 12 wherein said plurality of ground conductors includes at least two conductors,  
 said signal conductors being offset with respect to the longitudinal axis of said cable, I  
 said cable being folded upon itself between the end most termination pads whereby each said signal conductor is shielded by ground conductors on at least two sides.  
  17. A flat flexible cable according to claim 16 further comprising a plurality of conductors interconnecting each of said ground conductors.  
  18. A flat flexible cable according to claim 16 further comprising a plurality of conductor means interconnecting said ground conductors and the odd numbered signal conductors through said substrate whereby said flat flexible cable simulates a plurality of coaxial cables.