Flat flexible cable with pseudo-twisted conductors

Abstract: A flat flexible electrical cable includes a pair of pseudo-twisted conductors (3a, 3b) on a flexible dielectric substrate (2, d). Each conductor includes alternate straight (4) and oblique (5) sections. The straight sections (4) of the conductors are generally parallel to each other and of uniform width. The oblique sections (5) of the conductors cross each other at a crossover point (8). Each oblique section (5) of each conductor is reduced in width uniformly in a direction from the straight-to-oblique transfer point (7) of the respective conductor to the crossover point (8) of the conductors (3a, 3b).

Description: Field of the Invention This invention generally relates to the art of electrical cables and, particularly, to a flat flexible cable having at least a pair of pseudo-twisted conductors constructed to have a substantially constant impedance along the length of the cable. Background of the Invention High speed data transmission between complex electronic gear such as computers and their peripherals is commonly achieved through round multi-conductor cable assemblies. Inherent with high speed data transmission and multi-conductor cables are problems associated with electromagnetic interference. The interference between conductors can create "cross talk" and "noise", which can cause erroneous data transmission. One type of cable used to reduce such effects is "twisted pair" cable, where two adjacent conductors or differential pairs are twisted, thereby reversing the lateral position of each conductor at each twist. The benefits of this configuration are two fold. First, the position of the wires is constantly being reversed. Therefore any exterior magnetic or electric field has a generally uniform effect upon a given differential pair. This uniformity reduces the harmful effects of exterior electromagnetic fields and lessens the chances of erroneous data transmission. Second, when current runs through a conductor, an electromagnetic field is generated. The field's orientation is dependent upon the direction of the current flow. For a given differential pair, electric currents generally flow in opposite directions. Therefore, the electromagnetic field radiating from each conductor generally has a canceling effect upon the other. This canceling effect can substantially suppress the radiated emissions from a given differential pair. Flat flexible cable or circuitry (FFC) is another medium used for high speed data transmission between computers and their peripherals. A "pseudo-twisted" arrangement can be achieved with FFC by placing conductors of a given pair on opposite sides of the insulative carrier, with their paths slightly and oppositely offset with respect to a common nominal path locator, and then periodically reversing the offset at predetermined locations. A flat flexible cable according to the preamble of claim 1 is known from US-A-3,761,842. This patent discloses a pseudo-twisted cable having pairs of conductors, each pair having differing numbers of twists per unit length. Characteristic impedance for a given pair of conductors in a pseudo-twisted cable is also a function of the lateral width of the conductor paths. A reduction in the lateral width of the conductors increases the characteristic impedance for the given pair. US-A-3,761,842 also teaches that if the conductor width at the crossing point is decreased, the characteristic impedance at that point is increased. This relationship allows the user to construct a cable where the characteristic impedance per unit length is independent of the number of twists per unit length. Accordingly, a cable with many twists per unit length would have the lateral width of the conductors at the crossing point reduced more than a cable with fewer twists per unit length. While the pseudo-twisted cable of US-A-3,761,842 has some advantages over the prior art, because of the abrupt reduction in the width of the conductors, the impedance will also change abruptly. This is undesirable in many applications such as in high speed data transmission where it is desirable to have a cable whose characteristic impedance is constant through out the length of the cable. The present invention is directed to providing a pseudo-twisted cable having a characteristic impedance that is independent of the number of twists per unit length and substantially constant through out the length of the cable. The characteristic impedance of the flat flexible cable can affect the integrity of the signals travelling in the cable. One of the factors that influences the characteristic impedance of a given pair of conductors of a pseudo-twisted cable is the centerline distance "L" (Figure 3A) between conductors. In Figure 3A, cable "B" includes pseudo-twisted conductors "P" that are positioned on opposite sides of a flexible substrate "d" with adhesive layers "e" and "c" interposed therebetween. The conductors are covered by protective films "a" and "g" with adhesive layers "b" and "f" interposed therebetween. The lateral distance "D" between the conductors on opposite sides of a prior art pseudo-twisted cable varies along the length of the cable, as indicated by 0.2, 0.16 and 0 mm. If conductors "P" have constant widths and thicknesses along their lengths, the impedance "Z" decreases with a decrease in the conductor-to-conductor distance "D" (See Fig 3B). Therefore, at every crossing point or "twist" of the conductors, the characteristic impedance decreases. Accordingly, the more twists per unit length of the cable, the lower the characteristic impedance for that given length. Summary of the Invention An object, therefore, of the invention is to provide a new and improved flat flexible cable having at least a pair of pseudo-twisted conductors constructed to have a substantially constant impedance along the length of the cable whereby abrupt changes in the impedance lengthwise of the conductors are eliminated. The invention is defined in claim 1. In the exemplary embodiment of the invention, the pair of pseudo-twisted conductors are disposed on a flexible dielectric substrate, with one conductor being on one side of the substrate and the other conductor being on the other side of the substrate. Each conductor includes alternate straight. and oblique sections. The straight sections of the conductors are generally parallel to each other. The oblique sections of the conductors cross each other at a crossover point. Each oblique section of each conductor is reduced in width uniformly in a direction from the straight-to-oblique transfer point of the respective conductor to the crossover point of the conductors. By reducing the width of the oblique sections of the conductors uniformly from the straight-to-oblique transfer points to the crossover points, abrupt changes in the impedance lengthwise of the conductors are eliminated. The impedance, therefore, is substantially constant along the full length of the pair of pseudo-twisted conductors. Other objects, features and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings. Brief Description of the Drawings The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with its objects and the advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the figures and in which: FIG. 1 is an enlarged plan view of a pair of pseudo-twisted conductors along a length thereof involving two pseudo-twists of the conductors; FIG. 2 is a fragmented plan view of a flat flexible cable incorporating a plurality of pairs of pseudo-twisted conductors; FIG. 3A shows various generally schematic cross sections of a pair of cable conductors from a pseudo-twisted cable of the prior art; FIG. 3B is a graph comparing impedance with the lateral distance between conductors of the prior art pseudo-twisted cable of Figure 3A; FIG. 4 shows various cross sections similar to Fig. 3A but of a pair of cable conductors from a pseudo-twisted cable of the present invention; FIG. 5 is a fragmented perspective view of the pseudo-twisted cable of the present invention together with a connector to which it could be connected; and FIG. 6 is an enlarged, fragmented plan view of a schematic representation of a portion of a pair of conductors of a pseudo-twisted cable of the present invention. Detailed Description of the Preferred Embodiment Referring to the drawings in greater detail, and first to Figs. 1, 2 and 5, a flat flexible cable, generally designated 1, includes a plurality of pairs of pseudo-twisted conductors 3a and 3b. One conductor 3a is disposed on one side or surface of a flexible dielectric substrate 2, and the other conductor 3b is disposed on the other side or surface of the substrate. The cross-section of each cable is similar to that of the prior art pseudo-twisted cable B shown in Fig. 3A. Accordingly, like reference letters have been applied to Fig. 4 to designate like layers of the cable. In other words, each pair of conductors 3a and 3b is sandwiched between flexible substrate d and protective films a and g, respectively, therebetween to form a pair of pseudo-twisted conductors. As best seen in Fig. 1, each conductor 3a and 3b runs lengthwise of the cable in an oscillating pattern formed by alternate straight sections 4 and oblique sections 5. These patterns of conductors 3a and 3b are arranged symmetrically but opposite to each other on opposite sides or surfaces of the flexible substrate. The straight sections 4 of each conductor are generally parallel to each other but alternate along two parallel but spaced apart lines 4a and 4b as shown in Figs. 1 and 5. The oblique sections 5 of each conductor connects a pair of adjacent straight sections 4 and thus extends between lines 4a and 4b. Since the conductors 3a and 3b are arranged symmetrically but opposite to each other, the straight sections 4 of the conductors are generally parallel but spaced apart both vertically and horizontally. In other words, when the straight section 4 of one conductor 3a is aligned with line 4a, the straight section 4 of the other conductor 3b is aligned with line 4b but on opposite sides of substrate 2, and vice versa. As a result, the oblique sections 5 of the conductors 3a and 3b cross each other at crossover points 8. The straight sections 4 of conductors 3a and 3b have uniform or equal widths along the lengths thereof. As best shown in Figure 6, each oblique section 5 of each conductor decreases in width uniformly in a direction from the straight-to-oblique transfer point 7 of the respective conductor to the respective crossover point 8 of the conductors where it reaches its minimum width. Assuming that: 1) the thickness "t" (Fig. 4) of conductors 3a and 3b (i.e., in a direction perpendicular to the plane of the cable) is uniform along the full length of the conductors; 2) the vertical distance between the conductors remains constant; and 3) the dielectric constant of the material between the conductors remains constant, the impedance increases with the decrease in the width of each oblique section 5. The conductors 3a and 3b separately terminate in one of the pads 9 at an edge of flexible substrate 2. The pads of some of the conductors may include an associated through hole or via 10. These pads 9 are arranged at equal intervals along the edges of the flat flexible cable for engagement with equally spaced contacts (not shown) of an associated electrical connector 11 (Fig. 5) adapted for connecting to flat flexible cables. Therefore, either opposite edge of the flat flexible cable having the pseudo-twisted conductors can be inserted into the electrical connector to establish a required electrical connection. It can be understood that as the juxtaposed oblique sections 5 of conductors 3a and 3b in each pseudo-twisted pair get closer to each other at each crossover point 8, the conductor-to-conductor centerline distance "D" decreases and, thereby, the impedance decreases. However, since each oblique section 5 is uniformly reduced in width, the impedance is caused to uniformly increase. In other words, as the oblique sections 5 of each pair of conductors get closer to each other toward each crossover point 8 (thus decreasing the conductor-to-conductor centerline distance), the impedance is decreasing and, at the same time, as the width or lateral dimension "W" (Fig. 4) of the oblique sections decreases toward the crossover point, the impedance uniformly increases. In essence, the impedance decreases due to the reduction in the relative distance between the centerlines of the conductors and simultaneously increases due to the gradual tapering or narrowing of the conductors in a balancing condition or a net effect of zero such that the impedance substantially remains unchanged from the straight-to-oblique transfer points 7 to the crossover points 8 of the pseudo-twisted conductors 3a and 3b. The impedance is preferably set to be equal to the impedance in parallel sections 4 of the pseudo-twisted conductor patterns so that the pseudo-twisted cable has a constant impedance along its entire length. One example of a pseudo-twisted cable 1 according to the present invention is shown in Fig. 4 and was designed so as to have an impedance of 100 Ω. As shown in Case 1 of Fig. 4, the width of the straight sections W of each conductor 3a and 3b was 0.15 mm and each straight section was 1.0 mm long. Each oblique section 5 (Fig. 1) was 1.0 mm long. Therefore, the distance from a straight-to-oblique transfer point 7 to a crossover point 8 was 0.5 mm along. The distance D between the parallel straight sections was 0.22 mm (Case 1 of Fig. 4). This distance was determined in consideration of the data as shown in Fig. 3B. As shown in Case 3 of Fig. 4, the impedance was reduced in half to represent a zero distance between the conductors. The width of each oblique section 5 was reduced gradually or uniformly from 0.15 mm at the straight-to-oblique transfer point 7 to a width W" of 0.072 mm at the crossover point 8 as shown in Case 3 of Fig. 4. In Case 3 of Fig. 4, the distance between the centerlines of the conductors 3a and 3b is 0.0 mm. Case 2 of Fig. 4 shows a location between the straight-to-oblique transfer point 7 and crossover point 8 wherein the distance D between conductors 3a and 3b as well as their width W is between those dimensions of Cases 1 and 3 of Fig. 4. A pseudo-twisted cable thus obtained was found to have a constant or uniform impedance along its full length. It will be understood that the invention may be embodied in other specific forms without departing from the scope of the central characteristics thereof as defined in the claims. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein.

Claims: A generally planar flat flexible electrical cable, comprising: a plurality of pairs of first and second pseudo-twisted conductors (3a, 3b) generally positioned within a plane, said first and second conductors (3a, 3b) being spaced apart in direction perpendicular to said plane and having a flexible dielectric substrate (d) between said conductors (3a, 3b) and within said plane, said conductors (3a, 3b) having a periodic shape with said first conductors (3a) being shifted longitudinally relative to said second conductors (3b) by one half of a period of said shape, said first and second conductors (3a, 3b) including a crossover point (8) at which said first and second conductors of each pair cross over each other and are separated by said flexible dielectric substrate (d), said crossover points (8) of each pair defining a centerline with said first and second conductors reversing locations on opposite sides of said centerline, each said conductor including a transition section (7) extending from a peak location a maximum distance from its respective centerline to said crossover point (8), characterized in that each said conductor (3a, 3b) decreases in width uniformly in a direction from said peak location to said crossover point (8) whereby the width (w, w', w'') of each said conductor (3a, 3b) is at a maximum (w) at said peak location and at a minimum (w'') at said crossover point (8). The generally planar flat flexible electrical cable of claim 1, wherein each said conductor (3a, 3b) includes alternating straight (4) and oblique (5) sections, said oblique sections (5) crossing the centerline of each pair of conductors (3a, 3b) and said straight sections (4) of each conductor being alternatingly positioned on opposite sides of said centerline. The flat flexible electrical cable of claim 1 or 2, including dielectric flexible films (a, g) covering the outsides of the conductors (3a, 3b). The flat flexible electrical cable of claim 3, further including an adhesive (c, e) between said flexible dielectric substrate (d), said flexible conductors (3a, 3b) and said flexible films (a, g). The flat flexible electrical cable of claim 4, further including an adhesive (b, f) between said flexible dielectric substrate (d), said flexible conductors (3a, 3b) and said flexible films (a, g). The flat flexible electrical cable of any of claims 2 to 5, wherein the straight sections (4) of the conductors (3a, 3b) are generally parallel to each other and of uniform width. The flat flexible electrical cable of any of claims 2 to 6, wherein said crossover points (8) define a centerline generally parallel to said straight sections (4) of said conductors (3a, 3b), and said straight sections (4) of said first conductor (3a) being positioned in an alternating manner in two lines (4a, 4b) generally parallel to and on opposite sides of said centerline, said straight sections (4) of said second conductor (3b) being positioned in an alternating manner in two lines (4b, 4a) generally parallel to and on opposite sides of said centerline, each of the straight sections (4) of said first conductor (3a) being positioned on an opposite side of said centerline relative to said straight sections (4) of said second conductor (3b).

Inventors: HOSHINO TETSUO, NODA ATSUHITO, HOSHINO, TETSUO, NODA, ATSUHITO
Applicants: MOLEX INC, MOLEX INCORPORATED
Application year: 1997
Main classification: H01B7
IPC codes: H01B7, H05K1, H01B11