Title:
LOW ATTENUATION FLAT FLEXIBLE CABLE
United States Patent 3818117
Abstract:
A flat shielded cable comprising a plurality of parallel spaced-apart signal conductors in a first plane and a shield member in the form of a conductive ground plane facing the signal conductors and being positioned in a second plane, the shield member having preselected amounts of conductive material removed in the area opposite the signal conductors to form ground lines, which are parallel to the signal conductors to control the characteristic impedance of the cable, and also to form parallel, conductive shunting paths between the ground lines at predetermined intervals to correct for excessive attenuation caused by skewing of the signal conductors with respect to the ground lines.
US Patent References:
Shielded printed circuit electrical component
Wegner et al. - December 1960 - 2963535
Flexible multiconductor transmission line utilizing alternate conductors as crosstalk shields
Paulsen - March 1965 - 3179904
FLEXIBLE MULTIFLAT CONDUCTOR CHARACTERISTIC IMPEDANCE CABLE
Gerpheide - August 1969 - 3459879
FLEXIBLE FLAT CONDUCTOR CABLE OF VARIABLE ELECTRICAL CHARACTERISTICS
Thomas - October 1971 - 3612744
CONNECTOR FOR FLEXIBLE CIRCUITRY
Wilson - December 1971 - 3629787
Shielded printed circuit electrical component
Wegner et al. - December 1960 - 2963535
Flexible multiconductor transmission line utilizing alternate conductors as crosstalk shields
Paulsen - March 1965 - 3179904
FLEXIBLE MULTIFLAT CONDUCTOR CHARACTERISTIC IMPEDANCE CABLE
Gerpheide - August 1969 - 3459879
FLEXIBLE FLAT CONDUCTOR CABLE OF VARIABLE ELECTRICAL CHARACTERISTICS
Thomas - October 1971 - 3612744
CONNECTOR FOR FLEXIBLE CIRCUITRY
Wilson - December 1971 - 3629787
Inventors:
Reyner II, Emerson Marshall (Palmyra, PA)
Bogar, Jerry Hench (Harrisburg, PA)
Bogar, Jerry Hench (Harrisburg, PA)
Application Number:
05/353257
Publication Date:
06/18/1974
Filing Date:
04/23/1973
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
174/78, 174/117FF, 333/243, 174/268, 174/254, 174/117R
International Classes:
H01B7/08; H01B11/00; H05K1/02; H05K1/00; H01B7/08
Field of Search:
333/84M,84R,32,72S,81A 174/36,117F,117FF,78,117PC,88C
US Patent References:
Other References:
schun, A. G. Flat Flexible Cable and Wiring-Types, Materials, Constructions, and Features, Insulation/Circuits, Oct. 70, pp. 27-34..
Primary Examiner:
Gilheany, Bernard A.
Assistant Examiner:
Grimley A. T.
Claims:
What is claimed is
1. A flat cable for transmitting signals comprising: a plurality of transmission lines formed by a plurality of flat parallel spaced-apart signal conductors lying in a single plane and associated corresponding pairs of a plurality of parallel flat shield conductors lying in a plane parallel to the single plane of said signal conductors, said shield conductors extending parallel to said signal conductors and being laterally offset from said signal conductors, a plurality of parallel shunt conductors interconnecting said shield conductors at spaced intervals, and insulating material between said conductors and encasing said conductors, said shunt conductors preventing signal coupling between any individual transmission line and another transmission line formed by the pair of shield conductors associated with the individual transmission line.
2. A flat cable as recited in claim 1 wherein said shunt conductors are arranged substantially perpendicularly to said shield conductors.
3. A flat cable as recited in claim 2 wherein said shunt conductors and said shield conductors are formed from a single substantially continuous layer of conductive material.
4. A flat cable as recited in claim 2 wherein the spacing between said shunt conductors is selected to be a fraction of the wavelength of the signals to be transmitted by said cable.
5. A cable as recited in claim 4 wherein the spacing between said shunt conductors is selected to be substantially equal to L where:
6. A cable as recited in claim 4 wherein the spacing between said shunt conductors is selected to be substantially less than L' where:
7. A cable as recited in claim 2 wherein the ratio of the width of each said shunt conductors to the center-to-center spacing between adjacent shunt conductors in a direction parallel to said shield conductors is less than or equal to one tenth.
8. A terminated flat cable comprising: a plurality of parallel spaced-apart signal conductors lying in a single plane, a plurality of flat electrically conductive shielding means lying in a plane extending parallel to, and spaced from, said single plane, dielectric material between said signal conductors and said shielding means, a transversely extending zone adjacent to one end of said cable which is devoid of said conductive shielding means, and flat electrically conductive shunt means lying in said plane of said plurality of shielding means and interconnecting said plurality of shielding means at spaced intervals and being oriented in a direction substantially perpendicular to said parallel signal conductors.
9. A cable as recited in claim 2 wherein the spacing between shunt conductors is varied along the length of cable.
10. The structure as recited in claim 1, wherein said shield conductors are electrically ground.
11. A transmission cable comprising: at least one elongated signal conductor, an associated pair of shield conductors elongated and parallel to the signal conductor, providing a transmission line and a plurality of spaced shunt conducting paths joining the shield conductors at intervals along their lengths to prevent signal coupling between said transmission line and another transmission line formed by the two shield conductors.
1. A flat cable for transmitting signals comprising: a plurality of transmission lines formed by a plurality of flat parallel spaced-apart signal conductors lying in a single plane and associated corresponding pairs of a plurality of parallel flat shield conductors lying in a plane parallel to the single plane of said signal conductors, said shield conductors extending parallel to said signal conductors and being laterally offset from said signal conductors, a plurality of parallel shunt conductors interconnecting said shield conductors at spaced intervals, and insulating material between said conductors and encasing said conductors, said shunt conductors preventing signal coupling between any individual transmission line and another transmission line formed by the pair of shield conductors associated with the individual transmission line.
2. A flat cable as recited in claim 1 wherein said shunt conductors are arranged substantially perpendicularly to said shield conductors.
3. A flat cable as recited in claim 2 wherein said shunt conductors and said shield conductors are formed from a single substantially continuous layer of conductive material.
4. A flat cable as recited in claim 2 wherein the spacing between said shunt conductors is selected to be a fraction of the wavelength of the signals to be transmitted by said cable.
5. A cable as recited in claim 4 wherein the spacing between said shunt conductors is selected to be substantially equal to L where:
6. A cable as recited in claim 4 wherein the spacing between said shunt conductors is selected to be substantially less than L' where:
7. A cable as recited in claim 2 wherein the ratio of the width of each said shunt conductors to the center-to-center spacing between adjacent shunt conductors in a direction parallel to said shield conductors is less than or equal to one tenth.
8. A terminated flat cable comprising: a plurality of parallel spaced-apart signal conductors lying in a single plane, a plurality of flat electrically conductive shielding means lying in a plane extending parallel to, and spaced from, said single plane, dielectric material between said signal conductors and said shielding means, a transversely extending zone adjacent to one end of said cable which is devoid of said conductive shielding means, and flat electrically conductive shunt means lying in said plane of said plurality of shielding means and interconnecting said plurality of shielding means at spaced intervals and being oriented in a direction substantially perpendicular to said parallel signal conductors.
9. A cable as recited in claim 2 wherein the spacing between shunt conductors is varied along the length of cable.
10. The structure as recited in claim 1, wherein said shield conductors are electrically ground.
11. A transmission cable comprising: at least one elongated signal conductor, an associated pair of shield conductors elongated and parallel to the signal conductor, providing a transmission line and a plurality of spaced shunt conducting paths joining the shield conductors at intervals along their lengths to prevent signal coupling between said transmission line and another transmission line formed by the two shield conductors.
Description:
BACKGROUND OF THE INVENTION
This invention relates to flat multiple conductor cables and more particularly to the controlling of the characteristic impedance and attenuation of such cables.
Various flat multiple conductor cable configurations are known. These can be broadly classified into a first group comprising a plurality of round wires positioned in a parallel relationship to each other and embedded in a suitable dielectric and a second group comprised of flat ribbon like conductors also positioned in a parallel relationship to each other and embedded in a dielectric material. A description of one of such known cable configurations of the second group can be found in U.S. Pat. No. 3,703,604 issued Nov. 21, 1972.
The above-mentioned patent describes a shielded flat cable having a plurality of signal conductors in a parallel, spaced arrangement in a first plane and a conductive shield member in a second plane in confronting relationship with the signal conductors. The shield member or ground plane, as it is called, and the signal conductors are separated by a suitable dielectric. To control the cable impedance, portions of the ground plane facing the signal conductors are removed. Preferably, the deleted sections of the ground plane form thin, parallel slots running substantially the length of the signal conductors. This controlled removal of ground plane material decreases the signal conductor-to-ground plane capacitance to thereby alter the cable's characteristic impedance. By selectively varying the ratio of the width of the signal conductors to the width of the slots in the ground plane, the cable is effectively "turned" over a wide range of impedances.
It has been found that a problem exists during the manufacture of such flat cables. It is difficult, because of the photographic processes involved, to insure parallel registration of either the signal lines or the parallel ground lines associated with each signal line during the photo-etching process utilized to manufacture the desired conductor shapes. As a result, skewing of either the ground lines or the signal lines results. Skewing is a phenomenon whereby the ground lines or the signal lines are slightly serpentine rather than exactly straight and parallel as desired. Thus each signal line varies in distance relative to its associated two ground lines, thereby producing signal attenuation along the transmission line formed by the signal line and its two ground lines along the length of the signal line.
Each signal line is staggered with respect to the nearest ground lines so that there are two associated ground lines adjacent to each signal line. If a particular signal line is serpentine and at a particular point along the flexible cable it becomes relatively close to one of the associated pair of ground lines it also becomes correspondingly relatively further away from the other one of the pair of associated ground lines. Thus over a substantial length of the flexible cable the average impedance with respect to the pair of ground lines is constant. However, skewing does produce a number of problems, one of the most serious being that since the skewing is periodic, energy is coupled into the ground plane which produces high attenuation at wavelengths corresponding to the periodicity of the skewing. In such a case the two ground lines in themselves form a two wire transmission line. It is this "line" into which energy is coupled due to skewing. Such energy is thereby lost from the signal line.
Referring to FIG. 1, the attenuation of the cable as a function of the frequency of the transmitted signal is shown. The zero reference line is the output voltage derived from the signal generator without interposing a cable section. The attenuation is thus measured in dB with respect to the zero reference line. At a frequency which is in resonance with the skewing periodicity the attenuation increases, i.e., the distance to the zero reference line increases sharply as indicated in the figure.
The magnitude of this resonant attenuation is dependent to some degree on the length of the cable. In FIG. 2 it can be seen that, for example, the greatest attenuation occurs at approximately 103.7 MHz in a cable 2.20 meters long.
FIG. 3 shows a time domain reflectometry plot in which the abscissa is calibrated in time, which is directly related to the distance along the cable, and the ordinate is the reflection co-efficient. In general, time domain reflectometry (TDR) sends a fast rise time step along a line or circuit and looks at the reflection as it comes back. When measuring impedance, it produces a picture or graph representing the impedance of the line as it is averaged by the highest frequencies of the step along its length. Since the horizontal axis of the graph is distance along the line, it is possible to locate and identify the impedance of the elements of the line, such as lengths of cable, connectors and other electrical components. The vertical axis can be read as impedance or as reflection coefficient (γ) as marked.
As is known to those skilled in the art, the reflection coefficient (γ) is the ratio of the amplitude of the reflected voltage to the amplitude of the incident voltage. The reflection coefficient is related to the terminating impedance (Z L ) any point on the line by the equation:
Z L = 1 +γ/1 -γ × Z O ,
where Z O is the characteristic impedance of the cable. Thus the impedance at any point along the line is a second order function of the reflection coefficient at that point. A description of the techniques for producing such a TDR plot is given in Impedance Matched Printed Circuit Connectors by Homer E. Henschen and Emerson M. Reyner II, a paper presented at NEPCON '70.
The plots in FIG. 3 thus represent the impedance along a signal line of a prior art flexible cable under three conditions. Curve R 3 represents the impedance between the associated ground lines into which energy is coupled due to skewing, as explained above. Curve R 1 results from taking the impedance between one of the ground lines and the signal line. Curve R 2 shows the TDR curve resulting when the ground line responsible for curve R 1 is removed from the cable configuration and the effective impedance is provided by the other ground line of the pair of ground lines associated with the signal line. The curves R 1 , R 2 and R 3 thus represent the impedance as a function of length for the same cable. An inverse variation in the effective impedance is thus produced by skewing.
By combining the curves R 1 and R 2 it becomes obvious that since the impedance mismatch occurs at approximately the same location on the cable with respect to the two ground lines the impedance mismatches are of opposite magnitudes and tend to cancel each other out. This cancelling phenomenon has heretofore masked the problem of signal coupling due to skewing.
The phenomenon of the impedance mismatch due to skewing with respect to two ground lines was not readily discoverable. In fact when a number of cable prototypes were fabricated and tested by the applicant, the test results appeared to show a consistent impedance match over the lengths tested. The cable appeared ready to be marketed, and the assignee of the present application was sufficiently confident of its manufacturing processes to ready itself for supplying customer orders. However, the cable did not in fact perform satisfactorily.
Testing of randomly selected lengths of cable also did not readily discover the skewing effects present in the manufactured cable. The skewing effect was discovered only by continued testing of various lengths of the cable. It was startling and surprising to discover that certain lengths of cable selected and tested resulted in undesired attenuation measurements. This lead the applicant to further analyze those certain cable lengths to discover what caused the occurrence of high attenuation. His analysis indicated that skewing of either the ground lines or the signal lines, although nearly imperceptible in amplitude, occurred at some periodic rate which could not be easily eliminated during the manufacturing process.
It is speculated that any manufacturing process used would not be tolerant or accurate enough virtually to eliminate all skewing; and that skewing would have a probability of being present also in some periodic rate. This impedance mismatch in the cable caused by skewing would not be readily measurable or detected unless a resonant section of the cable were accidentally selected. In fact, even after measuring a length of cable, which would by coincidence be a resonant section, the exact cause of the resulting signal distortion would not be readily apparent. Accordingly, the problem existing in the prior art cable was not readily discoverable, and only through substantial analysis after such discovery was the actual cause found.
SUMMARY OF THE INVENTION
The above and other disadvantages are overcome by the present invention of a shielded flat cable having a plurality of signal conductors in a parallel, spaced arrangement in a first plane and a conductive shield member in a second plane in confronting relationship with the signal conductors and separated from them by a suitable dielectric. Portions of the shield member facing the signal conductors are removed to form ground lines, which are parallel to the signal conductors but generally spaced between them, and a plurality of parallel shunt paths oriented in a direction substantially perpendicular to the ground lines. The shunt paths are spaced apart by a predetermined distance selected to be a small fraction of the wavelengths of the signals which are expected to be carried by the flat cable.
It is therefore an object of the invention to provide an improved flat shielded cable design in which correction is made for excessive attenuation caused by skewing of the signal lines with respect to the ground lines by providing shunt paths between the ground lines at predetermined intervals.
It is another object of the invention to compensate for resonant coupling due to skewing of a periodic configuration whose wavelength corresponds to the length of the flat cable by providing parallel shunt paths between the ground lines.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of certain preferred embodiments of the invention, taken in conjunction with the accompanying drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is an R.F. attenuation curve of a prior art flat cable showing signal attenuation of the cable as a function of frequency;
FIG. 2 are R.F. signal attenuation plots of a prior art cable showing attenuation due to skewing as a function of cable length;
FIG. 3 are time domain reflectometry plots of the impedance characteristics of a prior art flat cable;
FIG. 4 is an R.F. signal attenuation curve, similar to the curve of FIG. 2, of a cable constructed according to the invention;
FIG. 5 is a plan view of the end of a flat cable according to one embodiment of the invention;
FIG. 6 is an enlarged section taken along the line 6--6 of FIG. 5; and
FIG. 7 is a horizontal view of the underside of the flat cable and depicted in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIGS. 5 and 6 a flexible flat conductor cable 10 according to one embodiment of the invention includes a plurality of parallel, spaced-apart ribbon-like signal conductors 12. Each of the conductor 12 terminates in a separate pad 11.
A ground plane 16, of electrically conductive material lies in a plane parallel to the plane of the spaced-apart signal conductors 12. The signal conductors 12 are separated from the ground plane 16 by a dielectric 13 of suitable material such as Mylar (polyethylene terephthalate). The Mylar is transparent and is best seen in the cross-section of FIG. 6. Two cover sheets 14 and 15 of similar material may encase the conductors 12 and 16 and the dielectric 13.
Referring now more particularly to FIG. 7 the underside of the flat cable 10 depicted in FIG. 5 is illustrated. The ground plane 16 contains slots 18 devoid of electrically conductive material. These narrow slots are opposite each of the signal conductors. By selectively altering the width of these slots, the capacitance between the signal conductors and the ground plane can be changed without requiring a change in the thickness of the dielectric 13 or a change in the dielectric constant of this material. Additionally the area of the ground plane 16 which is opposite the termination pads 11 is removed by etching to facilitate coupling of the cable 10 to a connector (not shown).
Across the width of the cable 10 the slots 18 which are devoid of electrically conductive material are parallel to each other and are separated by thin, parallel strips of conductive material, hereinafter referred to as the ground lines 20. The ground lines 20 are parallel to the signal lines 12 but substantially spaced between the signal lines 12 The orientation of the signal lines 12 and the ground lines 20 is that they run the length of the cable 10.
Along the length of the cable 10 the slots 18 are separated by a plurality of shunt lines 22 of electrically conductive material which remain after the etching of the slots 18 in a pattern such that the shunt paths 22 run substantially perpendicular to the ground lines 20. The width W of each of the shunt paths 22 is chosen such as to keep the resistance relatively low, for example, less then 0.01 ohms and also so that the capacitance of the lines are not greatly affected and the impedance made too low. For example, if L equals the center-to-center spacing between the shunt paths 22 along the length of the cable then a preferable ratio of W/L would be less than or equal to approximately 0.1.
The upper frequency limit may also be determined as a function of the spacing L between the shunt paths 22. When the spacing is such that the distance L is equal to a half wavelength then the upper frequency may be determined as follows:
f H = υ/λ = υ/2 L,
where υ = the speed of electromagnetic wave propagation in the cable
For example where W equals 20 mils and L equals 250 mils the upper frequency equals approximately 14.8 × 10 9 Hz. At this frequency all of the reflections on the signal line due to the shunt paths 22 combine in a manner which essentially produces a stopband. In actual operation the frequency transmitted by the flexible cable would be chosen to be well below this upper frequency limit. Conversely if L is chosen to be a quarter wavelength then the frequency f H ' = υ/4L is a passband frequency at which the signals transmitted by the cable will not be attenuated by the shunt paths.
In general the spacing between the shunt paths 22 should be chosen to be a small fraction of the shortest wavelength signal to be carried by the flexible cable. Based on the above formulae, in one embodiment the preferred center-to-center spacing between the shunt paths is chosen to be substantially less than
L' = υ/2f
and where
υ = the speed of electromagnetic wave propagation in the cable and
f = the highest frequency of the signals expected to be transmitted by the cable.
Although in the above description it has been assumed that substantially continuous signals are transmitted by the cable 10 it should be apparent to those skilled in the art that the cable of the invention is also suitable for transmitting pulse signals. In such case the equivalent frequencies (in GHz) of f and f H for use in the above described formulae may be obtained by dividing the factor 0.35 by the rise time of the pulses (in nanoseconds), as is known to those skilled in the art.
Referring now more particularly to FIG. 4, the R.F. attenuation curve of a 50 ohm flat cable constructed according to the invention is illustrated. The shunt paths were spaced 250 mils apart and the cable length was 5.5 feet. As is apparent from the figure, there is no resonant attenuation "dip" as in the curves of the prior art cables depicted in FIG. 2.
In order to reduce the effect of the shunt conductors at any one frequency it may be desirable for some applications to vary the spacing along the length of the cable.
This will extend the useful operating range of the cable by providing a solution to the problem recognized by Ragan in "Microwave Transmission Circuits" -- Radiation Laboratory Series No. 9 -- McGraw-Hill Book Company Incorporated, 1948, pages 160, 161.
The terms and expressions which have been employed here are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions, of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.
This invention relates to flat multiple conductor cables and more particularly to the controlling of the characteristic impedance and attenuation of such cables.
Various flat multiple conductor cable configurations are known. These can be broadly classified into a first group comprising a plurality of round wires positioned in a parallel relationship to each other and embedded in a suitable dielectric and a second group comprised of flat ribbon like conductors also positioned in a parallel relationship to each other and embedded in a dielectric material. A description of one of such known cable configurations of the second group can be found in U.S. Pat. No. 3,703,604 issued Nov. 21, 1972.
The above-mentioned patent describes a shielded flat cable having a plurality of signal conductors in a parallel, spaced arrangement in a first plane and a conductive shield member in a second plane in confronting relationship with the signal conductors. The shield member or ground plane, as it is called, and the signal conductors are separated by a suitable dielectric. To control the cable impedance, portions of the ground plane facing the signal conductors are removed. Preferably, the deleted sections of the ground plane form thin, parallel slots running substantially the length of the signal conductors. This controlled removal of ground plane material decreases the signal conductor-to-ground plane capacitance to thereby alter the cable's characteristic impedance. By selectively varying the ratio of the width of the signal conductors to the width of the slots in the ground plane, the cable is effectively "turned" over a wide range of impedances.
It has been found that a problem exists during the manufacture of such flat cables. It is difficult, because of the photographic processes involved, to insure parallel registration of either the signal lines or the parallel ground lines associated with each signal line during the photo-etching process utilized to manufacture the desired conductor shapes. As a result, skewing of either the ground lines or the signal lines results. Skewing is a phenomenon whereby the ground lines or the signal lines are slightly serpentine rather than exactly straight and parallel as desired. Thus each signal line varies in distance relative to its associated two ground lines, thereby producing signal attenuation along the transmission line formed by the signal line and its two ground lines along the length of the signal line.
Each signal line is staggered with respect to the nearest ground lines so that there are two associated ground lines adjacent to each signal line. If a particular signal line is serpentine and at a particular point along the flexible cable it becomes relatively close to one of the associated pair of ground lines it also becomes correspondingly relatively further away from the other one of the pair of associated ground lines. Thus over a substantial length of the flexible cable the average impedance with respect to the pair of ground lines is constant. However, skewing does produce a number of problems, one of the most serious being that since the skewing is periodic, energy is coupled into the ground plane which produces high attenuation at wavelengths corresponding to the periodicity of the skewing. In such a case the two ground lines in themselves form a two wire transmission line. It is this "line" into which energy is coupled due to skewing. Such energy is thereby lost from the signal line.
Referring to FIG. 1, the attenuation of the cable as a function of the frequency of the transmitted signal is shown. The zero reference line is the output voltage derived from the signal generator without interposing a cable section. The attenuation is thus measured in dB with respect to the zero reference line. At a frequency which is in resonance with the skewing periodicity the attenuation increases, i.e., the distance to the zero reference line increases sharply as indicated in the figure.
The magnitude of this resonant attenuation is dependent to some degree on the length of the cable. In FIG. 2 it can be seen that, for example, the greatest attenuation occurs at approximately 103.7 MHz in a cable 2.20 meters long.
FIG. 3 shows a time domain reflectometry plot in which the abscissa is calibrated in time, which is directly related to the distance along the cable, and the ordinate is the reflection co-efficient. In general, time domain reflectometry (TDR) sends a fast rise time step along a line or circuit and looks at the reflection as it comes back. When measuring impedance, it produces a picture or graph representing the impedance of the line as it is averaged by the highest frequencies of the step along its length. Since the horizontal axis of the graph is distance along the line, it is possible to locate and identify the impedance of the elements of the line, such as lengths of cable, connectors and other electrical components. The vertical axis can be read as impedance or as reflection coefficient (γ) as marked.
As is known to those skilled in the art, the reflection coefficient (γ) is the ratio of the amplitude of the reflected voltage to the amplitude of the incident voltage. The reflection coefficient is related to the terminating impedance (Z L ) any point on the line by the equation:
Z L = 1 +γ/1 -γ × Z O ,
where Z O is the characteristic impedance of the cable. Thus the impedance at any point along the line is a second order function of the reflection coefficient at that point. A description of the techniques for producing such a TDR plot is given in Impedance Matched Printed Circuit Connectors by Homer E. Henschen and Emerson M. Reyner II, a paper presented at NEPCON '70.
The plots in FIG. 3 thus represent the impedance along a signal line of a prior art flexible cable under three conditions. Curve R 3 represents the impedance between the associated ground lines into which energy is coupled due to skewing, as explained above. Curve R 1 results from taking the impedance between one of the ground lines and the signal line. Curve R 2 shows the TDR curve resulting when the ground line responsible for curve R 1 is removed from the cable configuration and the effective impedance is provided by the other ground line of the pair of ground lines associated with the signal line. The curves R 1 , R 2 and R 3 thus represent the impedance as a function of length for the same cable. An inverse variation in the effective impedance is thus produced by skewing.
By combining the curves R 1 and R 2 it becomes obvious that since the impedance mismatch occurs at approximately the same location on the cable with respect to the two ground lines the impedance mismatches are of opposite magnitudes and tend to cancel each other out. This cancelling phenomenon has heretofore masked the problem of signal coupling due to skewing.
The phenomenon of the impedance mismatch due to skewing with respect to two ground lines was not readily discoverable. In fact when a number of cable prototypes were fabricated and tested by the applicant, the test results appeared to show a consistent impedance match over the lengths tested. The cable appeared ready to be marketed, and the assignee of the present application was sufficiently confident of its manufacturing processes to ready itself for supplying customer orders. However, the cable did not in fact perform satisfactorily.
Testing of randomly selected lengths of cable also did not readily discover the skewing effects present in the manufactured cable. The skewing effect was discovered only by continued testing of various lengths of the cable. It was startling and surprising to discover that certain lengths of cable selected and tested resulted in undesired attenuation measurements. This lead the applicant to further analyze those certain cable lengths to discover what caused the occurrence of high attenuation. His analysis indicated that skewing of either the ground lines or the signal lines, although nearly imperceptible in amplitude, occurred at some periodic rate which could not be easily eliminated during the manufacturing process.
It is speculated that any manufacturing process used would not be tolerant or accurate enough virtually to eliminate all skewing; and that skewing would have a probability of being present also in some periodic rate. This impedance mismatch in the cable caused by skewing would not be readily measurable or detected unless a resonant section of the cable were accidentally selected. In fact, even after measuring a length of cable, which would by coincidence be a resonant section, the exact cause of the resulting signal distortion would not be readily apparent. Accordingly, the problem existing in the prior art cable was not readily discoverable, and only through substantial analysis after such discovery was the actual cause found.
SUMMARY OF THE INVENTION
The above and other disadvantages are overcome by the present invention of a shielded flat cable having a plurality of signal conductors in a parallel, spaced arrangement in a first plane and a conductive shield member in a second plane in confronting relationship with the signal conductors and separated from them by a suitable dielectric. Portions of the shield member facing the signal conductors are removed to form ground lines, which are parallel to the signal conductors but generally spaced between them, and a plurality of parallel shunt paths oriented in a direction substantially perpendicular to the ground lines. The shunt paths are spaced apart by a predetermined distance selected to be a small fraction of the wavelengths of the signals which are expected to be carried by the flat cable.
It is therefore an object of the invention to provide an improved flat shielded cable design in which correction is made for excessive attenuation caused by skewing of the signal lines with respect to the ground lines by providing shunt paths between the ground lines at predetermined intervals.
It is another object of the invention to compensate for resonant coupling due to skewing of a periodic configuration whose wavelength corresponds to the length of the flat cable by providing parallel shunt paths between the ground lines.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of certain preferred embodiments of the invention, taken in conjunction with the accompanying drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is an R.F. attenuation curve of a prior art flat cable showing signal attenuation of the cable as a function of frequency;
FIG. 2 are R.F. signal attenuation plots of a prior art cable showing attenuation due to skewing as a function of cable length;
FIG. 3 are time domain reflectometry plots of the impedance characteristics of a prior art flat cable;
FIG. 4 is an R.F. signal attenuation curve, similar to the curve of FIG. 2, of a cable constructed according to the invention;
FIG. 5 is a plan view of the end of a flat cable according to one embodiment of the invention;
FIG. 6 is an enlarged section taken along the line 6--6 of FIG. 5; and
FIG. 7 is a horizontal view of the underside of the flat cable and depicted in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIGS. 5 and 6 a flexible flat conductor cable 10 according to one embodiment of the invention includes a plurality of parallel, spaced-apart ribbon-like signal conductors 12. Each of the conductor 12 terminates in a separate pad 11.
A ground plane 16, of electrically conductive material lies in a plane parallel to the plane of the spaced-apart signal conductors 12. The signal conductors 12 are separated from the ground plane 16 by a dielectric 13 of suitable material such as Mylar (polyethylene terephthalate). The Mylar is transparent and is best seen in the cross-section of FIG. 6. Two cover sheets 14 and 15 of similar material may encase the conductors 12 and 16 and the dielectric 13.
Referring now more particularly to FIG. 7 the underside of the flat cable 10 depicted in FIG. 5 is illustrated. The ground plane 16 contains slots 18 devoid of electrically conductive material. These narrow slots are opposite each of the signal conductors. By selectively altering the width of these slots, the capacitance between the signal conductors and the ground plane can be changed without requiring a change in the thickness of the dielectric 13 or a change in the dielectric constant of this material. Additionally the area of the ground plane 16 which is opposite the termination pads 11 is removed by etching to facilitate coupling of the cable 10 to a connector (not shown).
Across the width of the cable 10 the slots 18 which are devoid of electrically conductive material are parallel to each other and are separated by thin, parallel strips of conductive material, hereinafter referred to as the ground lines 20. The ground lines 20 are parallel to the signal lines 12 but substantially spaced between the signal lines 12 The orientation of the signal lines 12 and the ground lines 20 is that they run the length of the cable 10.
Along the length of the cable 10 the slots 18 are separated by a plurality of shunt lines 22 of electrically conductive material which remain after the etching of the slots 18 in a pattern such that the shunt paths 22 run substantially perpendicular to the ground lines 20. The width W of each of the shunt paths 22 is chosen such as to keep the resistance relatively low, for example, less then 0.01 ohms and also so that the capacitance of the lines are not greatly affected and the impedance made too low. For example, if L equals the center-to-center spacing between the shunt paths 22 along the length of the cable then a preferable ratio of W/L would be less than or equal to approximately 0.1.
The upper frequency limit may also be determined as a function of the spacing L between the shunt paths 22. When the spacing is such that the distance L is equal to a half wavelength then the upper frequency may be determined as follows:
f H = υ/λ = υ/2 L,
where υ = the speed of electromagnetic wave propagation in the cable
For example where W equals 20 mils and L equals 250 mils the upper frequency equals approximately 14.8 × 10 9 Hz. At this frequency all of the reflections on the signal line due to the shunt paths 22 combine in a manner which essentially produces a stopband. In actual operation the frequency transmitted by the flexible cable would be chosen to be well below this upper frequency limit. Conversely if L is chosen to be a quarter wavelength then the frequency f H ' = υ/4L is a passband frequency at which the signals transmitted by the cable will not be attenuated by the shunt paths.
In general the spacing between the shunt paths 22 should be chosen to be a small fraction of the shortest wavelength signal to be carried by the flexible cable. Based on the above formulae, in one embodiment the preferred center-to-center spacing between the shunt paths is chosen to be substantially less than
L' = υ/2f
and where
υ = the speed of electromagnetic wave propagation in the cable and
f = the highest frequency of the signals expected to be transmitted by the cable.
Although in the above description it has been assumed that substantially continuous signals are transmitted by the cable 10 it should be apparent to those skilled in the art that the cable of the invention is also suitable for transmitting pulse signals. In such case the equivalent frequencies (in GHz) of f and f H for use in the above described formulae may be obtained by dividing the factor 0.35 by the rise time of the pulses (in nanoseconds), as is known to those skilled in the art.
Referring now more particularly to FIG. 4, the R.F. attenuation curve of a 50 ohm flat cable constructed according to the invention is illustrated. The shunt paths were spaced 250 mils apart and the cable length was 5.5 feet. As is apparent from the figure, there is no resonant attenuation "dip" as in the curves of the prior art cables depicted in FIG. 2.
In order to reduce the effect of the shunt conductors at any one frequency it may be desirable for some applications to vary the spacing along the length of the cable.
This will extend the useful operating range of the cable by providing a solution to the problem recognized by Ragan in "Microwave Transmission Circuits" -- Radiation Laboratory Series No. 9 -- McGraw-Hill Book Company Incorporated, 1948, pages 160, 161.
The terms and expressions which have been employed here are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions, of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.