WO1996034400A1

WO1996034400A1 - Low skew transmission line

Info

Publication number: WO1996034400A1
Application number: PCT/US1996/005708
Authority: WO
Inventors: Jerry Kulaga; Albert Kelley, Jr.; Donald Dombrowsky
Original assignee: Tensolite Company
Priority date: 1995-04-26
Filing date: 1996-04-24
Publication date: 1996-10-31

Abstract

A high speed, low skew transmission line (10) for the propagation of high speed electrical signals in computer systems, for example, has a low density thermoset insulator (14) that provides an equal effective dielectric constant along the length of two conducting wires (12). A methodology for making such a wire in accordance with this invention incorporates an extrusion process that vaporizes, stretches and sinters the insulator in a single pass through an extrusion machine, which forms the insulator about the two conducting wires.

Description

LOW SKEW TRANSMISSION LIRE

BACKGROUND OF THE INVERTIOR

1. Field of the Invention.

This invention relates generally to transmission lines for the propagation of electrical signals. More specifically, this invention relates to a low skew transmission line used for the propagation of high-speed electrical signals in computer applications.

2. Description of the Prior Art.

Various transmission lines have been used for communicating electrical signals between hardware devices for many years. Transmission lines of varying capabilities and characteristics are available depending on the needs of a particular application. Modern day computer systems, for example, require transmission lines that are capable of carrying electrical signals at relatively high speeds. In some applications, it is necessary to have two lines or conducting wires within a transmission line. In such applications it is necessary to have a minimum amount of skew of signal propagation time between the two wires of the transmission line.

Consider a two-wire transmission line that has a beginning and an ending point. Each of the two wires has an equal physical length between the beginning and ending points. Electrical signals, such as a square wave, travel or are conducted along the two wires at a speed that is determined, in part, by the impedance of each wire. If the two wires have relatively varying impedances, skew will result in signal propagation. Skew is the difference in time it takes an electrical signal, such as a square wave, to traverse one wire compared to the second wire.

Conventional two-wire transmission lines are made by a process whereby the two conductors are insulated in separate manufacturing processes and then are paired together to make a finished, balanced line construction. The significant drawback associated with such wires is that any shift or variation in the effective dielectric constant that is not matched in equal amounts in both insulated wires results in excessive and undesirable skew.

Taking a balanced transmission line as an example, it is recognized that the impedance of the each wire within the line can be represented by the equation:

Z₀ = (120/(e) )ln(2V(l-s²)/(l+s²))

where: V = the distance between the centers of the two wires divided by the diameter of the wires (assuming the two wires have an equal diameter);

s = the distance between the two wires divided by the overall diameter of the insulation surrounding both wires; and

e = the effective dielectric constant of the wires and the insulation system.

For a balanced transmission line, the distance between the two conducting wires is typically 1/2 the overall diameter of the insulating system, therefore, s = .5. By substitution:

Z. = (120/(e)^%)ln(1.2(h/d)) where h = the distance between the respective centers of the two conducting wires; and

d = the diameter of the conducting wires.

Since the time delay in signal propagation along each wire is a function of the effective dielectric constant of the wire and insulation system, it follows that if the dielectric constant (e) of one line is not equal to the other line from the beginning point to the ending point undesirable skew occurs. In other words, the two conducting wires will have different effective electrical lengths.

In high speed signal transmission, skew is problematic. Therefore, this invention provides an improved two-wire transmission line for use in transmitting high speed electrical signals in computer systems, for example. This invention uses a special process, which incorporates the use of microporous polytetrafluoroethylene, for providing an insulator about two conducting wires such that each wire has an equal effective dielectric constant over their respective lengths.

SUMMARY OF THE INVENTION

In general terms, this invention is a low skew transmission line that is made up of a pair of wires for conducting electrical signals. The wires have equal lengths with corresponding points along those lengths. A low density insulator is disposed about and contacts the wires. The insulator has an axial length and a uniform density in a radially directed cross-sectional plane taken at any point along the length of the insulator. This uniform density provides the wires with an equal effective dielectric constant for each of the corresponding points along the lengths of the wires. /34400 PC17US96/ 5

-4-

The preferred methodology associated with this invention for making a low skew transmission line includes several basic steps. First, an extrudable mixture of unsintered high reduction ratio rating polytetrafluoroethylene is extruded through a die to thereby form a tube. A pair of conducting wires are fed through the die such that the wires are spaced from each other and disposed within the tube. The linear speed of the tube exiting the die is controlled in order to maintain a volumetric rate of the tube that is approximately equal to a volumetric rate of the mixture passing through the die. Next, the tube is heated to remove extrusion aid. Then the tube is heated to a higher temperature and it is stretched at a stretching rate that exceeds the linear speed of the tube as it is exiting the die. The stretched tube is then preferably sintered by heating the stretched tube to a sintering temperature.

The various features and advantages associated with the low skew transmission line and methodology associated with this invention will become apparent in the following detailed description of the preferred embodiments along with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross-sectional view of a low skew transmission line designed in accordance with this invention.

Figure 2 is a diagrammatic schematic representation of a preferred process for making a low skew transmission line in accordance with this invention.

Figure 3 is a cross-sectional view of a portion of the process Illustrated by Figure 2 taken along the line 3-3 in Figure 2. Figure 4 is a partial cross-sectional view of a portion of the process diagrammatically illustrated in- Figure 2.

Figures 5-7 are cross-sectional views of respective alternative embodiments of low skew transmission lines designed in accordance with this invention.

Figure 8 is a cross-sectional view of a low skew transmission line designed in accordance with this invention that includes an outer shield and jacket.

Figures 9 and 10 are cross-sectional view of low skew transmission lines designed in accordance with this invention, which correspond to the cores illustrated in Figures 6 and 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMEHTS

Figure 1 illustrates, in cross-sectional view, a low skew transmission line 10. Low skew transmission line 10 includes a pair of conducting wires 12. Wires 12 are preferably made of copper. Wires 12 are disposed within insulator 14. Insulator 14 is preferably a porous, thermoset material, commonly known as a porous solid dielectric material. The most preferred material for insulator 14 is polytetrafluoroethylene (PTFE). Low density, sintered polytetrafluoroethylene, which is microporous, is described, for example in United States Patent No. 3,953,566. The materials for insulator 14 and the wires 12 are commercially available. Low skew transmission line 10 provides an essentially equal effective dielectric constant along the entire length of each conducting wire 12. This is provided, largely in part, because insulator 14 has a uniform density, when viewed in a radial cross-sectional plane along the length of transmission line 10. The uniform density ensures that the effective dielectric constant for each conducting wire 12 is the same. In the most preferred embodiment, the density of insulator 14 is also uniform longitudinally along the entire length of transmission line 10. It is to be understood, however, that variations in density can occur along the length of transmission line 10 provided that the effective dielectric constant for each wire remains the same. That is, the density of insulator 14 near the beginning of transmission line 10 could, theoretically, be different than that near the end of transmission line 10 without introducing undesirable skew provided that each wire has an overall effective dielectric constant that is the same. One way to achieve equal overall effective dielectric constant is to ensure that each wire has uniform insulation for each corresponding point along the length of each conducting wire 12.

Transmission line 10 is preferably made from a process that provides for uniform density in insulator 14 and a uniform spacing between conducting wires 12. This is preferably accomplished by assembling the wires and insulator 14 in a single process. This process preferably includes the use of expanded PTFE, which provides for signal velocities of approximately 85% the speed of light. Expanded PTFE is the preferred material because it provides the ability to establish a uniform density in insulator 14 while also facilitating high speed signal transmission.

Referring to Figure 2, the preferred methodology of manufacturing transmission line 10 will be described. Extrusion machinery 20 is used to extrude PTFE about a pair of conductive wires in the formation of low density sintered PTFE insulation. Machinery 20 physically includes an extruder 22, which has a die 24 disposed at one end, a vaporizing oven 26, a stretching oven 28 and a sintering oven 30.

Extruder 22 is a conventional ram extruder for inline extrusion of PTFE extrusion paste, which shall be described below, shaped into an annular cylindrical preform 32. Conducting wires 12 are fed through the center of the barrel of extruder 22, in which preform 32 is located, and out through die 24 located at the exit end of extruder 22.

Extruder 22 is illustrated vertically positioned with die 24 at its upper end, such that the conducting wires 12 and extruded tubing 34 of extrusion paste, which overlies conducting wires 12 as it exits extruder 22, are drawn upwardly through tubular vaporizing oven 26. The extruded tubing 34 is then drawn upwardly to tubular stretching oven 28, which is vertically aligned above vaporizing oven 26 and extruder 22. For practical reasons, sintering oven 30, which is also tubular, is positioned parallel to vaporizing oven 26 and stretching oven 28. Accordingly, as tubing 34 and conducting wires 12 issue from the upper end of oven 28, they are preferably carried on turn around wheel 36 to reverse the direction of travel such that tubing 34 (and conducting wires 12) are brought vertically downward through the center of sintering oven 30.

Ovens 26, 28 and 30, which are convection ovens, are provided with internal electrical resisting heating units typified at 26A, 28A and 30A, respectively. The internal electrical resistance heating units are preferably controlled to produce internal oven air temperatures in excess of 700°F. Ovens 26 and 28 are preferably operated at 500°F and oven 30 is preferably operated at 700°F, such that volatile extrusion aid in the extrusion paste is driven off in oven 26 and the remaining PTFE is super heated to between 400 and 650°F in oven 28 and then sintered in oven 30.

At the lower end (according to the drawing) of sintering oven 30, the low density sintered PTFE insulated cable 10 is drawn by a fleeter capstan 40. Capstan 40 has a drum 42 and a fleeter wheel 44. Drum 42 is driven by a motor 46. Drum 42 and wheel 44 are mounted to rotate on parallel axes with their surfaces spaced apart slightly (as shown in the illustration). Cable 10 is wound in a figure-8 fashion in peripheral grooves on drxim 42 and wheel 44 and then taken off to a storage wheel or the like.

In order to produce low density sintered polytetrafluoroethylene Insulated transmission cable in accordance with this invention, machinery 20 also includes a pair of pinch rolls 50 and 52. Rolls 50 and 52 are driven in counterroatation by a motor 54. As shown in Figure 3, rolls 50 and 52 are peripherally grooved, preferably as indicated by the reference numerals 56 and 58, respectively, to receive the exterior of extruded tubing 34 between rolls 50 and 52. Motor 54 is preferably connected to rolls 50 and 52 to drive them at the same counterrotating speeds such that the confronting surfaces of grooves 56 and 58 travel at the same speed, in the same direction for passing tubing 34 between rolls 50 and 52.

Rolls 50 and 52 are positioned adjacent the exit of die 24 and in front of vaporizing oven 26. Rolls 50 and 52 are preferably aligned with the path of travel of extruding tubing 34 such that tubing 34 is carried between grooves 56 and 58, which lightly contact the surface of tubing 34 (as can be seen in the cross-sectional view of Figure 3) to nip and control the speed of tubing 34. Any pressure exerted by grooves 56 and 58, however, must be less than would restrain the relative movement of conducting wires 12.

As shown in Figure 4, extruder 22 includes a long sleeve 60, which serves as a guide tube, sized to receive the two conducting wires 12 and to carry them through the center of preform 32. Sleeve 60 normally terminates with a needle tip 62 within die 24 but short of the land 64 of the die. Although the illustration in Figure 4 appears to show only one wire, it is to be appreciated that sleeve 60 and needle tip 62 receive and carry the two conducting wires 12 in ^the preferred radial spacing that the two wires will have within insulator 14. It is possible to use a single, long sleeve 60 with two openings for the two conducting wires, or alternatively, to use two sleeves 60 each carrying one of the wires 12. Until wires 12 reach land 64, their rate of speed is preferably greater in excess of the rate of speed of the paste from preform 32, as the latter is extruded toward die 24. Sleeve 60 functions to permit the relatively higher speed of wires 12 to be unimpeded by the slower movement of the paste.

Sleeve 60 is also utilized to form an opening in the extruded paste as the latter enters land 64, thereby forming a tubing 34 of the extruded paste and functioning as a mandrel. As can be seen in Figure 4, which is a longitudinal section through die 24, needle tip 62 of sleeve 60 is located within die before land 64 and, because of its slightly greater thickness than wires 12, forms a bore in tubing 34 which has a diameter greater than the outside diameter of wires 12.

Sleeve 60 also facilitates maintaining a controlled spacing between wires 12. The spacing between wires 12 is preferably predictably controlled in order to reduce any variation in the impedance characteristics of the two wires 12.

In forming low density sintered polytetrafluoroethylene insulated cable in accordance with this invention, the peripheral rate of rolls 50 and 52 are the same as the linear speed of extrusion of tubing 34. The peripheral speed of drum 42 and wheel 44 and capstan 40, however, are at a rate substantially in excess of the rate of rolls 50 and 52 such that cable 10 is drawn at a rate substantially exceeding that of extrusion. Turn around wheel 36 is normally free to turn as drawn by tubing 34 in contact with it. The tension on tubing 34 is sufficient to bind tubing 34 against the conducting wires 12 such that no relative movement between tubing 34 and wires 12 at the location of wheel 36 can take place. Wheel 36 thus turns with a peripheral speed determined by the speed of wires 12, and hence of capstan 40. Tubing 34 is thus stretched between pinch rolls 50 and 52 and turn around wheel 36 by an amount percentage-wise equal to the percentage difference in ^' speed of rolls 50 and 52 and turn around wheel 36.

In the preferred embodiment, as illustrated, rolls 50 and 52 are used to control the linear speed of tubing 34 as it is drawn from die 24, however, it is possible to eliminate the use of rolls 50 and 52.

Tubing 34 is therefore stretched while under tension imposed by the difference in linear speeds of tubing 34 between rolls 50 and 52 or die 24 and wheel 36. This introduces porosity, which is set upon sintering in oven 30. The degree of porosity achieved is directly proportional to the degree of stretch. Therefore, if the cable is drawn by capstan 40 about wheel 36 at a linear rate equal to twice the speed of extrusion and wires 12 are fed to extruder 22 at twice the rate of extrusion, the resultant cable 10, as illustrated in Figure 1 for example, is insulated with polytetrafluoroethylene insulation 14, which has 50% volume of micropores.

It is preferred to preheat the extruder and the die to a temperature in the range between about 150°F (93°c) to 250°F (177°c). It has been found that such preheating raises the temperature of the paste above room temperature and promotes smooth cell structure. Preheating, however, should be limited, as temperatures of the paste above approximately 200°F correspond to a reduction of the tubing diameter and hence, lower porosity of the final product. The reduction ratio at which a PTFE powder can be extruded depends on the manner in which it is manufactured. The extruder reduction ratio is determined by the following formula:

RR = (A_c - A_r)/(A₁ - A₀)

where: A_c equals the total cross-sectional area of the extruder barrel, which has a diameter substantially equal to the outer diameter of the polytetrafluoroethylene preform 32;

A_r equals the total cross-sectional area occupied by the extruder guide tube 16 (ignoring the area of any small opening therein);

A^ equals the cross-sectional area of the die 24 opening; and

A_Q equals the cross-sectional area of the bore (if any) in the article, which area is equal to the cross-sectional area of the conductor or wires 12 when the PTFE is extruded on wires 12 as shown in the drawings.

With the type of extrusion described, in which the length of a wire to be insulated is limited by the volume of the preform, it usually is desirable to have a high extruder reduction ratio in order to be able to produce long lengths of insulated wire. If a polytetrafluoroethylene powder rated at a low reduction ratio were used in an extruder with a high reduction ratio, the high reduction pressure involved could potentially damage the extruder. Furthermore, the PTFE fibers may be fractured, thereby causing insulation surface roughness and non-uniform dielectric properties along the length of the insulated wire. Accordingly, it is conventional to use PTFE powders that have high reduction ratio ratings (i.e., greater than 1,000/1) and an extruder with a high reduction ratio (i.e., greater than 1,000/1) when extruding insulation over a conductor or conducting wires. After stretching and sintering, the resulting insulation preferably has a high matrix tensile strength (i.e., greater than 10,000 psi).

Contrary to conventional practices, the process of this invention uses a PTFE powder with a reduction ratio rating of at least 300/1, and preferably at least 2,500/1, and an extruder having a reduction ratio of substantially less than (i.e., preferably not more than 1/4 of) the reduction ratio rating of the powder and not greater than the PTFE preform. Such system characteristics yield an article that has a relatively low tensile strength is obtained. Preferably, the extruder reduction ratio is in the range from 200/1 to 3000/1, and a matrix tensile strength of the resulting polytetrafluoroethylene is in the range of 2,000 psi to 8,000 psi. The matrix tensile strength, however, may be as low as 900 psi and can be as high as 16,000 psi. The reduction ratio ratings of the polytetrafluoroethylene powders that are used in the methodology associated with this invention are always substantially higher than the extruder reduction ratio. When the extruder reduction ratio is low, the powder reduction ratio may be at the low end of the preferred range but as the extruder reduction ratio increases, the reduction ratio rating of the PTFE power should be increased. Preferably, the extrusion pressure on the PTFE mass in the extruder is in the range of 2,000/4,000 psi. With lower pressures, the extruded product does not properly hold together and with higher pressures (i.e., above about 6,000 psi) there is an undesirable increase in matrix tensile strength of the product and/or a tendency for the extruded product to break undesirably.

Further, it is desirable to improve the properties of the extruded product, particularly for facilitating the handling of electrical signals, by increasing the porosity of the product by stretching it. Porosity increases with an increased amount of stretching. In the methodology associated with this invention, the extruded product is stretched at a low rate prior to sintering, which is preferably in the range from 20% to 600%. Preferably, the stretching is in the range from approximately 100% to 400%, and the percentage voids or pores in the sintered product is at least 50%. As discussed above and generally understood in the art, the electrical characteristics of the sintered polytetrafluoroethylene are improved by increasing porosity. The typical signal propagation velocity of solid PTFE is approximately 70% of the speed of light. By increasing the porosity of the PTFE, in accordance with this invention, the signal propagation velocity in the sintered, porous polytetrafluoroethylene product can be as high as 93% of the speed of light. The amount of stretching is preferably selected so that the signal propagation velocity is at least 75% and, more preferably, in the range from approximately 85 to 90%. Density of the product is preferably no greater than 1.9g/cm³ and preferably it is in the range from approximately .45 to 1.9g/cm³.

In an alternate embodiment, which includes a full density dielectric, signal velocity is approximately 70% the speed of light in a vacuum. A fully density dielectric, according to this specification, has a density in the range from 2.1 to 2.2 g/cm³.

Low skew transmission lines designed and made in accordance with this invention, will be characterized by a continuous insulation 14 that has a uniform density from the center to the outer surface when viewed in a radially directed cross-sectional plane. The surface of insulation 14 is preferably smooth and, although it preferably contains 70% voids, the appearance in a cross-sectional view is homogenous. Further details regarding the methodology of manufacturing a low density, sintered polytetrafluoroethylene insulator such as insulator 14 are contained in United States Patent No. 4,826,725 issued on May 2, 1989, and having common assignee of interest with this application. The teachings of United States Patent No. 4,826,725 are hereby incorporated into this specification by reference.

Figures 5-7 illustrate, in cross-sectional view, three alternative embodiments of low skew transmission line 10. The embodiment of Figure 5 is the currently most preferred embodiment because it provides a uniform electromagnetic field about each wire 12. Due to practical assembly reasons, the embodiments of Figures 6 and 7 are also highly preferred. When an outer shield is placed about insulator 14, the shield is evenly displaced from conductors 12. Such an outer shield, when employed, is typically and preferably grounded. The embodiment of Figure 5 has a generally oval or elliptical cross-sectional configuration. The embodiment of Figure 6 has an elongated, generally elliptical cross-sectional configuration. The embodiment of Figure 7 has a cross-sectional configuration that appears as two intersecting circles. In the embodiments of Figure 6 and 7, drain wires are preferably placed on the outside of insulator 14. Although the embodiment of Figure 5 is most preferred as stated above, the embodiments of Figures 6 and 7 are also highly preferred because of practicalities in assembling a complete transmission line as shown in Figures 9 and 10.

Figure 8 shows, in cross-sectional view, a low skew transmission line 10 (as illustrated in Figure 1) that further includes a shield 70 and a jacket 72 disposed about insulator 14 in a conventional manner. Shield 70 is preferably made of an electrically conductive material and is connected to ground. Jacket 70 is a conventional jacket that would be employed in making electrical signal transmission lines for computer systems, for example. The embodiment of Figure 5 would be similarly finished with a shield and jacket.

Figures 9 and 10 show in cross-sectional view alternative, also preferred embodiments of low skew transmission lines that include a drain wire 74, a shield 76 and a finish jacket 78. Drain wire 74 is a conventional wire utilized to ground a transmission line. Shield 76 preferably is made from a polyester foil or tape also known as an aluminized polyester foil. Shield 76 has the aluminized side facing inward, toward dielectric 14. Finish jacket 78 is a conventional outer jacket.

The above decsription is exemplary rather than limiting in nature. Variations and modifications will become to those skilled in the art that do not depart from the purview and spirit of this invention. The scope of this invention is to be limited only by the appended claims and all fair legal equivalents thereof.

Claims

CLAIMSWhat is claimed is:

1. A low skew transmission line, comprising: a pair of wires for conducting electrical signals and having equal lengths with corresponding points along said lengths; a low density insulator disposed about and contacting said wires such that said wires are separated, said insulator having an axial length and having a uniform density in a radially directed cross-sectional plane taken at any point along the length of said insulator such that said wires have an equal effective dielectric constant for each of said corresponding points along said wire lengths.

2. The transmission line of claim 1, wherein said insulator further comprises a longitudinally uniform density such that said conducting wires have a uniform equal effective dielectric constant along said lengths.

3. The transmission line of claim 1, wherein said insulator comprises a thermoset material.

4. The transmission line of claim 1, wherein said insulator comprises polytetrafluoroethylene.

5. The transmission line of claim 4, wherein said polytetrafluoroethylene is expanded such that a density of said polytetrafluoroethylene is reduced.

6. The transmission line of claim 4, wherein said insulator has a density in the range from approximately 0.45 g/cm³ to approximately 1.9 g/cm³.

7. The transmission line of claim 4, wherein said insulator is substantially homogenous and has a tensile strength that is less than 16,000 psi.

8. The transmission line of claim 1, wherein a speed of propagation of electrical signals along said wires is between about 75% and 95% of the speed of light.

9. The transmission line of claim 1, wherein said insulator has a uniform cross-section along said length of said insulator.

10. The transmission line of claim 1, wherein said line is a balanced transmission line and wherein said insulator has a generally circular cross-section and wherein said wires are equally radially spaced from a center of said insulator.

11. The transmission line of claim 1, wherein said insulator has a generally elliptical cross-section and wherein said wires are spaced from each such that each wire is surrounded by an equal volume of said insulator.

12. The transmission line of claim 1, wherein said insulator has a cross-sectional configuration consisting of two intersecting circles and wherein said wires are disposed in a center of said circles, respectively.

13. The transmission line of claim 1, wherein said wires are spaced in a controlled manner such that an impedance characteristic associated with each wire is generally consistent.

14. A method of making a low skew transmission line, which is adapted to conduct high speed electrical signals, comprising the steps of:

(A) extruding an extrudable mixture of unsintered polytetraflouroethylene through a die to thereby form a tube;

(B) feeding a pair of conducting wires through the die such that the wires are spaced from each other and disposed within the tube;

(C) controlling a linear speed of the tube exiting the die to thereby maintain a volumetric rate of the tube approximately equal to a volumetric rate of the mixture passing through the die;

(D) removing extrusion aid from the tube by heating the tube to a removal temperature;

(E) heating the tube to a temperature above that in step (D) and below a sintering temperature of the polytetrafluoroethylene; and

(F) stretching the tube at a stretching rate that exceeds the linear speed of step (C).

15. The method of claim 14, further comprising the step of sintering the tube after performing steps (E) and (F) by heating the tube to a sintering temperature.

16. The method of claim 14, further comprising the substeps of controlling the linear speed of the tube such that the linear speed is the same while performing step (D) as it is in step (C).

17. The method of claim 14, wherein step (C) is performed by the substeps of nipping an outer surface of the tube and propelling the tube along at the linear speed without restraining relative linear movement between the wires and the tubes. -19-

18. The method of claim 14, wherein step (B) is performed by the substeps of feeding the wires at an equal and unchanging rate through a needle tip disposed linearly in front of the die.

19. The method of claim 14, wherein step (E) is performed by the substep of heating the tube to a temperature in the range from about 400° to about 650° F.

20. The method of claim 14, wherein the polytetrafluoroethylene has a reduction ratio rating of at least 2500/1 and wherein a reduction ratio brought about by performing steps (C) through (F) is in the range from about 200/1 to about 3000/1.