BACKGROUND OF THE INVENTION

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 each wire within the line can be represented by the equation:

Z ₀ = (120/(e) ^% )ln(2V(l-s ² )/(l+s ² ))

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 ₀ = (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 process whereby two wires are simultaneously placed within a thermoplastic insulator 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 thermoplastic 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.

The various features and advantages associated with the inventive low skew transmission line will become apparent from 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.

Figures 2-4 are cross-sectional views of other respective embodiments of low skew transmission lines designed in accordance with this invention.

Figure 5 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 6 and 7 are cross-sectional views of low skew transmission lines designed in accordance with this invention, which correspond to the cores illustrated in Figures 3 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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 made from foamed fluorocarbons or foamed polyolefins. The preferred fluorocarbons used include fluorinated ethylene propylene (FEP) and perfluoroalkoxy (PFA). The preferred polyolefins include polyethylene or polypropylene. 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 constants 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. The process includes conventional techniques for foaming the thermoplastic insulator material. The process also includes feeding insulator wires 12 in a controlled manner such that equal spacing between the wires is ensured along the entire length of transmission line 10.

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 the appearance in a cross-sectional view preferably is homogenous.

Figures 2-4 illustrate, in cross-sectional view, three embodiments of low skew transmission line 10. The embodiment of Figure 2 is the currently most preferred embodiment because it provides a uniform electromagnetic field about each wire 12. The embodiments of Figures 3 and 4 are also highly preferred for practical reasons. 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 2 has a generally oval or elliptical cross-sectional configuration. The embodiment of Figure 3 has an elongated, generally elliptical cross-sectional configuration. The embodiment of Figure 4 has a cross-sectional configuration that appears as two intersecting circles. In the embodiments of Figure 3 and 4, drain wires are preferably placed on the outside of insulator 14. Although the embodiment of Figure 2 is most preferred as stated above, the embodiments of Figures 3 and 4 are also highly preferred because of practicalities in assembling a complete transmission line as shown in Figures 6 and 7.

Figure 5 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 2 would be similarly finished with a shield and jacket.

Figures 6 and 7 show, in cross-sectional view, alternatively 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 apparent 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.