Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
1999-10-28
2002-06-11
Reichard, Dean A. (Department: 2834)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
C174S1130AS, C174S036000
Reexamination Certificate
active
06403887
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to data transmission cables and more specifically to a high speed data transmission cable which has low signal skew and attenuation, is mechanically durable and is able to deliver more consistent data signals at high data rates.
BACKGROUND OF THE INVENTION
There is currently a demand for high speed data transmission cables which are capable of high-fidelity signal transmission at minimal signal attenuation. The ever-increasing use of high speed computer equipment and telecommunications equipment has increased such demand.
One existing cable product capable of high data rate transmission is fiber-optic cable which has good bandwidth performance over long distances. Furthermore, fiber-optic cables provide very low attenuation and little interference or noise with the transmitted signal. However, despite their desirable signal transmission qualities, fiber-optic cables are still very expensive. Furthermore, when transmission of signals over shorter distances is required, fiber-optic cables become even less desirable from an economic standpoint. For high speed data transmission over relatively short distances, such as up to 50 meters, copper based, differential signal transmission cables are the predominant choice in the industry.
Differential signal transmission involves the use of a cable having a pair of conductors wherein the information or data which is transmitted is represented by a difference in voltage between each of the conductors of the pair. The data is represented in transmission by polarity reversals on the conductor pair, and the receiver or other equipment coupled to the receiving end of the cable determines the relative voltage difference between the conductors. The voltage difference is analyzed to determine its logical value, such as a 0 or 1. Differential pairs may be shielded or unshielded. Shielded differential pairs generally perform better than unshielded pairs because the internal and external environments of the conductors are isolated. Improved attenuation performance also usually results with shielded cables.
Differential signal transmission cables have a variety of desirable electrical characteristics, including immunity to electrical noise or other electrical interferences. Since the differential signals transmitted are 180° out of phase to provide a balanced-signal in the cable, and are considered to be complementary to one another, any noise will affect both of the conductors equally. Therefore, the differences in the signals between the conductor pairs due to external electrical noise and interference are generally negated, particularly for shielded pairs. It may also be true for unshielded differential pairs as well by varying the twisting of the pairs, for example. Differential signal transmission cables are also immune to cross-talk, that is, interference between the individual cables due to the signals on other cables which are bundled together into a multi-cable structure. Again, shielded differential pairs will generally outperform unshielded pairs with respect to cross-talk. Multiple differential signal cables in a larger overall cable structure are referred to as primary cables.
Since differential signal transmission relies upon parallel transmission of the data signal and comparison of the differences in those signals at the receiving end of the cable, it is desired that the corresponding signals of each pair arrive at the receiving end at the same time. Because of insulative property differences experienced by each conductor of a cable pair, such as differences due to dielectric inconsistencies in the insulation and/or simply the physical characteristics of the cable, differential signal transmission cables are subject to signal skew. Signal skew is defined as the time delay of the arrival of one of the corresponding or complimentary signals at the receiving end of a conductor with respect to the other signal on the other conductor of the pair. In simpler terms, one complimentary signal arrives at the receiving end of the cable faster than the other signal. This signal skew condition is exaggerated as cable length increases. Generally, a signal skew budget is designed into data transmission systems, and the cables which link the systems are allowed only a portion of the budget. Therefore, signal skew is an important parameter which must be considered when using a differential signal transmission cable.
As will be appreciated, it is desirable to keep signal skew in a cable to a minimum to prevent errors in communication. Furthermore, low signal skew is necessary for proper cancellation of noise, because if the two opposing signals do not arrive at the receiving end at the same time, a certain amount of the noise in the cable will not be cancelled. A lower signal skew will also minimize jitter, which is the amount of real time it takes for the signal rising and falling edges to cross, which allows a differential signal transmission cable to be utilized at greater lengths or distances. It is therefore desirable to utilize a data transmission cable having a relatively low signal skew.
Another desirable characteristic in differential signal data transmission cables is low attenuation. Attenuation will generally be affected by the
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physical structure of the cable defining its impedance and including the shield type and design, the dielectric material used as insulation, the position of the conductors, and the electrical interaction between the conductors of the cable. If the cable is poorly constructed, the dielectric material properties, conductor-to-dielectric geometry, and hence impedance characteristics of the cable, may vary along its length, thus increasing its signal attenuation or loss characteristics when the cable is subjected to use. Accordingly, it is desirable to utilize a cable which has low attenuation characteristics at the desired operating frequency, so that cable length can be maximized, and also to utilize a cable which maintains constant, low attenuation characteristics during use.
To that end, it is further desirable to maintain the conductors in consistent positions within the cable insulation and in consistent positions with respect to one another. It is also desirable to maintain consistent dielectric properties of the cable insulation along its length to reduce impedance variations and hence reduce attenuation and signal skew. At the same time, high speed data transmission cables should still be flexible and able to withstand the mechanical and physical abuses associated with usage.
For example, the distance between the conductors, as well as the distance from the center of each conductor to the outer surface of the dielectric, should be consistent along the length of the cable.
Data transmission cables have been designed to address various of the concerns discussed above and to reduce signal skew while maintaining a durable and cost-effective cable. For example, the cable disclosed in U.S. Ser. No. 08/991,730, filed Dec. 16, 1997, which is commonly owned with the present application, discloses a cable with low signal skew and a robust design. U.S. Ser. No. 08/991,730 is incorporated herein by reference in its entirety. Such a cable requires particular attention to the placement of the cable elements during its formation, and specifically requires attention to the concentricity of each wire and tension of the cable and the proper placement of the shield for reducing skew. While the cable has produced desirably low skew figures, it is still an objective to improve upon its design.
Specifically, it is noted that varying electric charge on the cable and between the conductors will degrade the skew performance of the cable.
The rising and falling edges of the differential signal are affected by such charge variation. Particularly, the edge degradation of the signal's rising and falling edges may vary between the conductors (often referred to as slew) due to charge variation. The slew characteristics of a cable directly affect the skew characteristics of tha
Kebabjian Matthew T.
Kulaga Jerry J.
Mayo III William H.
Tensolite Company
Wood Herron & Evans LLP
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