Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
2000-11-10
2002-09-17
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
C174S036000
Reexamination Certificate
active
06452107
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 multiple primary cable pairs combined together into a larger cable structure.
BACKGROUND OF THE INVENTION
There is currently a demand for high speed data transmission cables which are capable of high-fidelity data 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 particularly less desirable from an economic standpoint. As a result, 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 individual conductors wherein the information or data which is transmitted is represented by a difference in voltage between the individual conductors. 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 difference is then 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 generally 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 conductors of the pair 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. It is common to twist the individual conductors of a pair together along the longitudinal axis of the pair. The cables are then referred to as twisted pair cables. The main advantage of such cables is increased mechanical flexibility. However, there are considerable disadvantages to twisted pair cables; two important ones being size increase and high group skew.
Differential signal transmission cables are also generally 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, or multi-pair, structure. Again, shielded differential pairs will generally outperform unshielded pairs with respect to cross-talk. The multiple differential signal cables bundled together into a larger overall cable structure are referred to as primary cables of the overall, larger cable construction.
Since differential signal transmission relies upon parallel transmission of the data signals through the conductors of a pair, and then comparison of the differences between those signals at the receiving end of the cable, it is desired that the complementary signals of each pair arrive at the receiving end of the cable at the same time. However, properties of the cable affect the propagation speed of the signals along the conductors and therefore introduce delays between the signals of a differential pair. For example, because of insulative property differences experienced by each conductor of a cable pair, such as differences due to dielectric inconsistencies and/or physical characteristics of the cable, differential signal transmission cables are subject to propagation differences between the individual conductors. Variances in the effective length of one conductor with respect to the other conductor of a pair also create such differences. The difference in signal propagation between the conductors of a differential pair and the delays associated therewith is referred to as signal skew. Signal skew is defined as the delay of the arrival of one of the corresponding or complimentary signals at the receiving end with respect to the other signal. In simpler terms, one complimentary signal arrives at the receiving end faster than the other signal, a condition which 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.
Within a single differential pair, the skew is determined between the two individual conductors of the pair and is referred to as within-pair skew. In some cable applications, multiple differential pairs are bundled together to form a larger overall cable. Skew is then measured for each pair as a time delay for the differential balanced signal of the cable pair. The measure of time difference between the fastest and slowest signals for each of the multiple pairs, with each pair being considered to provide a single signal, is defined as a pair-to-pair or group skew.
More specifically, with a signal of one conductor considered M
1
, and the signal of another conductor considered M
2
, a differential pair will have a propagation delay associated not only with each signal M
1
, M
2
individually, but also with the propagation of the differential balanced signal (M
1
−M
2
). The differential balanced signal takes into account the differences in potential along the length of the whole line, the reference limit being zero. As differences in the individual conductors are encountered, each individual conductor of a pair contributes different potentials to the (M
1
−M
2
) balanced signal. The (M
1
−M
2
) signal fluctuates about zero. The group skew measurement is then the time delay difference between the fastest differential signal (M
1
−M
2
) and the slowest of such signals in a group of pairs in a multi-pair cable. That is, (M
1
−M
2
) is measured for each pair in a multi-pair cable and then the difference between the maximum time delay and the slowest time delay defines group skew.
Therefore, within-pair and group signal skews are important parameters which must be considered when using a differential signal transmission cable which incorporates multiple differential pairs. As will be appreciated, it is desirable to keep the in-pair signal skew characteristics of 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.
Another important characteristic for a differential signal cable is signal jitter. Signal jitter is defined as the amount of real time it takes for the differential signals' rising and falling edges to cross over when they transition. Low jitter, or rapid rising and falling edges, is desirable.
Attenuation should also be minimized in a differential cable. All cables will inherently reduce or attenuate the level of the signal transmitted thereon, due to the impedance qualities of the cable. Attenuation is generally affected by the physical structure of the cable, which includes the shield type and design, the dielectric insulation material type, the
Mayo III William H.
Reichard Dean A.
Tensolite Company
Wood Herron & Evans L.L.P.
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