Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
2001-05-21
2003-12-16
Mayo, III, William H. (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C174S10500R
Reexamination Certificate
active
06664466
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electrical cables. More particularly, the present invention relates to shielded electrical cables capable of preventing radiation from signals contained within, while also avoiding the creation of undesirable ground loops. In general, ground loop formation is an unintentional side effect of the process of cable shield connection to the terminating devices.
BACKGROUND OF THE INVENTION
The use of shielded electrical cables for establishing suitable electromagnetic compatibility (EMC) margins in commingled communications, or other electronic equipment environments, is nearly ubiquitous. In such equipment settings, isolated single purpose connections are commonly utilized to bond conductive equipment shells to supporting frames and structures, and then through these, to earth or ground potential. This is primarily done to prevent hazardous voltage differences from developing between the exposed surfaces of the various entities so interconnected, and to improve signal integrity between equipment communicating over an electrical path. Nonetheless, easily measurable, and operationally problematic, voltage differences can result from any number of a variety of factors such as, for example, local fault currents, or external influences such as lightning, power system induction or faults, or even the ramifications of ambient magnetic disturbances created by solar storms.
In an effort to achieve suitable EMC margins, a shield conductor of a connecting cable is frequently connected at each end to an equipment shell. This practice, however, leads to the undesirable result of creating a complete electrical loop, which in the present context, is called a ground loop. Specifically, in this example, the ground loop consists of the preexisting equipment bonding mechanisms and the interconnecting cable shield.
FIG. 1
provides a schematic representation of this condition. In
FIG. 1
, cable shield
10
is connected at opposite ends to a first equipment shell
12
and a second equipment shell
14
, and thereby to area bonding network
16
to form a complete electrical loop or ground loop
18
.
Often, the effects of the group loop are benign because there is little or no potential difference between cable ends, as a result of no external currents and a relatively small loop area as defined by the enclosing ground loop path. In other cases, however, a ground loop formed incidentally by the shield connections of the cable can create serious problems. For example, even though potential differences can be controlled by bonding system design to no more than a few volts, such a voltage can produce unintended cable shield currents of many amperes. This unintended current can, in turn, induce disturbances in other proximally located cables and, due to imperfections of shield construction, disturb the signals carried within the offending cable itself. Unreliable communication between interconnected equipment can result, and in rare instances, destructive levels can occur.
Therefore, because it is difficult to establish the immediate and future ramifications of incidental cabling ground loops, the routine creation of ground loops is to be avoided. Present practice is to avoid the creation of cable shielding ground loops by establishing a shield connection at only one end of the subject cable. By doing so, the continuous ground loop may be broken and the incidental and unwanted current flow in the shield interrupted.
This solution, however, is contrary to EMC best practices. In this regard, connecting the cable shielding at only one end of the cable gives rise to a number of other problems. These, in particular, include signal leakage radiation, and susceptibility to external radio frequency and other electromagnetic ambient conditions. To elaborate, shielding is used when it is desirable to prevent conducted signal leakage and resultant radiation from cabling. In a reciprocal manner, external electromagnetic fields are intended to induce currents on the cable shield, as opposed to the signal conductors contained within. Any discontinuity in the shield, such as intentionally disconnecting one end from the equipment shell at that end, to interrupt a ground loop path, for example, allows a voltage differential to develop across the discontinuity, with attendant undesirable coupling between external fields and the intentional signal currents.
To this date, the devices of the prior art have not been effective in addressing these and other problems. Current cabling designs alone cannot directly satisfy the contradictory goals of providing a continuous, and thus potentially effective, radio frequency (RF) shielding, and in the same instance, provide a discontinuous ground path, thus avoiding the formation of a ground loop. A well known, but rarely practiced solution because of induced mechanical complexities, and consequent cost penalties in equipment design, is to incorporate a discrete capacitor which is in series between the conductive equipment shell and each of the corresponding cable end shield connection means, in at least one of the devices to be interconnected, taken two at a time. For this purpose, a blocking capacitor typically in the order of 0.1 microfarads is selected, which must, along with its mounting means, possess very low stray inductive and resistive effects to avoid materially affecting shield RF performance as a result of its introduction.
A typical prior art cable
20
used for telecommunications equipment interconnections, which employs a metallized film shielding means is shown in cross-section in FIG.
2
A. The metallized film used as the shield itself is shown in cross-section in FIG.
2
B. Referring first to
FIG. 2B
, shield
28
is composed of a strip of nonconductive or insulating material
44
with a metallized layer
48
formed on one side. Referring next to
FIG. 2A
, shield
28
is helically or longitudinally wrapped around a plurality of conductors or signal leads
24
. One edge of the metallized film shield, essentially parallel to the cable axis, is folded
40
so that when the shield material is formed around leads
24
and overlapped, the metallized surfaces so overlapped connect, forming an electrically continuous shield circumferentially. An uninsulated wire or drainwire
36
, in turn, is wound in a widely spaced helix around shield
28
along its entire length in such a manner that it is in continuous contact with metallized outer layer
48
. Drainwire
36
serves the purpose of mitigating the effects of the unavoidable shield seam, and when exposed at each cable end, provides a convenient means of connection to the cable shield. An insulating jacket
38
surrounds the shield
28
and the drainwire
36
.
As in the case with this and any other form of shielded cable lacking isolation, connection of the shield to equipment enclosure at both ends in an environment where the enclosures are otherwise connected, in most instances by a grounding network, undesirably creates a ground loop.
Thus, prior art does not provide an economical or routine way to achieve simultaneously good cable RF shielding and avoidance of ground loop creation during interconnection of electronic equipment. Consequently, a need exists for a cabling mechanism which directly and economically addresses both performance goals at one time.
SUMMARY OF THE INVENTION
To address these and other needs of the prior art, it is an object of the present invention to provide a shielded cable capable of connecting, communications equipment in a manner that avoids the formation of undesirable ground loops while also avoiding signal radiation and unwanted external radio frequency and electromagnetic interference.
It is another object of the present invention to provide a shielded cable that incorporates a blocking capacitor within the shield construction itself.
It is yet another object of the present invention to provide a shielded cable that possesses the characteristics of capacitively coupled yet electrically isolated parallel shield surfaces.
It is
Bateman Andrew J.
Mayo III William H.
Spirent Communications of Rockville, Inc.
Villacorta Gilberto M.
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