Electricity: conductors and insulators – With fluids or vacuum – Conduits – cables and conductors
Reexamination Certificate
2000-03-22
2002-05-14
Nguyen, Chau N. (Department: 2831)
Electricity: conductors and insulators
With fluids or vacuum
Conduits, cables and conductors
C174S1130AS, C174S116000
Reexamination Certificate
active
06388188
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to electric cables, and in particular, although not exclusively, to electric cables for audio, hi-fi, video or computer applications.
It is a well-known technique to twist a pair of wires carrying an electrical signal to improve the noise rejection of the pair.
For computer applications, such as connecting peripherals to interface card, cables are known which comprise adjacent sets of twisted pairs, each pair consisting of one signal line and one ground wire. One configuration of such a cable is a ribbon-like flat cable in which there are flat untwisted regions at regular intervals along the cable for easy connection to crip-on connectors of the type used for ordinary ribbon cable. Because of the strobed data transfer protocol used on computer buses and in connections to peripherals, it generally is not necessary to use twisted pairs for all signal lines, instead just for the synchronising pulses and other strobing or enabling lines.
In audio systems applications it is a well-known technique to twist the pairs of conductors carrying differential signals in interconnecting leads (between for example the CD player and amplifier) and in speaker cables to improve the noise rejection of the cables. Spurious RF signals which would degrade the sound quality are rejected by the twisted geometry. In the case of speaker cables, it becomes particularly desirable to twist pairs of wires carrying the signals for each channel when the total cable length is great.
A variety of known audio cable geometries are shown in FIG.
1
. In FIG.
1
(
a
), two insulated wires
1
,
2
with conductive cores
11
,
21
are twisted together and encased in a flexible dielectric jacket
6
. FIG.
1
(
b
) shows a known geometry comprising two wires twisted together with a non-conductive strand
3
, and FIG.
1
(
c
) shows the cross section of a known geometry comprising two wires
1
,
2
twisted together with two non-conductive strands
3
,
3
a.
In general, the greater the twist (i.e. the greater the number of twists per unit length) the greater the noise rejection property of the cable. However, as the number of twists per unit length is increased, there comes a point when the cable has a strong tendency to bunch even under tension, and once released from the spool on which it has been wound it becomes unmanageable. This has been referred to as the “elastic band” effect.
Partly for this reason, in the audio cable industry a twist frequency of 5 per inch has been seen as the benchmark for cables comprising twisted jacketed (i.e. insulated) conductors. It is a compromise figure, giving good noise rejection in conjunction with ease of manufacture.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an electrical cable which overcomes some of the problems associated with the prior art.
It is a further object of the present invention to provide an improved method of manufacturing an electrical cable.
According to a first aspect of the present invention there is provided an electrical cable comprising a first strand, a second strand, and a third strand, wherein said first and second strands are electrically conductive and electrically insulated from each other, said third strand is electrically non-conductive, and said first, second, and third strands are braided together.
Advantages of a cable in accordance with this first aspect of the present invention are numerous, and include:
1. The inductance per unit length of the cable is lower than that of a cable comprising the same conductive strands but in a twisted pair configuration (with the same crossover frequency). The “braided” geometry reduces the self inductances of the individual conductive strands.
2. The noise rejection of the cable is significantly better than that of an equivalent twisted pair. In particular the “braided” cable, whilst performing similarly to the twisted pair at rejecting noise caused by fluctuations in the component of background magnetic field transverse to the cable, is intrinsically better at rejecting noise caused by fluctuations in the longitudinal component.
3. The “braided” geometry can lead to significant reductions in the attenuation of signals along the cable compared with the twisted pair.
4. Braided cables in accordance with the present invention may exhibit lower resistance than equivalent cables comprising the same conductive strands but in the form of twisted or parallel pairs. This reduced resistance is due to reduced interaction between the adjacent “go” and “return” currents in a pair of conductive strands used to carry a differential signal, or, in other applications ac or dc power.
The “go” and “return” currents are moving in opposite directions and electromagnetic interaction between them distorts the current distributions in each conductive strand. This reduces the effective cross sectional area of the conductive part of the strand and so results in an increase in resistance. It should be noted that this is a real increase in the resistance of the cable, separate from any increase in the magnitude of the impedance of the cable due to any increase in its self inductance which may also result from changes in current distributions.
In a twisted pair or a closely spaced parallel pair the conductive strands carrying the “go” and “return” currents are in close proximity to each other along their entire lengths. The interaction between these currents is therefore strong, and results in increased resistance.
In contrast, in the inventive braided cables, the is conductive strands are repeatedly separated along the length of the cable by the non-conductive strand (or strands). This reduces the interaction between the go and return currents and so reduces any resultant resistance increase.
The thickness of the non-conductive strand or strands may result in a repeated spacing of the conductive strands between their “crossover” points that is sufficiently large to make resistance increases due to inter-strand interaction negligible, or even zero.
In a parallel or twisted pair arrangement, the separation of the conductive strands can be increased to reduce inter-strand interaction, but results in increased inductance and susceptibility to noise.
The inventive braided cables reduce inter-strand interaction, resulting in reduced cable resistance, whilst retaining low inductance geometry and improved noise rejection.
Interaction between adjacent “go” and “return” currents in cables has also been termed “the proximity effect”, and for ac signals has been seen to result in increases in cable resistance with frequency. In contrast to twisted or parallel pairs, the inventive braided cables may render such resistance increases negligible, or even zero, for signal frequencies of interest (e.g. up to 20 KHz for audio applications).
In general, the proximity effect is more significant for low-resistance cables (i.e. incorporating heavier guage conductors) such as speaker cables, as lateral current mobility, which enables distortion of the current profile in the conductor, is greater. With larger diameter conductors, there is greater scope for current distribution distortion. Thus, in audio applications, the resistance reducing aspect of the inventive braided cables is particularly advantageous in speaker cables.
The strands may be encased in a flexible jacket of dielectric material. This jacket may comprise one or more materials from a list including PTFE, PE, and PVC, and may be a composite. Of course, a wide variety of other materials may be used.
As a result of the “braided” geometry lowering the self inductance of the cable, increased capacitance can be tolerated. Thus the cable designer has a wider choice of materials to use for the jacket, and so has more freedom to tailor the LCR properties of the cable to the particular application. For audio cables, the designer thus has more freedom to alter the “sound” of the cable.
Also, because higher cable capacitances can be tolerated, less expensive materials can be used for the jacket.
For cer
Ixos Limited
Nguyen Chau N.
Westman Nickolas E.
Westman Champlin & Kelly
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