A Cable Assembly

Technical Field

The present invention relates to a cable assembly.

Disclosure of the Invention

According to the present invention there is provided a cable assembly comprising a set of signal lines, a sub-set of which transmit signals over a high frequency range and the remaining set of which transmit low frequency signals and/or are electrically neutral, wherein the high frequency signal lines are separated from one another by the low frequency signal and/or electrically neutral lines.

Preferably, the number of high frequency transmission lines is even comprising sets of twisted pairs of lines, each pair twisting over a different length to the remaining pairs.

Preferably, the transmission lines each have the following specified properties of attenuation, crosstalk and reflection co-efficient for the stated signal frequency range, cable length and terminating impedance namely

Attenuation < 5dB @ lOMhz

Crosstalk > 56dB from 2 to 8 Mhz

Reflection Co-efficient < 0.09 from 2 to 12 Mhz

Cable Length 100m Terminating Impedance 145Ω

Preferably, each of the lines is insulated with an electrically insulating material which is coloured to facilitate identification of the individual lines.

Brief Description of the Drawings

Figure 1 is a cross-sectional view of a cable assembly according to the invention;

Figure 2 is a cross-sectional view of a cable assembly according to a second embodiment of the invention; and

Figure 3 is a cross-sectional view of a cable assembly according to a third embodiment of the invention.

Description of the Preferred Embodiments

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Referring now to the drawings wherein similar numerals have been used to indicate like parts, there is shown a cable assembly generally indicated at 10 according to the invention. The cable assembly 10 according to a first embodiment of the invention comprises a pair of power lines 11; two twisted pairs of transmission lines 12; and four twisted pairs of transducer signal lines 13. The cable assembly 10 according to a second embodiment of the invention comprises four twisted pairs of transmission lines 12; four mono-filament fillers 14; and a continuity conductor 15. The cable assembly 10 according to a third embodiment of the invention comprises two twisted pairs

of transmission lines 12; and two double pairs of transducer signal lines 13. Each cable assembly 10 is insulated by a polyurethane outer jacket 16.

The purpose of the power pair 11 is to supply direct current power through the cable assembly 10. The individual wires of the pair 11 comprise 61 x 0.2mm diameter of pure copper conductors bunched together. The insulating material is polyethylene giving the wires an outside diameter of 2.8mm. The electrical properties of the power pair 11 are not critical and the amount of dye used to colour the wires for the purposes of identification is 2%.

When the cable assembly 10 according to the present invention is used for measuring seismic signals, the transducer signal lines 13 pick up information from reflected seismic waves in the ground via a geophone, which is essentially an electromagnetic transducer device. Although an important part of the cable 10, their electrical properties are not critical as they are required to transport low voltage signals over a short distance to a nearby seismic station unit (max distance - 200m) . Each wire of the pair 13 comprises 7 x 0.14mm bronze conductors. The pair 13 is insulated with polyethylene, with a dye content of 2%, giving the wires an outside diameter of 1.1mm. There may be up to four transducer pairs 13 used in a cable assembly 10.

The crucial elements of the cable are the transmission lines 12. Their purpose is to transmit all information obtained throughout a grid of seismic sensors and stations back to a system central control unit. Each transmission line wire comprises 7 x 0.2mm silver plated copper conductors stranded together. The wires are insulated with polyethylene, with a dye

content of 0.5%, giving the wires an outside diameter of 1.95 to 2.00mm.

In order for the cable assembly 10 to operate, the transmission lines 12 must meet the specification of a number of electrical properties. The first of these is attenuation, which is a measure of signal power or amplitude loss over a distance of cable. The essential criterion governing attenuation, is that the longer a length of line, the more attenuated a signal will become. Typically, the longest lengths of cable involved between seismic station units are up to 800m.

Attenuation is a logarithmic unit expressed in decibels (dB) . In terms of power (P) , the formula for attenuation may be expressed as lOlog _! Q (Pout/Pin) dB a loss of 3dB representing a reduction of 50% in power.

The main factors influencing attenuation are: the type of conductor used, the amount and type of insulating material used and the amount of dye used in the insulating material.

The specification is that over 100m of cable, a loss or drop of no less than 5dB shall occur. Using the materials indicated above, typical results achieved are - 4.6dB with cable assemblies according to the invention.

Crosstalk is the signal from one pair which is inadvertently 'crossed' into another pair. 'Near-end' crosstalk is measured by sending a signal down one transmission pair 12 and measuring the crossed signal on the near end of another 'dormant' transmission pair 12, both pairs 12 being terminated in equal value

resistances.

Crosstalk is expressed in dB, being the value of the crossed signal relative to the transmitted signal. The specification is - 56 dB or '56 dB down' on the transmitted signal. This represents an extremely small value of crossed signal, however, this is necessitated by the large dynamic range of signals transmitted.

The criteria defining crosstalk are difficult to determine, but the chief criteria are the respective transmission pair's 12 proximity and the dissimilarity of their lay-lengths. Lay-length is the length of one twist of a pair.

In the first embodiment of the invention the power lines 11 are used to separate the transmission pairs 12. In the second embodiment, where there are four transmission pairs 12, lay-lengths of 25, 30, 35 and 40 mm are used, and mono-filament fillers 14 and a continuity conductor 15, which also help to 'round' the cable shape, are used to keep the transmission pairs 12 apart. In the third embodiment of the invention, the contact surface of the transmission pairs 12 is minimised by placing double pairs of transducer signal lines 13 between the transmission pairs 12.

It is important to note that any cross-section taken along the length of the cable assembly 10 differs only in that the twisted pairs rotate within the circles circumscribing each pair. Using the configurations described above a crosstalk value of 58dB is achieved.

Reflection co-efficient represents the amount of signal which 'bounces back' along any pair on which a signal is transmitted. It is an inherent part of any

communications system and the co-efficient must be kept as low as possible. The co-efficient is calculated by dividing the reflected signal value by the transmitted signal value to give a dimensionless result. It is specified as 0.09 within the given signal frequency range, which means that at no frequency should the reflected signal be above 9% of the signal transmitted.

The criterion on which the reflection co-efficient most depends is impedance. Also, greater amounts of insulation material (to a practical limit) contribute to degrading of the reflection co-efficient.

Impedance represents a complex combination of the resistance, capacitance and inductance associated with any given length of transmission line. It is again, like attenuation, affected directly by the nature of the conductor used and the amount of insulating material employed. Generally, an increase in impedance will mean an improvement in attenuation. Impedance (Z) is expressed in ohms (Ω) and can be calculated by the formula

Z = (X2 ₊ R2) where R = resistance; X = 27T/L - l/(2ιr/C)

L = inductance; C = capacitance; / = signal frequency

It is to preferable to keep impedance as low as possible, while still maintaining the specified attenuation. Using the configurations described above a reflection co-efficient of 0.06 is achieved.

The properties of attenuation, crosstalk and reflection co-efficient are specified for certain signal frequency ranges. All of the properties generally degrade as the signal frequency increases. The

specifications are:-

Attenuation : < 5dB @ lOMhz Crosstalk : > 56dB from 2 to 8 Mhz Reflection Co-eff. : < 0.09 from 2 to 12 Mhz

using a terminating impedance of 145Ω and a cable length of 100m.

The problem is balancing all of these parameters. To improve, say, attenuation, the diameter of the insulation material is increased. This, however, increases the impedance which adversely affects the reflection co-efficient. The increase adversely affects the crosstalk. The balance of these specifications is extremely fine and the slightest change in any of the parameters could cause any of the other parameters to fall outside the specified limits. It will be seen that the geometry and configuration of the cable assemblies 10 described herein overcome these problems.

It will be realised that the geometry and configuration of the cable assembly 10 according to the invention also fall within the mechanical requirements of weight, flexibility and outside diameter specified for the cable.