Communications – electrical: acoustic wave systems and devices – Distance or direction finding – By combining or comparing signals
Reexamination Certificate
1999-03-22
2001-08-21
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Distance or direction finding
By combining or comparing signals
C367S118000, C379S008000
Reexamination Certificate
active
06278655
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fault location toning method for submarine networks and in particular, a method of increasing the propagation distance of a low frequency tone.
BACKGROUND TO THE INVENTION
It is known to use a low frequency tone superimposed on the normal working current at the power feed of a submarine cable system to enable a cable ship to locate the cable for inspection or repair using a submerged narrow band detector. It is normal to use a sine wave of typically between 4 and 25 Hz having a peak to peak amplitude of around 400 mA. The frequency response of the submarine cable is such that the toning signal is attenuated with distance and therefore not normally detectable beyond 1000 kilometres. The effect of attenuation is illustrated in
FIGS. 7A
to
7
D for a sine wave toning signal which propagates through a series of four optical repeaters at a distance of 40 km (FIG.
7
B), 80 km (
FIG. 7C
) and 160 km (
FIG. 7D
) from the powerfeed, respectively. As shown, after the fourth repeater the peak to peak amplitude of the toning signal is reduced to only around 3 mA.
The maximum amplitude of a toning signal generated by the power feed equipment is limited by the electrical characteristics of amplifiers within optical repeaters provided at intervals along the length of the submarine cable, which are sensitive to the negative half cycle of the toning signal. In future, repeaters will incorporate state of the art amplifiers which operate at much lower currents so that the expected maximum toning amplitude will be around 200 mA peak to peak. A toning signal at this amplitude would not be expected to propagate any great distance and still be detectable. Since ship time is expensive any decrease in toning distance could be costly.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, a method of locating a submarine cable comprises the step of transmitting a toning signal along the submarine cable, wherein the toning signal comprises a first signal waveform which is to be detected combined with at least a second signal waveform, wherein the second signal waveform interferes with the first signal waveform to limit the peak to peak amplitude of the toning signal for transmission.
Preferably, each of the first signal waveform and second signal waveform is sinusoidal. More preferably, the second signal waveform is of a higher frequency than the first signal waveform.
Preferably, the toning signal is a square wave, wherein the first signal waveform corresponds to the fundamental frequency and the second and any subsequent signal waveforms corresponds to a respective one of the harmonic frequencies.
Preferably, a number of the harmonics of the square wave are pre-emphasised by a predetermined amount to take account of the frequency response of the submarine cable.
According to a second aspect of the present invention, there is provided the use of a square wave as a toning signal in a submarine communications system.
Preferably, a number of the harmonics of the square wave are pre-emphasised by a predetermined amount to take account of the frequency response of a submarine cable.
According to a third aspect of the present invention, a power feed circuit for a submarine cable comprises a toning signal control circuit configured to modulate an electrical output of the power feed circuit to provide a toning signal, the toning signal comprising a first signal waveform which is to be detected combined with at least a second signal waveform, wherein the second signal waveform interferes with the first signal waveform to limit the peak to peak amplitude of the toning signal for transmission.
Preferably, the toning signal control circuit comprises a digital signal processor which is arranged to cycle through a look-up table stored in a memory and output a series of sample values which are used to control the electrical output of the power feed circuit.
Preferably, the contents of the look-up table stored in memory define a complete cycle of a square wave. More preferably, the sample values in the look-up table are effective to provide a degree of pre-emphasis to the harmonics of the square wave toning signal.
The present invention provides a way of offsetting the loss of transmission distance due to the use of state of the art optical repeaters. It is proposed to optimise the shape of the toning signal so that more current can be launched into the submarine cable without damaging the repeaters and thereby extend significantly the useful toning distance. If a square wave is used, the amplitude of the fundamental is around 20% higher than the peak of the total wave. A further improvement can be obtained by utilising the fact that the higher frequency components of the square wave toning signal will be attenuated more strongly by the submarine cable than those at a lower frequency. In particular, the toning signal wave shape is changed significantly between the power feed and first repeater and this can be compensated for by increasing the amplitude of the higher harmonics so that by the time it gets to the repeater it has a maximum acceptable amplitude square wave.
REFERENCES:
patent: 4820991 (1989-04-01), Clark
patent: 5754053 (1998-05-01), Bourdeau
patent: 5977773 (1999-11-01), Medelius et al.
patent: 5995588 (1999-11-01), Crick
Lilly et al., “Submarine Cable Systems—Recent Developments”, Electronics & Communication Engineering Journal, pp. 88-96, Apr. 1996, vol. 8 Issue: 2.
Lancaster Philip C
Webb Stephen M
Alcatel
Lobo Ian J.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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