FR2830385A1 - METHOD AND DEVICE FOR DETECTING AND LOCATING A SYMMETRY FAULT ON A TELEPHONE LINE - Google Patents

Abstract

The invention relates to a method of detecting and locating a symmetry defect on a telephone line (2) comprising, at least, one pair of first (a) and second (b) wires. The inventive method is characterised in that it comprises at least the following steps: first and second common-mode signals are injected simultaneously on the first (a) and second (b) wires respectively; and, subsequently, the analogous difference (Va-Vb) between the first signal, which propagates along wire a, and the second signal, which propagates along wire b, is measured in real time. The invention also relates to the device used to carry out said method, said device comprising at least an impedance transformer (3) or a combination of two impedance transformers (5 and 6) or a differential operational amplifier (7).

Description

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 The present invention relates to a method and a device for detecting and locating a symmetry fault on a telephone line.

 By telephone line is meant cable with symmetrical pairs or two-wire lines.

 Currently, when a symmetry defect affects a telephone line, echometric measurements are then carried out on this line by an operator, either in situ, by means of an echometric measuring device, or remotely, by means of a system. centralized measurement. In the latter case, the operator has the option of physically switching the telephone line on a measuring robot installed in the centralized system.

 In a manner known as such, measurements of this type consist in injecting a signal at one end of the line and in an interpretation of the signal reflected or echoed at the other end of the line. Generally, these echometric measurements are carried out to refine the diagnosis of the telephone line, in addition to the conventional electrical measurements, such as in particular the measurements of alternating and direct voltages, the insulation measurements and the capacitive measurements.

 These echometric measurements are generally carried out in the following manner with reference to FIG. 1.

 This figure shows an echometer designated by the general reference numeral 1, which echometer is connected to the two wires a and b of a telephone line 2, respectively by their corresponding first ends e1, e'1.

 Initially, the echometer 1 injects at the end e1 of the wire a signal in common mode Va, for example a pulse. The injected signal is reflected at a second end e2 of the wire a, distant from the end e1, this reflection resulting in an echo. The signal Va then returns to the end e1.

 In a second step, the echometer 1 injects at the end e'1 of the wire

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 b a common mode signal Vb of the same type as the signal Va. The injected signal Vb is reflected at a second end e'2 of the wire b, distant from the end e'1, this reflection resulting in an echo. The signal Vb then returns to the end e'1.

 The level, the shape and the position of the echo on the wires a and b are then measured, then a calculation of the difference between the echo on the wire a and the echo on the wire b is carried out to allow establish information on the quality of telephone line 2 (type of fault, fault location).

- la mesure de la différence entre l'écho sur le fil a et l'écho sur le fil b est entachée d'une part, d'un premier type de bruit dit stationnaire , c'est à dire constitué principalement par des ondes radio correspondant aux fréquences d'émission d'appareils du type émetteurs hertziens de télévision, téléphones sans fil, émetteurs à ondes courtes, postes de radioamateurs, etc.., et, d'autre part, d'un second type de bruit dit impulsif , c'est à dire généré chez le client par des décharges électriques dues, par exemple, à l'allumage d'un tube néon ou d'un équipement électroménager ; - lorsque la différence entre les deux échos ci-dessus est faible par rapport à l'un des signaux de mode commun Va, Vb, il en résulte que la sensibilité des mesures échométriques n'est pas suffisamment élevée. En outre, la mesure d'une si faible différence nécessite l'utilisation de convertisseurs analogique/numérique d'une très grande précision et dont le coût est prohibitif pour déduire de chaque signal Va et Vb le bruit stationnaire et le bruit Such two-stage echometric measurements have the following drawbacks: - when one of the common mode signals, for example the signal Va, is injected onto the wire a, account must be taken of the common mode impedance of the wire b and the differential mode impedance between wires a and b. This results from the fact that the wires a and b of the line are twisted, which makes them interdependent; any synchronization fault in the injection of the common mode signal Va or Vb results in an error when the difference between the echo on wire a and the echo on wire b is calculated;

- the measurement of the difference between the echo on wire a and the echo on wire b is tainted on the one hand, by a first type of so-called stationary noise, that is to say constituted mainly by radio waves corresponding to the emission frequencies of apparatuses of the radio-television transmitters type, cordless telephones, short-wave transmitters, amateur radio sets, etc., and, on the other hand, of a second type of so-called impulsive noise, that is to say generated at the customer by electric shocks due, for example, to the lighting of a neon tube or of household electrical appliances; - When the difference between the two above echoes is small compared to one of the common mode signals Va, Vb, it follows that the sensitivity of the echometric measurements is not sufficiently high. In addition, the measurement of such a small difference requires the use of analog / digital converters of a very high precision and whose cost is prohibitive to deduce from each signal Va and Vb the stationary noise and the noise.

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 impulsive.

 The object of the present invention is in particular to remedy the abovementioned drawbacks by proposing a method for detecting and locating a symmetry defect on a telephone line comprising at least one pair of first and second wires, said method being characterized in that it comprises at least the following steps: - the first and second common mode signals are injected simultaneously on the first wire and on the second wire respectively, - the analog difference between the first signal which propagates along the line is measured in real time wire and the second signal that propagates along the second wire.

 Such an arrangement thus makes it possible to overcome stationary noise and impulsive noise which are systematically canceled during the measurement in real time of the analog difference.

 In addition, it offers the advantage of requiring the use of an analog / digital converter of much simpler design than that of the converters used in the prior art since: - the signal to be processed in this converter is a signal of difference and no longer two independent signals, - stationary noise and impulsive noise, systematically canceled during the measurement in real time, are no longer to be deduced by the converter.

 In a preferred embodiment of the method according to the invention, use is made of one and / or the other of the following arrangements: - the first common mode signal has an amplitude value equal to that of the second signal common mode in the absence of symmetry fault on the telephone line; - The first common mode signal has an amplitude value equal and opposite to that of the second common mode signal in the absence of symmetry defect on the telephone line; - based on knowledge of the length of the line

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 telephone and the speed of propagation of a common mode signal on said line, the location of a fault on a wire of this line consists in dividing the time of appearance of an echo on said wire by the speed of propagation of the common mode signal which propagates on said wire.

 The present invention also relates to a device for implementing the method according to the invention.

 According to a first embodiment, the device is characterized in that it comprises a 1/1 voltage ratio impedance transformer connected to the first and second wires of said telephone line so that when a common mode signal is injected at the midpoint of a winding of the transformer, said signal is divided into a first common mode signal which propagates on the first wire and a second common mode signal which propagates on the second wire, the analog difference between the first and second signals being obtained at one end of another winding of the transformer.

 According to a second embodiment, the device is characterized in that it comprises a first impedance transformer connected to the first wire and a second impedance transformer connected to the second wire, said first and second transformers each having a ratio of voltage 1 / k and being coupled together so that when a signal is injected at a first end of a winding of the first transformer, said signal splits into a first common mode signal which propagates along the first wire and a second common mode signal which propagates along the second wire, the analog difference between the first signal and the second signal being measured at a first end of a winding of the other transformer.

 According to a third embodiment, the device is characterized in that it comprises: - an impedance bridge connected to the telephone line and having an input intended to receive a common mode signal, said bridge being designed so as to deliver at the output a signal dividing into a first common mode signal which propagates along the first wire and a second mode signal

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 common which propagates along the second wire, - and a differential operational amplifier having a first input connected to the first wire and a second input connected to the second wire so as to output the analog difference between the first signal and the second signal.

 Other characteristics and advantages of the invention will appear during the following description with reference to the appended drawings in which: - Figure 1 is a block diagram illustrating how an echometer is used in the prior art; - Figure 2 is a block diagram of a first embodiment of the device according to the invention; - Figure 3 is a block diagram of a second embodiment of the device according to the invention; - Figure 4 is a block diagram of a third embodiment of the device according to the invention; FIG. 5 is a diagram representing the results obtained using the device and the detection and localization method according to the invention, the symmetry defects affecting a telephone line of a first length; - Figure 6 is a diagram showing the results obtained using the device and the detection and localization method according to the invention, the symmetry defects affecting a telephone line of a second length.

 Referring to FIG. 2, a first embodiment of a device for detecting and locating a symmetry fault on a telephone line is shown.

 For the sake of simplification, the reference numbers in FIG. 1 are shown in FIG. 2.

 More specifically, the injection of common mode signals is carried out by an echometer 1 which, in the example shown, is a signal generator.

 The telephone line, on which is intended to be detected and located

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 a defect in symmetry, is designated by the reference numeral 2. Line 2 comprises at least a pair of two wires, a first wire a and a second wire b, said wires a and b having respectively two first ends e1, e'1 adjacent to each other and two second ends e2, e'2 also adjacent to each other.

 In this figure is also shown a transformer 3 of Balun type comprising: - a secondary winding S3 which has first and second ends S3, S'3 connected respectively to the first ends e1, e'1 respective of the first and second wires a and b of the telephone line 2, - and a primary winding P3 which has a first end P3 connected to a device 4 for measuring the analog difference between the first signal Va and the second signal Vb and a second end P'3 connected to a reference potential, for example the earth.

 The transformer 3 also has a voltage ratio of 1/1, the primary windings P3 and secondary 83 being designed so as to respect this ratio. The primary winding P3 is also adapted to the impedance of the measuring device 4. The secondary winding S3 is, in turn, preferably adapted to the impedance of the pair of wires a and b.

 The echometer 1 has an output OUT which is connected to the midpoint M of the secondary winding S3 of the transformer 3 so that a common mode signal of value V injected by the latter splits into a first signal. common mode Va which propagates on the first wire a and a second common mode signal Vb which propagates on the second wire b, said signal Vb, taking into account the configuration of the assembly, being of the same value and of the same sign as the signal Va, in the absence of symmetry fault on line 2.

 In the example shown, the signals Va and Vb each have an amplitude and a duration which are adapted to the pair of wires a and b and to the nature of the fault to be detected and located.

 The assembly described above works as follows. At an instant to = 0, the echometer 1 injects a common mode signal on the midpoint

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 M of transformer 3. At this instant, the signal is divided: - into the first common mode signal Va which traverses a first half of the secondary winding S3 to reach an instant t1 at the end e1 of wire a, - and in the second common mode signal Vb which traverses a second half of the secondary winding S3 to reach the instant ti at the end e'1 of the wire b, assuming that the secondary winding S3 is ideal.

 The signal Va traverses the wire a and is reflected at an instant t2 at the end e2 of the wire a, which results in an echo received at an instant ts. The signal Va then returns at an instant t4 to the end e1 of the wire a.

 The signal Vb traverses the wire b and is reflected at an instant t'2 at the end e'2 of the wire b, which results in an echo received at an instant t'3. The signal Vb then returns at a time t'4 to the end e'1 of the wire b.

 Thanks to the configuration of the assembly, the measuring device 4 is able to measure in real time an analog difference between the common mode signal Va and the common mode signal Vb received at the end P3 of the primary winding P3, which difference is designated by Va-Vb.

 If the telephone line 2 has no defect, the analog difference Va-Vb is zero taking into account the conventions of the assembly.

 If, on the other hand, the telephone line 2 has a fault, for example on the wire a, the analog difference Va-Vb obtained at the end P3 of the winding P3 is not zero.

 It should be noted that this analog difference, prior to its processing in an analog / digital converter (not shown), is deduced at all times from stationary noise and impulsive noise affecting the telephone line 2, the stationary and impulsive noises respectively affecting the wires a and b advantageously canceling out during the analog difference Va-Vb.

 We will now describe with reference to FIG. 3 a second embodiment of a device for detecting and locating a symmetry fault on a telephone line.

 In the example shown in Figure 3, the assembly used is

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 differentiated mainly from that of Figure 2 by the fact that two impedance transformers are used instead of one. These two transformers are identical. They are designated respectively by the reference numerals 5 and 6.

 More specifically, the transformer 5 comprises two secondary windings 85 and S'5 and a primary winding P5, and the transformer 6 comprises, in a similar manner to the transformer 5, two secondary windings S6 and S'6 and a primary winding P6.

 The transformers 5 and 6 are coupled so that: the secondary winding 85 is mounted in series with the secondary winding S6 and the secondary winding S'5 is mounted in series with the secondary winding S'6, - the secondary winding S5 has a first end S5 connected to the first end e1 of the wire a and a second end s's connected to a first end Se of the secondary winding S6, - the secondary winding S'5 has a first end s "5 connected to earth and a second end S" '5 connected to a second end s' "e of the secondary winding S'e, - the secondary winding S6 has a second end retained to earth, - the secondary winding S'e has a first end s "e connected to a first end e'1 of the wire b, - the primary winding P5 has a first end p5 connected to the output OUT of the echometer 1 and a second end p'5 connected to earth, - the winding primary Pe has a first end pe connected to the measuring device 4 of the analog difference between the first signal Va and the second signal Vb and a second end P'6 connected to ground.

 Each transformer 5,6 also has a voltage ratio 1 / k.

 The primary winding Ps is preferably adapted to the impedance of the echometer 1. The primary winding Pe is, for its part, adapted to

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 preferably the impedance of the measuring device 4. The series connection of the secondary windings S5 and S6 is adapted to the impedance of the wire a, and, similarly, the series connection of the secondary windings S'5 and S'6 is adapted to the impedance of the wire b.

 In addition, each secondary winding S5, S6, S'5 and S'6 has the same number of turns.

 Thus, thanks to the configuration of such an arrangement, the common mode signal injected by the echometer 1 is divided into a first common mode signal Va which propagates on the first wire a and a second common mode signal Vb which propagates on the second wire b, said signal Vb being, in this example, of the same value as the signal Va, but of opposite sign, in the absence of symmetry defect on line 2.

 In the example shown, the signals Va and Vb each have an amplitude and a duration which are adapted to the pair of wires a and b and to the nature of the fault to be detected and located.

 The assembly described above works as follows. At an instant to = 0, the echometer 1 injects at the end p5 of the primary winding? 5 of transformer 5 a common mode signal of value kV. Taking into account that the transformer 5 has a voltage ratio of 1 / k, the potential difference at the ends S5, s'5 of the secondary winding S5, as well as at the ends s "5, s'" 5 of the secondary winding S'5 will be equal to V / 2.

 At an instant t1, the signal is divided: - into the first common mode signal Va injected from the end e1 of the wire a, - and into the second common mode signal Vb injected from the end e ' 1 of wire b.

 The signal Va traverses the wire a and is reflected at an instant t2 at the end e2 of the wire a, which results in an echo received at an instant ts. The signal Va then returns at an instant t4 to the end e1 of the wire a. Then the signal Va travels successively through the secondary windings S5 and Se to arrive at an instant t5 at the end s'e of the winding S6.

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 The signal Vb traverses the wire b and is reflected at an instant t'2 at the end e'2 of the wire b, which results in an echo received at an instant t'3. The signal Vb then returns at a time t'4 to the end e'1 of the wire b. Then the signal Vb successively traverses the secondary windings 6 and S'5 to arrive at an instant t'5 at the end S "5 of the winding S'5.

 Thanks to the configuration of the assembly, the measuring device 4 is able to measure in real time the analog difference Va-Vb received at the end pe of the primary winding Pc.

montage, la différence analogique Va-Vb obtenue à l'extrémité po de

l'enroulement primaire P6 a pour va ! eur k---= 0. If the telephone line 2 has no defect, the voltages recorded at the respective ends of each secondary winding S5, Se, S'5, S'6 all have the same value, ie V / 2. These voltages are shown in solid lines in Figure 3. Due to the sign convention adopted in this

mounting, the analog difference Va-Vb obtained at the po end of

the primary winding P6 goes! eur k --- = 0.

f-p & l l'extrémité pe de l'enroulement primaire Pea pour valeur k----0. 2
In the where, on the other hand, the telephone line 2 has a fault, for example on wire a, the voltages noted at the respective ends of each secondary winding S5 and S6 have the same positive value, that is Wa / 2, and the voltages noted at the respective ends of each secondary winding S'e, S'6 have the same negative value, ie-Wb / 2. These voltages are shown in broken lines in FIG. 3. Due to the sign convention adopted in this arrangement, the analog difference Va-Vb obtained at

fp & l the pe end of the primary winding Pea for value k ---- 0.

2 1
As in the embodiment of FIG. 2, this analog difference, prior to its processing in an analog / digital converter (not shown), is deduced at all times from stationary noise and from impulsive noise affecting the telephone line 2, the noises stationary and impulsive, affecting the wires a and b respectively, canceling each other out advantageously during the analog difference Va-Vb.

 We will now describe with reference to Figure 4 a third mode

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 production of a device for detecting and locating a symmetry fault on a telephone line.

 In the example shown in FIG. 4, the assembly used differs mainly from that of FIGS. 2 and 3 by the fact that an operational amplifier, designated by the reference numeral 7, is used in place of one or two impedance transformers.

 The amplifier 7 is of the differential type. It includes an inverting input 7a connected to the wire a, a non-inverting input 7b connected to the wire b, and an output 7c intended to deliver a differential signal which is none other than the analog difference Va-Vb. The output 7c of the amplifier 7 is connected to the measuring device 4.

 The amplifier 7 is adapted to be able to measure low differential voltages in the presence of high common mode voltages.

 The echometer 1 used in the assembly of FIG. 4 comprises an output OUT which is connected on the one hand to the first end e1 of the wire a via a first impedance 8, and, on the other hand, to the first end e'1 of the wire b via a second impedance 9.

 It is made so that the ohmic value of the impedance 8 is equal to that of the impedance 9.

 The impedances 8 and 9 are preferably adapted to the respective impedances of the wires a and b.

 Thus, thanks to the configuration of the assembly of FIG. 4, the common mode signal injected by the echometer 1 is divided into a first common mode signal Va which propagates on the first wire a and a second common mode signal Vb which propagates on the second wire b, said signal Vb being, in this example, of the same value as the signal Va, in the absence of symmetry defect on line 2, as in the case of FIG. 2.

 In the example shown, the signals Va and Vb each have an amplitude and a duration adapted to the pair of wires a and b and to the nature of the fault to be detected and located.

 The assembly described above works as follows. Has a

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 instant to = 0, the echometer 1 injects at output OUT a common mode signal of value V. At this instant the signal V divides: - into the first common mode signal Va which arrives at an instant t1 at the first end e1 of wire a, - and in the second common mode signal Vb which reaches instantaneous t'1 at the first end e'1 of wire b.

 The signal Va traverses the wire a and is reflected at an instant t2 at the end e2 of the wire a, which results in an echo received at an instant ts. The signal Va then returns at an instant t4 to the end e1 of the wire a.

 The signal Vb traverses the wire b and is reflected at an instant t'2 at the end e'2 of the wire b, which results in an echo received at an instant t'3. The signal Vb then returns at a time t'4 to the end e'1 of the wire b.

 Thanks to the configuration of the assembly, the measuring device 4 is able to measure in real time an analog difference Va-Vb obtained at output 7c of amplifier 7.

 If the telephone line 2 has no defect, the analog difference Va-Vb is zero taking into account the conventions of the assembly.

 If, on the other hand, telephone line 2 has a fault, for example on wire a, the analog difference Va-Vb is not zero.

 Also in this embodiment, the analog difference VaVb, prior to its processing in an analog / digital converter (not shown), is deduced at all times from stationary noise and impulsive noise affecting the telephone line 2, stationary and impulsive noises , respectively affecting the wires a and b, canceling each other out advantageously during the analog difference Va-Vb.

 We will now describe, with reference to FIGS. 5 and 6, respectively two examples of results of the detection and the localization of resistive and capacitive faults on the telephone line 2.

 In the example shown in Figure 5, the telephone line has a length of 1km and a diameter of 4/10 mm.

 In abscissa, are carried in them (microseconds) the instants

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 of resistive faults and, on the ordinate, are given in mV (millivolts) the amplitude of the analog voltage Va-Vb.

 The experiment was carried out with the following four faults: a resistive fault (in bold line) of 100 Ohms at 1000 meters from the ends e1, e'1 of the pair of wires a and b, - a resistive fault (in bold line) dotted line) from 100 Ohms to 500 meters from the ends e1, e'1 of the pair of wires a and b, - a capacitive fault (in thin line) of 3. 2 uF of common mode at 1000 meters from the ends e1, e ' 1 of the pair of wires a and b, - a capacitive fault (in dotted thin lines) of 3.2 jjF of common mode at 500 meters from the ends e1, e'1 of the pair of wires a and b.

 In the example shown in Figure 6, the telephone line 2 has a length of 3 km and a diameter of 4/10 mm.

 On the abscissa, are plotted in; j. s (microseconds) the instants of appearance of resistive faults and, on the ordinate, are plotted in mV (millivolts) the amplitude of the analog voltage Va-Vb.

- un défaut résistif (en trait gras pointillé) de 100 Ohms à 1500 mètres des extrémités e1, e'1 de la paire de fils a et b, - un défaut capacitif (en trait fin) de 9. 6 jjF de mode commun à 3000 mètres des extrémités e1, e'1 de la paire de fils a et b, - un défaut capacitif (en trait fin pointillé) de 9.6 F de mode commun à 1500 mètres des extrémités e1, e'1 de la paire de fils a et b. As in the example in Figure 5, the experiment was carried out with four faults which are respectively: - a resistive fault (in bold line) of 100 Ohms at 3000 meters from the ends e1, e'1 of the pair of wires a and b,

- a resistive fault (in bold dotted line) of 100 Ohms at 1500 meters from the ends e1, e'1 of the pair of wires a and b, - a capacitive fault (in thin line) of 9. 6 ddF of common mode to 3000 meters from the ends e1, e'1 of the pair of wires a and b, - a capacitive fault (in dotted thin lines) of 9.6 F in common mode at 1500 meters from the ends e1, e'1 of the pair of wires a and B.

 The method for locating such resistive or capacitive faults is known as such. It is sufficient for a person skilled in the art to know: - the length of the telephone line, ie 1 km and 3 km, respectively in the examples shown in FIGS. 5 and 6.

 - and the propagation speed of the common mode signal V injected by

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 the echometer 1, that is, in the examples shown in the figures, of the order of 10I-Ls / km.

 the nature of the fault by electrical measurements known to those skilled in the art, such as, on the one hand, the measurement of the insulation resistance of each of the wires a and b relative to the ground, and, on the other hand, the measurement of the common mode capacity of each of the wires a and b.

 As can be seen in Figures 5 and 6, the impact of each resistive or capacitive fault results in an echo whose position at a time t in the time domain makes it possible to locate the fault by dividing this time by the speed of propagation of the common mode signal V.

 It should be noted that a resistive defect results in an echo only positive and that a capacitive defect results in an echo either positive or negative.

Claims (7)

1. Method for detecting and locating a symmetry defect on a telephone line (2) comprising at least a pair of first (a) and second (b) wires, said method being characterized in that it comprises at least the following steps: - the first and second common mode signals are simultaneously injected respectively on the first wire (a) and on the second wire (b), - the analog difference (Va-Vb) between the first is measured in real time signal that propagates along the wire (a) and the second signal that propagates along the second wire (b).  2. Method according to claim 1, in which the first common mode signal has an amplitude value equal to that of the second common mode signal in the absence of symmetry defect on the telephone line (2).  3. Method according to claim 1, wherein the first common mode signal has an amplitude value equal to and opposite to that of the second common mode signal in the absence of symmetry defect on the telephone line (2).  4. Method according to any one of claims 1 to 3, wherein on the basis of the knowledge of the length of the telephone line (2) and the speed of propagation of a common mode signal on said line, the locating a fault on a wire (a or b) of this line consists in dividing the time of appearance of an echo on said wire by the speed of propagation of the common mode signal which propagates on said wire.  5. Device for implementing the method according to claims 1 and 2, characterized in that it comprises an impedance transformer (3) of voltage ratio 1/1 connected to the first (a) and second (b) wires of said telephone line in such a way that when a common mode signal (V) is injected on the midpoint (M) of a winding (sus) of the transformer (3), said signal is divided into a first signal common mode <Desc / Clms Page number 16>  which propagates on the first wire (a) and a second common mode signal which propagates on the second wire (b), the analog difference (Va-Vb) between the first and the second signal being obtained at one end (P3 ) another winding (P3) of the transformer.  6. Device for implementing the method according to claims 1 and 3, characterized in that it comprises a first impedance transformer (5) connected to the first wire (a) and a second impedance transformer (6) connected to the second wire (b), said first and second transformers each having a voltage ratio 1 / k and being coupled together so that when a signal (kV) is injected at a first end (p5) of a winding (P5) of the first transformer (5), said signal is divided into a first common mode signal (V / 2) which propagates along the first wire (a) and a second common mode signal (V / 2) which propagates along the second wire (b), the analog difference (Va-Vb) between the first signal and the second signal being obtained at a first end (stink) of a winding (stink) of the other transformer ( 6).  7. Device for implementing the method according to claim 1, characterized in that it comprises: - an impedance bridge (8,9) connected to the telephone line (2) and having an input intended to receive a common mode signal (V), said bridge being designed to output a first common mode signal which propagates along the first wire (a) and a second common mode signal which propagates along the second wire (b), - and a differential operational amplifier (7) having a first input (7a) connected to the first wire (a) and a second input (7b) connected to the second wire (b) so as to output the analog difference (Va-Vb) between the first common mode signal and the second common mode signal.