Title of Invention	A PROCESS FOR COMPENSATING A MAGNETIC FIELD IN A COUPLING UNIT
Abstract	The invention relates to a device and method for low-frequency magnetic field compensation in an inductive signal coupling unit. According to the invention, the coupling unit comprises a ferromagnetic core (30) which surrounds a conductor through which a low-frequency current (1) flows, in order to inject a high-frequency signal therein. The invention is characterised in that it consists in detecting the current (1) flowing through the conductor or the magnetic field (3) generated in the coupling unit, producing a supply current from the value obtained during the aforementioned detection process and injecting the supply current produced in the coupling unit via a low pass filter (9), in order to generate a magnetic field (4) that is equal, but in the opposite direction to that produced by the current (1) from the conductor in the above-mentioned coupling unit, such as to prevent saturation of the magnetic core (30) without adding insertion losses. In general, the invention can be used for communication systems that require the injection of a radiofrequency signal in a conductor and, in particular, in systems using the electrical network as a transmission means.

Title of Invention

A PROCESS FOR COMPENSATING A MAGNETIC FIELD IN A COUPLING UNIT

Abstract

The invention relates to a device and method for low-frequency magnetic field compensation in an inductive signal coupling unit. According to the invention, the coupling unit comprises a ferromagnetic core (30) which surrounds a conductor through which a low-frequency current (1) flows, in order to inject a high-frequency signal therein. The invention is characterised in that it consists in detecting the current (1) flowing through the conductor or the magnetic field (3) generated in the coupling unit, producing a supply current from the value obtained during the aforementioned detection process and injecting the supply current produced in the coupling unit via a low pass filter (9), in order to generate a magnetic field (4) that is equal, but in the opposite direction to that produced by the current (1) from the conductor in the above-mentioned coupling unit, such as to prevent saturation of the magnetic core (30) without adding insertion losses. In general, the invention can be used for communication systems that require the injection of a radiofrequency signal in a conductor and, in particular, in systems using the electrical network as a transmission means.

Full Text	OBJECT OF THE INVENTION The present invention, as stated in the heading to this descriptive specification, relates to a process and a device that allow to compensate the low frequency- magnetic field which is produced in an inductive coupling unit. The invention is preferably applicable for injecting high frequency signals on the electricity grid by means of an inductive coupling unit, and its object is to minimize the insertion loss of a high frequency signal on the electricity grid, to which end it avoids the effect of magnetic saturation which occurs in the inductive coupling unit. BACKGROUND OF THE INVENTION In telecommunications systems, once the signal has been produced in the transmitter it is necessary to inject it into the transmission medium, in order that it reachs the receiver. The way of carrying out the injection depends mainly on the transmission medium employed. In the case of injecting signals onto the electricity grid, this can be carried out with inductive coupling units which are usually constituted by current transformers which are placed around the power conductors in which it is desired to induce the signal. In these cases the operation is based on the principle that currents which vary in time generate magnetic fields which vary in time and viceversa. In other words, the radiofrequency (RF) current it is desired to inject, produces a variable magnetic field (in time, and therefore a magnetic flux also variable in time) inside the ferromagnetic core of the coupling unit and, besides, the variation of magnetic flux around a conductor, induces therein a current proportional to this variation. Based on this principle, it is possible to induce signals between conductors enclosed by the same magnetic core. Because there are high electric currents circulating through the power conductors, and therefore, the magnetic field strength around the cables is very high, the problem arises that if a ferromagnetic core is placed around the cables, the magnetic core can become saturated and unusable for carrying out the function of signal injection. In the state of the art it is known that this effect can be overcome by including an air gap in the magnetic circuit so that the reluctance of the magnetic circuit is considerably increased and the magnetic flux is decreased dramatically. This implies an increase in the insertion loss of the signal, which is not desirable in communication applications, wherein the loss signifies an effective reduction in the range of the communications system. To counteract this effect the usual practice is to lengthen the coupling unit in order to try to cover more flux, but this makes the coupling unit more bulky and more expensive, which is not desirable; ,and all this while seeking a compromise solution between size, air gap, and insertion loss. The present invention allows the inductive coupling to be carried out without saturation of the ferromagnetic core taking place and without additional insertion loss occurring. DESCRIPTION OF THE INVENTION To avoid the drawbacks indicated in the foregoing paragraphs, the invention comprises a process for compensating the low frequency magnetic field in an inductive signal coupling unit, wherein the coupling unit comprises a magnetic core which surrounds a conductor through which a low frequency current is circulating in order to inject a high frequency signal therein. The process of the invention is characterised in that it foresees a selective detection of the current which is circulating through the conductor or of the magnetic field produced in the coupling unit, in order to obtain thereafter a compensating current based on the value obtained in the detection performed, said injection current being produced with an external source, by the actual equipment which wishes to transmit the high frequency signal over the grid or automatically by induction on a winding around the coupler. Subsequently the injection of the compensating current obtained in the coupling unit is produced through a low pass filter which offers a high impedance to the high frequency signal it is desired to inject. The action of injecting the compensating current results in that, in the inductive coupling unit, a magnetic field is produced equal and opposite to that produced by the current which is circulating through the conductor, whereby saturation is avoided in the magnetic core of the inductive coupling unit without adding insertion loss for the desired signal it is to transmit, and maximum efficiency is obtained. The invention has been specially conceived to send high frequency signals over the electricity grid in which a low frequency current is circulating. In one embodiment of the invention the detection of the magnetic field is carried out in the ferromagnetic core of the inductive signal coupling unit. The magnetic field detected is compared with a reference signal in order to obtain the compensating current, so that the signal of the measure of the magnetic field is maintained practically equal to the reference signal, wherein said reference signal lies between 0 and a value such that the coupling unit is prevented from becoming saturated, that is, the signal inside the ferromagnetic core of the coupling unit never surpasses the maximum flux density it is capable of withstanding. In another embodiment of the invention, the detection is achieved by measuring the current which is circulating in the electricity grid. The injection of the compensating current obtained, is carried out by means of a compensation winding which is applied on the ferromagnetic core of the inductive signal coupling unit through a low pass filter which offers a high impedance for the high frequency signals it is desired to inject. In an embodiment of the invention the measurement of the current which is circulating through the conductor is carried out by means of a winding arranged on a ferromagnetic core which is independent of the inductive signal coupling unit. The invention also foresees that the detection, the obtaining of the compensating current, and the injection of said current are carried out simultaneously by locating a winding with a single winding around a double core which forms the ferromagnetic core of the coupling unit. The invention also refers to a low frequency magnetic field compensating device in an inductive signal coupling unit which works according to the aforementioned process. To this end the device of the invention comprises a detector which is selectively constituted by a current detector which is circulating through the conductor or by a detector of the magnetic field produced in the coupling unit. Moreover the device of the invention comprises a control module by means of which a compensating current is calculated and produced, based on the value obtained in the detector. The current produced by the control module is injected in the conductor by means of a compensation winding arranged on the ferromagnetic core of the coupling unit, and through a filter offering high impedance to high frequencies which prevents the high frequency signal to be injected from being affected by the injection of the compensating current. The injection of the compensating current produces a magnetic field in the ferromagnetic core, equal and opposite to that produced by the current which is circulating through the conductor, so that saturation is avoided of the magnetic core of the inductive coupling unit, whereby insertion loss is not added and the maximum efficiency is obtained in the transmission of the signals over the electricity grid. In an embodiment of the invention the detector is constituted by means of a device for measuring the magnetic field inside the ferromagnetic core of the coupling unit. The signal provided by the detector is compared with a reference signal, which preferably is zero, and is applied to the control module so that the latter generates the compensating current to obtain a signal from the detector equal to the reference signal. The injection of the current produced by the control module is carried out through a low pass filter and by means of a compensation winding which is arranged around the ferromagnetic core of the inductive signal coupling unit. The device of the invention also foresees that it comprises a single winding around a double core which constitutes the ferromagnetic core of the coupling unit in order to carry out simultaneously the detection, the obtaining of the compensating current and the injection of said current. Next, to assist in a better understanding of this descriptive specification and being an integral part thereof, some figures are appended wherein by way of illustration and not restrictively, the object of the invention has been represented. BRIEF DESCRIPTION OF THE FIGURES Figure 1 illustrates a schematic form of an inductive coupler of the state of the art in which the problem of saturation of the ferrite core arises. Figure 2 illustrates a circuit diagram of a device carrying out magnetic field detection in order to inject the compensating current in accordance with an embodiment of the present invention. Figure 3 illustrates a perspective view of the device in which the current is measured in the conductor and the compensating current is automatically injected in accordance with an embodiment of the present invention. Figure 4 illustrates a perspective view of the device in which the current is measured in the cable and the compensating current is injected automatically in accordance with another embodiment of the present invention. Figure 5 illustrates a perspective view of the device, as in the case of previous figure, in which the current is measured in the conductor and the compensating current is automatically injected in accordance with another embodiment of the present invention. Figure 6 illustrates a perspective view of the device, in which the detection, the obtaining of the compensating current and the injection of said current are carried out simultaneously by means of a single winding in accordance with another embodiment of the present invention. DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION A description is made below of several embodiments of the invention, making reference to the numbering adopted in the figures. The embodiments of the invention refer to the injection of a specific signal onto a conductor of the electricity grid, over which a low frequency current 1 is circulating, and onto which conductor a high frequency signal is desired to be injected. Based on figure 1 a generic description is provided of the problem presented in the state of the art. The employ of inductive couplers is known for injecting a determined signal onto a conductor through which a current 1 is circulating. To this end the inductive coupler is constituted by a ferromagnetic core 3 0 arranged around the conductor onto which one desires to inject the signal, and in which is included a winding 31 to which a current 2 is applied that produces a magnetic field 4 in the ferromagnetic core 30, which induces a current in the conductor. This device has the drawback that the current 1 which is circulating through the conductor also produces a magnetic field 3 in the ferromagnetic core 30, so that if said magnetic field 3 is high, saturation of the ferromagnetic core 30 occurs, whereby the current 2 is unable to increase the value of the magnetic field and hence no current is induced in the conductor, and therefore the desired signal is not injected. To overcome this drawback, an embodiment is shown in figure 2 wherein a Hall-effect sensor 5 is mounted on the ferromagnetic core 30, which sensor requires an external power supply in order to be able to work (not shown in the figure). In this case the current 1 which is circulating through the conductor likewise produces a magnetic field 3 in the ferromagnetic core 30, which is detected by the sensor 5, the signal obtained being applied to a low pass filter 6 which smoothes the obtained signal. The detected and filtered signal has been referred to with the number 20, which signal is applied to a comparator 32 which in turn receives a reference signal 21, and which moreover is connected to a control module 7, which calculates and generates a compensating current to maintain the signal 2 0 equal to the value of the reference signal 21. The compensating current is applied through a power stage 8 and a radiofrequency choke 9 to a winding 33 arranged on the ferromagnetic core 30, producing in the latter a magnetic field 4 equal and opposite to the field 3 produced by the current 1, so that a compensation of the magnetic field 3 occurs, so that the ferromagnetic core 3 0 is not saturated, whereby the current 2 is induced in the conductor without additional insertion loss. By means of the radiofrequency chokes 9 low pass filters are obtained which impede the passage of the high frequencies produced by the signal to be injected. In another embodiment of the invention, as shown in figure 3, the current 1 which circulates through the conductor is obtained by means of an external ferromagnetic core 11 on which a coil 34 has been arranged wherein, from the magnetic field produce by current 1 in the ferromagnetic core 11, said current 1 is produced in the coil 34, this current 1 being applied, through the radiofrequency choke 9 to the winding 33, the compensation of the magnetic field being produced in the manner described for the example in the previous figure. Lastly in figures 4 and 5 other examples are shown for compensation of the magnetic field 3 produced by current 1. Thus, in the example of figure 4 the employ is foreseen of two inductive couplers constituted by a double ferromagnetic core 16 and 17 that form the ferromagnetic core 30. The ferromagnetic core 16 is a power transformer, not suitable for radiofrequency, that is, it has a very high magnetic permeability at low frequencies and very low at high frequencies; whilst the ferromagnetic core 17 is suitable for radiofrequency; that is, it has a very low magnetic permeability at low frequency and very high at high frequency. In this case the detection and obtaining of the compensating current is carried out simultaneously by locating a winding around the double ring of the coupler. The magnetic induction, produced by the current which is passing through the cable, is detected by the winding and produces therein the current which later serves to perform the compensation. To this end, n coils are mounted which act as radiofrequency chokes 18 and which enclose both ferromagnetic cores 16 and 17, a smaller nominal current being achieved in each winding 19, so that said current carries out the desired compensation of the magnetic field induced by the current 1 which is circulating through the conductor. In this way the magnetic field 3 is only induced in the ferromagnetic core 16 and the magnetic field 4 in the ferromagnetic core 17 preventing the aforesaid saturation from taking place. In figure 5 an alternative embodiment is shown in which n windings are mounted short-circuited by means of a radiofrequency choke 15. This case is equivalent to the previous one, but it has the advantage of reducing the number of choke coils necessary to carry out the compensation. In figure 6 another alternative embodiment is shown of a device which implements the process of the invention. In this device a single winding 35 is used around the double ferromagnetic core 16 and 17 that forms the ferromagnetic core 30 of the coupling unit. This case is similar to that presented in figure 5, but with the advantage of reducing the number of windings and without having to use choke coils for the compensation. By means of said winding 3 5 the detection and obtaining of the compensating current are carried out simultaneously. Moreover, and because the winding has a high impedance at low frequencies and has a low impedance at high frequencies, the injection of the compensating current is carried out simultaneously by mounting said winding 35, whereby a low cost solution is obtained. We claim: 1. A process for compensating a magnetic field (3) in a coupling unit, the coupling unit is arranged around a conductor, current (1) in the conductor produces the magnetic field (3), and the process is characterized in that the process comprises: detecting the magnetic field (3) in a ferromagnetic core (16 and 17) of the coupling unit to obtain a signal; generating a compensating current based on the signal; and injecting the compensating current through a low pass filter (9 or 18) and into at least one winding (19 or 35) on the ferromagnetic core (16 and 17) to generate a magnetic field (4) and compensate the magnetic field (3), wherein the low pass filter (9 or 18) is in series with the at least one winding (19 or 35) on the ferromagnetic core (16 and 17), and wherein the at least one winding (19 or 35) detects the magnetic field (3). 2. The process as claimed in claim 1, wherein: the low pass filter (9) provides impedance to a high frequency signal produced by the magnetic field (4); and the magnetic field (4) is equal to and opposite the magnetic field (3). 3. The process as claimed in claim 1, further comprising a signal winding (2) on the ferromagnetic core (16 and 17), wherein the signal winding (2) injects a signal in the ferromagnetic core (16 and 17), and wherein the signal injected in the ferromagnetic core (16 and 17) induces a current in the conductor. 4. The process as claimed in claim 1, further comprising filtering the signal obtained from the detecting of the magnetic field (3) to generate a filtered signal (20). 5. The process as claimed in claim 4, further comprising generating the compensating current to maintain the filtered signal (20) equal to a value of a reference signal (21). 6. The process as claimed in claim 5, further comprising selecting the reference signal (21) to prevent flux density of the ferromagnetic core (16 and 17) from surpassing a maximum flux density of the ferromagnetic core (16 and 17). 7. The process as claimed in claim 1, wherein the ferromagnetic core (16 and 17) is a double magnetic core and comprises a ferromagnetic core (16) and a ferromagnetic core (17). 8. The process as claimed in claim 7, wherein: the ferromagnetic core (16) is a power transformer with a magnetic permeability that is high at low frequencies and low at high frequencies; and the ferromagnetic core (17) has a magnetic permeability that is low at high frequencies and high at low frequencies. 9. The process as claimed in claim 1, wherein the injecting of the compensating current is performed simultaneously with the detecting of the magnetic field (3). 10. The process as claimed in claim 1, wherein: the at least one winding (19) comprises n coils mounted on the ferromagnetic core (16 and 17), where n is an integer greater than 1; and the n coils perform as radio frequency chokes (18). 11. The process as claimed in claim 1, wherein: the at least one winding comprises only a single winding (35); and the ferromagnetic core (16 and 17) is a double magnetic core and comprises a ferromagnetic core 16 and a ferromagnetic core 17. J ABSTRACT "A PROCESS FOR COMPENSATING A MAGNETIC FIELD IN A COUPLING UNIT" The invention relates to a device and method for low-frequency magnetic field compensation in an inductive signal coupling unit. According to the invention, the coupling unit comprises a ferromagnetic core (30) which surrounds a conductor through which a low-frequency current (1) flows, in order to inject a high-frequency signal therein. The invention is characterised in that it consists in detecting the current (1) flowing through the conductor or the magnetic field (3) generated in the coupling unit, producing a supply current from the value obtained during the aforementioned detection process and injecting the supply current produced in the coupling unit via a low pass filter (9), in order to generate a magnetic field (4) that is equal, but in the opposite direction to that produced by the current (1) from the conductor in the above-mentioned coupling unit, such as to prevent saturation of the magnetic core (30) without adding insertion losses. In general, the invention can be used for communication systems that require the injection of a radiofrequency signal in a conductor and, in particular, in systems using the electrical network as a transmission means.

Full Text

OBJECT OF THE INVENTION
The present invention, as stated in the heading to this
descriptive specification, relates to a process and a
device that allow to compensate the low frequency-
magnetic field which is produced in an inductive coupling
unit.
The invention is preferably applicable for injecting high
frequency signals on the electricity grid by means of an
inductive coupling unit, and its object is to minimize
the insertion loss of a high frequency signal on the
electricity grid, to which end it avoids the effect of
magnetic saturation which occurs in the inductive
coupling unit.
BACKGROUND OF THE INVENTION
In telecommunications systems, once the signal has been
produced in the transmitter it is necessary to inject it
into the transmission medium, in order that it reachs the
receiver.
The way of carrying out the injection depends mainly on
the transmission medium employed.
In the case of injecting signals onto the electricity
grid, this can be carried out with inductive coupling
units which are usually constituted by current
transformers which are placed around the power conductors
in which it is desired to induce the signal. In these
cases the operation is based on the principle that
currents which vary in time generate magnetic fields
which vary in time and viceversa. In other words, the
radiofrequency (RF) current it is desired to inject,
produces a variable magnetic field (in time, and

therefore a magnetic flux also variable in time) inside
the ferromagnetic core of the coupling unit and, besides,
the variation of magnetic flux around a conductor,
induces therein a current proportional to this variation.
Based on this principle, it is possible to induce signals
between conductors enclosed by the same magnetic core.
Because there are high electric currents circulating
through the power conductors, and therefore, the magnetic
field strength around the cables is very high, the
problem arises that if a ferromagnetic core is placed
around the cables, the magnetic core can become saturated
and unusable for carrying out the function of signal
injection.
In the state of the art it is known that this effect can
be overcome by including an air gap in the magnetic
circuit so that the reluctance of the magnetic circuit is
considerably increased and the magnetic flux is decreased
dramatically. This implies an increase in the insertion
loss of the signal, which is not desirable in
communication applications, wherein the loss signifies an
effective reduction in the range of the communications
system.
To counteract this effect the usual practice is to
lengthen the coupling unit in order to try to cover more
flux, but this makes the coupling unit more bulky and
more expensive, which is not desirable; ,and all this
while seeking a compromise solution between size, air
gap, and insertion loss.
The present invention allows the inductive coupling to be
carried out without saturation of the ferromagnetic core
taking place and without additional insertion loss
occurring.
DESCRIPTION OF THE INVENTION
To avoid the drawbacks indicated in the foregoing

paragraphs, the invention comprises a process for
compensating the low frequency magnetic field in an
inductive signal coupling unit, wherein the coupling unit
comprises a magnetic core which surrounds a conductor
through which a low frequency current is circulating in
order to inject a high frequency signal therein.
The process of the invention is characterised in that it
foresees a selective detection of the current which is
circulating through the conductor or of the magnetic
field produced in the coupling unit, in order to obtain
thereafter a compensating current based on the value
obtained in the detection performed, said injection
current being produced with an external source, by the
actual equipment which wishes to transmit the high
frequency signal over the grid or automatically by
induction on a winding around the coupler. Subsequently
the injection of the compensating current obtained in the
coupling unit is produced through a low pass filter which
offers a high impedance to the high frequency signal it
is desired to inject.
The action of injecting the compensating current results
in that, in the inductive coupling unit, a magnetic field
is produced equal and opposite to that produced by the
current which is circulating through the conductor,
whereby saturation is avoided in the magnetic core of the
inductive coupling unit without adding insertion loss for
the desired signal it is to transmit, and maximum
efficiency is obtained.
The invention has been specially conceived to send high
frequency signals over the electricity grid in which a
low frequency current is circulating.
In one embodiment of the invention the detection of the
magnetic field is carried out in the ferromagnetic core
of the inductive signal coupling unit. The magnetic field

detected is compared with a reference signal in order to
obtain the compensating current, so that the signal of
the measure of the magnetic field is maintained
practically equal to the reference signal, wherein said
reference signal lies between 0 and a value such that the
coupling unit is prevented from becoming saturated, that
is, the signal inside the ferromagnetic core of the
coupling unit never surpasses the maximum flux density it
is capable of withstanding.
In another embodiment of the invention, the detection is
achieved by measuring the current which is circulating in
the electricity grid.
The injection of the compensating current obtained, is
carried out by means of a compensation winding which is
applied on the ferromagnetic core of the inductive signal
coupling unit through a low pass filter which offers a
high impedance for the high frequency signals it is
desired to inject.
In an embodiment of the invention the measurement of the
current which is circulating through the conductor is
carried out by means of a winding arranged on a
ferromagnetic core which is independent of the inductive
signal coupling unit.
The invention also foresees that the detection, the
obtaining of the compensating current, and the injection
of said current are carried out simultaneously by
locating a winding with a single winding around a double
core which forms the ferromagnetic core of the coupling
unit.
The invention also refers to a low frequency magnetic
field compensating device in an inductive signal coupling
unit which works according to the aforementioned process.
To this end the device of the invention comprises a
detector which is selectively constituted by a current

detector which is circulating through the conductor or by
a detector of the magnetic field produced in the coupling
unit.
Moreover the device of the invention comprises a control
module by means of which a compensating current is
calculated and produced, based on the value obtained in
the detector.
The current produced by the control module is injected in
the conductor by means of a compensation winding arranged
on the ferromagnetic core of the coupling unit, and
through a filter offering high impedance to high
frequencies which prevents the high frequency signal to
be injected from being affected by the injection of the
compensating current.
The injection of the compensating current produces a
magnetic field in the ferromagnetic core, equal and
opposite to that produced by the current which is
circulating through the conductor, so that saturation is
avoided of the magnetic core of the inductive coupling
unit, whereby insertion loss is not added and the maximum
efficiency is obtained in the transmission of the signals
over the electricity grid.
In an embodiment of the invention the detector is
constituted by means of a device for measuring the
magnetic field inside the ferromagnetic core of the
coupling unit. The signal provided by the detector is
compared with a reference signal, which preferably is
zero, and is applied to the control module so that the
latter generates the compensating current to obtain a
signal from the detector equal to the reference signal.
The injection of the current produced by the control
module is carried out through a low pass filter and by
means of a compensation winding which is arranged around
the ferromagnetic core of the inductive signal coupling

unit.
The device of the invention also foresees that it comprises a single winding around a
double core which constitutes the ferromagnetic core of the coupling unit in order to
carry out simultaneously the detection, the obtaining of the compensating current and the
injection of said current.
Next, to assist in a better understanding of this descriptive specification and being an
integral part thereof, some figures are appended wherein by way of illustration and not
restrictively, the object of the invention has been represented.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 illustrates a schematic form of an inductive coupler of the state of the art in
which the problem of saturation of the ferrite core arises.
Figure 2 illustrates a circuit diagram of a device carrying out magnetic field detection in
order to inject the compensating current in accordance with an embodiment of the present
invention.
Figure 3 illustrates a perspective view of the device in which the current is measured in
the conductor and the compensating current is automatically injected in accordance with
an embodiment of the present invention.
Figure 4 illustrates a perspective view of the device in which the current is measured in
the cable and the compensating current is injected automatically in accordance with
another embodiment of the present invention.
Figure 5 illustrates a perspective view of the device, as in the case of previous figure, in
which the current is measured in the conductor and the compensating current is
automatically injected in accordance with another embodiment of the present invention.
Figure 6 illustrates a perspective view of the device, in which the detection, the obtaining
of the compensating current and the injection of said current are carried out
simultaneously by means of a single winding in accordance with another embodiment of
the present invention.

DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION
A description is made below of several embodiments of the
invention, making reference to the numbering adopted in
the figures.
The embodiments of the invention refer to the injection
of a specific signal onto a conductor of the electricity
grid, over which a low frequency current 1 is
circulating, and onto which conductor a high frequency
signal is desired to be injected.
Based on figure 1 a generic description is provided of
the problem presented in the state of the art. The employ
of inductive couplers is known for injecting a determined
signal onto a conductor through which a current 1 is
circulating. To this end the inductive coupler is
constituted by a ferromagnetic core 3 0 arranged around
the conductor onto which one desires to inject the
signal, and in which is included a winding 31 to which a
current 2 is applied that produces a magnetic field 4 in
the ferromagnetic core 30, which induces a current in the
conductor. This device has the drawback that the current
1 which is circulating through the conductor also
produces a magnetic field 3 in the ferromagnetic core 30,
so that if said magnetic field 3 is high, saturation of
the ferromagnetic core 30 occurs, whereby the current 2
is unable to increase the value of the magnetic field and
hence no current is induced in the conductor, and
therefore the desired signal is not injected.
To overcome this drawback, an embodiment is shown in
figure 2 wherein a Hall-effect sensor 5 is mounted on the
ferromagnetic core 30, which sensor requires an external
power supply in order to be able to work (not shown in
the figure). In this case the current 1 which is
circulating through the conductor likewise produces a

magnetic field 3 in the ferromagnetic core 30, which is
detected by the sensor 5, the signal obtained being
applied to a low pass filter 6 which smoothes the
obtained signal.
The detected and filtered signal has been referred to
with the number 20, which signal is applied to a
comparator 32 which in turn receives a reference signal
21, and which moreover is connected to a control module
7, which calculates and generates a compensating current
to maintain the signal 2 0 equal to the value of the
reference signal 21. The compensating current is applied
through a power stage 8 and a radiofrequency choke 9 to a
winding 33 arranged on the ferromagnetic core 30,
producing in the latter a magnetic field 4 equal and
opposite to the field 3 produced by the current 1, so
that a compensation of the magnetic field 3 occurs, so
that the ferromagnetic core 3 0 is not saturated, whereby
the current 2 is induced in the conductor without
additional insertion loss.
By means of the radiofrequency chokes 9 low pass filters
are obtained which impede the passage of the high
frequencies produced by the signal to be injected.
In another embodiment of the invention, as shown in
figure 3, the current 1 which circulates through the
conductor is obtained by means of an external
ferromagnetic core 11 on which a coil 34 has been
arranged wherein, from the magnetic field produce by
current 1 in the ferromagnetic core 11, said current 1
is produced in the coil 34, this current 1 being applied,
through the radiofrequency choke 9 to the winding 33, the
compensation of the magnetic field being produced in the
manner described for the example in the previous figure.
Lastly in figures 4 and 5 other examples are shown for
compensation of the magnetic field 3 produced by current

1. Thus, in the example of figure 4 the employ is
foreseen of two inductive couplers constituted by a
double ferromagnetic core 16 and 17 that form the
ferromagnetic core 30. The ferromagnetic core 16 is a
power transformer, not suitable for radiofrequency, that
is, it has a very high magnetic permeability at low
frequencies and very low at high frequencies; whilst the
ferromagnetic core 17 is suitable for radiofrequency;
that is, it has a very low magnetic permeability at low
frequency and very high at high frequency.
In this case the detection and obtaining of the
compensating current is carried out simultaneously by
locating a winding around the double ring of the coupler.
The magnetic induction, produced by the current which is
passing through the cable, is detected by the winding and
produces therein the current which later serves to
perform the compensation. To this end, n coils are
mounted which act as radiofrequency chokes 18 and which
enclose both ferromagnetic cores 16 and 17, a smaller
nominal current being achieved in each winding 19, so
that said current carries out the desired compensation of
the magnetic field induced by the current 1 which is
circulating through the conductor. In this way the
magnetic field 3 is only induced in the ferromagnetic
core 16 and the magnetic field 4 in the ferromagnetic
core 17 preventing the aforesaid saturation from taking
place.
In figure 5 an alternative embodiment is shown in which n
windings are mounted short-circuited by means of a
radiofrequency choke 15. This case is equivalent to the
previous one, but it has the advantage of reducing the
number of choke coils necessary to carry out the
compensation.
In figure 6 another alternative embodiment is shown of a

device which implements the process of the invention. In
this device a single winding 35 is used around the double
ferromagnetic core 16 and 17 that forms the ferromagnetic
core 30 of the coupling unit. This case is similar to
that presented in figure 5, but with the advantage of
reducing the number of windings and without having to use
choke coils for the compensation. By means of said
winding 3 5 the detection and obtaining of the
compensating current are carried out simultaneously.
Moreover, and because the winding has a high impedance at
low frequencies and has a low impedance at high
frequencies, the injection of the compensating current is
carried out simultaneously by mounting said winding 35,
whereby a low cost solution is obtained.

We claim:
1. A process for compensating a magnetic field (3) in a coupling unit, the coupling
unit is arranged around a conductor, current (1) in the conductor produces the magnetic
field (3), and the process is characterized in that the process comprises:
detecting the magnetic field (3) in a ferromagnetic core (16 and 17) of the
coupling unit to obtain a signal;
generating a compensating current based on the signal; and
injecting the compensating current through a low pass filter (9 or 18) and into at
least one winding (19 or 35) on the ferromagnetic core (16 and 17) to generate a magnetic
field (4) and compensate the magnetic field (3),
wherein the low pass filter (9 or 18) is in series with the at least one winding (19
or 35) on the ferromagnetic core (16 and 17), and
wherein the at least one winding (19 or 35) detects the magnetic field (3).
2. The process as claimed in claim 1, wherein:
the low pass filter (9) provides impedance to a high frequency signal produced by
the magnetic field (4); and
the magnetic field (4) is equal to and opposite the magnetic field (3).
3. The process as claimed in claim 1, further comprising a signal winding (2) on the
ferromagnetic core (16 and 17), wherein the signal winding (2) injects a signal in the
ferromagnetic core (16 and 17), and wherein the signal injected in the ferromagnetic core
(16 and 17) induces a current in the conductor.
4. The process as claimed in claim 1, further comprising filtering the signal
obtained from the detecting of the magnetic field (3) to generate a filtered signal (20).
5. The process as claimed in claim 4, further comprising generating the
compensating current to maintain the filtered signal (20) equal to a value of a reference
signal (21).

6. The process as claimed in claim 5, further comprising selecting the reference
signal (21) to prevent flux density of the ferromagnetic core (16 and 17) from surpassing
a maximum flux density of the ferromagnetic core (16 and 17).
7. The process as claimed in claim 1, wherein the ferromagnetic core (16 and 17) is
a double magnetic core and comprises a ferromagnetic core (16) and a ferromagnetic core
(17).
8. The process as claimed in claim 7, wherein:
the ferromagnetic core (16) is a power transformer with a magnetic permeability
that is high at low frequencies and low at high frequencies; and
the ferromagnetic core (17) has a magnetic permeability that is low at high
frequencies and high at low frequencies.
9. The process as claimed in claim 1, wherein the injecting of the compensating
current is performed simultaneously with the detecting of the magnetic field (3).
10. The process as claimed in claim 1, wherein:
the at least one winding (19) comprises n coils mounted on the ferromagnetic core
(16 and 17), where n is an integer greater than 1; and
the n coils perform as radio frequency chokes (18).
11. The process as claimed in claim 1, wherein:
the at least one winding comprises only a single winding (35); and
the ferromagnetic core (16 and 17) is a double magnetic core and comprises a
ferromagnetic core 16 and a ferromagnetic core 17.

J

ABSTRACT

"A PROCESS FOR COMPENSATING A MAGNETIC FIELD IN A COUPLING
UNIT"
The invention relates to a device and method for low-frequency magnetic field
compensation in an inductive signal coupling unit. According to the invention, the
coupling unit comprises a ferromagnetic core (30) which surrounds a conductor through
which a low-frequency current (1) flows, in order to inject a high-frequency signal
therein. The invention is characterised in that it consists in detecting the current (1)
flowing through the conductor or the magnetic field (3) generated in the coupling unit,
producing a supply current from the value obtained during the aforementioned detection
process and injecting the supply current produced in the coupling unit via a low pass
filter (9), in order to generate a magnetic field (4) that is equal, but in the opposite
direction to that produced by the current (1) from the conductor in the above-mentioned
coupling unit, such as to prevent saturation of the magnetic core (30) without adding
insertion losses. In general, the invention can be used for communication systems that
require the injection of a radiofrequency signal in a conductor and, in particular, in
systems using the electrical network as a transmission means.

A PROCESS FOR COMPENSATING A MAGNETIC FIELD IN A COUPLING UNIT

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