Filter circuit

Abstract

Filter circuit (1), comprising: a core (10) made of magnetic material with a through-opening through which a connecting line can be inserted; a wound wire (11, 12) wound around the core (10) of magnetic material through the through-hole of the core (10) of magnetic material; and a variable impedance unit (20) connected to the wound wire (11, 12) and having an impedance that is variable depending on a current or a voltage introduced into the wound wire (11, 12) by the connecting line, wherein the variable impedance unit (20) comprises: an element (21) having properties in which the impedance of the element (21) is adaptively variable depending on the current or voltage introduced into the wound wire (11, 12) by the connecting line; an electrical line (26) connecting the element and the two ends of the wound wire (11, 12); and a resistor (25) connected to the electrical line (26), and the impedance of the variable impedance unit (20) changes depending on at least one of the following factors: type of element (21), number of turns of the wound wire (11, 12), type of wound wire (11, 12), type of electrical line (26) and constant of the resistance (25), and wherein the element (21) is a junction field effect transistor (21), the wound wire (11, 12) comprises a first wound wire (11), a first end (11a) of the first wound wire is connected to a drain terminal of the junction field effect transistor (21), a second end (11b) of the first wound wire (11) is connected to a source terminal of the junction field effect transistor (21), and the impedance of the variable impedance unit (20) is equivalent to a ratio between a drain-source voltage of the junction field effect transistor (21) generated when a current or voltage is introduced from the connecting line to the first wound wire (11) and a drain-source current of the junction field effect transistor (21) flowing when the current or voltage is introduced from the connecting line to the first wound wire (11).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a filter circuit. BACKGROUND TO THE STATE OF THE ART

[0002] As a measure against electromagnetic noise propagating through a cable (connector) connected to electrical and electronic equipment, in some cases a filter circuit is attached to the cable to reduce the propagation of electromagnetic noise. A tubular ferrite core, which can be retrofitted to a cable, is often used as such a filter circuit. For example, inserting a cable into the hollow part of a tubular ferrite core enhances the cable's magnetic field and increases the cable's self-inductance to achieve high cable impedance at high frequencies, thereby interrupting the propagation of electromagnetic noise to the cable.

[0003] However, since an additional influence of a ferrite core, namely the impedance characteristic of the ferrite core, is determined solely by physical constants such as the length of the ferrite core, the ratio between the inner diameter and the outer diameter of the tube, and the material properties of the ferrite, there are only limited methods for obtaining specific impedance characteristics, and it has been difficult to master many types of cables.

[0004] Therefore, for example, Patent Literature 1 discloses a device having a structure in which the impedance characteristic is not determined by physical constants of a ferrite core. A noise attenuation device according to Patent Literature 1 has a structure in which a special wound wire is wound around a tubular part of the ferrite core attached to a cable that is a target for noise mitigation, so that the special wound wire penetrates the hollow portion of the tubular part, and a resistance part is attached in series in the path of the wound wire. Note that the resistance part may be formed of a variable resistor.With such a configuration, it is possible to select an optimal resistance value for an electronic device in an electromagnetic noise interference environment, which can reduce and eliminate the noise interference and improve the electromagnetic environment at the installation site of the electronic device. JP 2018 - 37 942 A describes a filter for the highly efficient suppression of an interference wave. JP 2019 - 149 675 A describes an active noise filter that can suppress common-mode noise over a wide range. REFERENCE LISTPATENT LITERATURE

[0005] Patent literature 1: WO 2011 / 136 232 A1 SUMMARY OF THE INVENTIONTECHNICAL PROBLEM

[0006] In a case where the optimal resistance value for reducing and eliminating noise interference is selected for the noise attenuation device shown in Patent Literature 1, the resistance value is often higher than the characteristic impedance of a cable. Accordingly, an impedance mismatch may occur at a portion of the cable where the ferrite core is attached during signal transmission. When a transmission signal, such as a digital communication signal or a clock signal with a periodic square wave, is transmitted through the cable, waveform distortion or ringing of the transmission signal occurs, and the signal transmission quality deteriorates undesirably.

[0007] An object of the present disclosure is to provide a filter circuit that makes it possible to reduce both the propagation of electromagnetic noise through a cable and the deterioration of signal transmission quality. SOLUTION TO THE TASK

[0008] A filter circuit according to the present disclosure comprises: a core made of magnetic material having a through-hole through which a connecting line can be inserted; a coiled wire wound around the core made of magnetic material through the through-hole of the core made of magnetic material; and a variable impedance unit connected to the coiled wire and having an impedance that is variable depending on a current or a voltage introduced into the coiled wire from the connecting line, the variable impedance unit comprising: the element having characteristics in which the impedance of the element is adaptively variable depending on the current or voltage introduced from the power line to the coiled wire; an electrical line connecting the element and the two ends of the coiled wire; and a resistor,which is connected to the electrical line, and the impedance of the variable impedance unit changes depending on at least one of the following factors: type of element, number of turns of the wound wire, type of wound wire, type of electrical line, and resistance constant, and wherein the element is a junction field-effect transistor, the wound wire comprises a first wound wire, a first end of the first wound wire is connected to the drain terminal of the junction field-effect transistor, a second end of the first wound wire is connected to the source terminal of a junction field-effect transistor, and the impedance of the variable impedance unit is equivalent to a ratio between a drain-source voltage of the junction field-effect transistor generated when a current or a voltage is introduced from the connecting line to the first wound wire,and a drain-source current of the junction field-effect transistor that flows when the current or voltage is introduced from the connecting line to the first wound wire. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0009] By the configuration described above, the present disclosure makes it possible to reduce both the propagation of electromagnetic noise through a cable and the deterioration of signal transmission quality. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a configuration example of a filter circuit according to a first embodiment. Fig. 2 is a diagram showing a configuration example of a variable impedance unit in the first embodiment. Fig. 3 is a diagram showing the electrical connection of the filter circuit according to the first embodiment. Fig. 4 is a diagram showing the electrical characteristics of a junction field effect transistor of the first embodiment. DESCRIPTION OF THE EMBODIMENTS

[0010] In the following, an embodiment is explained in detail with reference to the figures. First embodiment.

[0011] Fig. 1 is a diagram showing a configuration example of a filter circuit 1 according to a first embodiment. As shown in Fig. 1, the filter circuit 1 according to the first embodiment comprises a core made of magnetic material or magnetic material core 10, a first wound wire 11, a second wound wire 12 and a variable impedance unit 20.

[0012] As in Fig. As shown in Figure 1, the magnetic material core 10 is tubular. Furthermore, a through-hole is formed through the magnetic material core 10. The first wound wire 11 and the second wound wire 12, as well as a cable (not shown), which is a connecting line and is a target for a noise reduction measure, can be inserted through the through-hole.

[0013] The first wound wire 11 and the second wound wire 12 are wound around the magnetic material core 10 through the connection hole formed by the magnetic material core 10. Furthermore, both ends (ends 11a and 11b) of the first wound wire 11 and both ends (ends 12a and 12b) of the second wound wire 12 are connected to the variable impedance unit 20, respectively.

[0014] The variable impedance unit 20 is connected to both ends (ends 11a and 11b) of the first coiled wire 11 and to both ends (ends 12a and 12b) of the second coiled wire 12. The variable impedance unit 20 is configured such that the impedance of the variable impedance unit 20 is variable depending on a current or voltage introduced from the cable into the first coiled wire 11 and the second coiled wire 12 when a transmission signal such as a digital communication signal or a clock signal having a periodic square wave is transmitted through the cable.

[0015] Fig. 2 is a diagram showing a configuration example of the variable impedance unit 20 in the first embodiment. As shown in Fig. 2, the variable impedance unit 20 comprises an element 21, a peripheral circuit 22, winding wire terminals 23 and winding wire terminals 24.

[0016] Element 21 has properties such that the impedance of element 21 adaptively varies depending on a current or voltage introduced from the cable into first coiled wire 11 and second coiled wire 12 when a transmission signal, such as a digital communication signal or a clock signal with a periodic square wave, is transmitted through the cable. Element 21 is, for example, a junction field-effect transistor (JFET), which is a type of field-effect transistor.

[0017] The peripheral circuit 22 is a circuit for electrically connecting the element 21 to the winding wire terminals 23 and the winding wire terminals 24. The peripheral circuit 22 comprises, for example, a resistor 25 and a conductor wire or electrical line 26 (see Fig. 3).

[0018] The winding wire terminals 23 are terminals connected to the ends 11a and 11b of the first wound wire 11. As shown in Fig. 2, for example, two winding wire terminals 23 are provided at the corners of the peripheral circuit 22.

[0019] The winding wire terminals 24 are terminals connected to the ends 12a and 12b of the second wound wire 12. As shown in Fig. 2, for example, two winding wire terminals 24 are provided at locations that are located at the corners of the peripheral circuit 22, but different from the locations where the winding wire terminals 23 are provided.

[0020] It is sufficient if the number of winding wire terminals provided for the peripheral circuit 22 matches the number of wound wires. For example, since two wound wires are used in the first embodiment, the total number of winding wire terminals provided is also four.

[0021] Next, the electrical connection of the filter circuit 1 according to the first embodiment will be explained. Fig. Figure 3 is a diagram showing the electrical connection of the filter circuit 1 according to the first embodiment. In an example case explained here, the two coiled wires (the first coiled wire 11 and the second coiled wire 12) are used, and an N-channel junction field-effect transistor is employed as element 21 of the variable impedance unit 20.

[0022] The two winding wire terminals 23 are connected to the ends 11a and 11b of the first wound wire 11.

[0023] In addition, a first of the two winding wire terminals 23 (e.g., one connected to the end 11a of the first wound wire 11) is connected to a drain terminal of the junction field-effect transistor 21 via the electrical line 26 and the resistor 25. In addition, a second of the two winding wire terminals 23 (e.g., one connected to the end 11b of the first wound wire 11) is connected to a source terminal of the junction field-effect transistor 21 via the electrical line 26 and the resistor 25.

[0024] Furthermore, the two winding wire terminals 23, i.e., the drain terminal and the source terminal of the junction field-effect transistor 21, are connected to each other via the electrical line 26 and the resistor 25. An electrical potential difference generated in the first wound wire 11 is applied as a voltage between the drain terminal and the source terminal of the junction field-effect transistor 21. This voltage is also referred to below as the "drain-source voltage."

[0025] The two winding wire terminals 24 are connected to the ends 12a and 12b of the second wound wire 12.

[0026] A first of the two winding wire terminals 24 (e.g., one connected to the end 12a of the second wound wire 12) is connected to a gate terminal of the junction field-effect transistor 21 via the electrical line 26 and the resistor 25. Furthermore, a second of the two winding wire terminals 24 (e.g., one connected to the end 12b of the second winding wire 12) is connected to the source terminal of the junction field-effect transistor 21 and the second of the two winding wire terminals 23 via the electrical line 26 and the resistor 25. An electrical potential difference generated in the second wound wire 12 is applied as a voltage between the gate terminal and the source terminal of the junction field-effect transistor 21. This voltage is also referred to below as the "gate-source voltage."

[0027] Next, an operational example and advantageous effects of the filter circuit 1 according to the first embodiment will be explained.

[0028] First, an operator attaches the filter circuit 1 to the cable that serves as the target for noise reduction. Specifically, the operator inserts the cable into the above-mentioned through-hole of the magnetic material core 10. Note that the cable is designed to transmit a transmission signal such as a digital communication signal or a clock signal with a periodic rectangular wave, and not only electromagnetic noise but also the transmission signal is transmitted through the cable.

[0029] When a voltage or current associated with the electromagnetic noise or the transmission signal increases, the magnetic field generated by the cable at the magnetic material core 10 increases. This not only generates an electric potential difference based on the number of turns of the first coiled wire 11 in the first coiled wire 11 wound around the magnetic material core 10, but also generates an electric potential difference based on the number of turns of the second coiled wire 12 in the second coiled wire 12. For this reason, the drain-source voltage and the gate-source voltage are applied to the junction field-effect transistor 21.

[0030] The electrical properties of the junction field effect transistor 21 in the first embodiment are shown in Fig. 4. In Fig. Figure 4 shows the electrical properties of a typical N-channel junction field-effect transistor. Fig. 4, the horizontal axis represents the drain-source voltage applied to the junction field-effect transistor 21, and the vertical axis represents a current flowing between the drain terminal and the source terminal of the junction field-effect transistor 21 (hereinafter also simply referred to as “drain current”).

[0031] As in Fig. As shown in Figure 4, the drain current flows depending on the applied drain-source voltage in the junction field-effect transistor 21 in the first embodiment. For example, in the junction field-effect transistor 21, when the applied drain-source voltage is increased from 0 V, the drain current also increases. However, after the applied drain-source voltage falls into the range on the right side of the dashed line in Fig. 4, the drain current no longer increases, although the drain-source voltage is increased. Note that the area on the left side of the dashed line in Fig. 4 as a resistance area and the area on the right side of the dashed line in Fig. 4 is called the saturation region.

[0032] In the junction field-effect transistor 21, the drain current value can be changed depending on the applied drain-source voltage value when the drain-source voltage is within the resistance range on the left side of the dashed line. That is, in the junction field-effect transistor 21, if the relationship between the applied drain-source voltage value and the drain current value is considered as the resistance between the drain terminal and the source terminal (hereinafter also referred to as "drain-source resistance"), it is possible to change the drain-source resistance value within the resistance range by changing the applied drain-source voltage.

[0033] In addition, the drain current characteristic can be Fig. 4, can also be changed by the value of the gate-source voltage applied to the junction field-effect transistor 21.

[0034] For example, the characteristic of the drain current is a locus denoted by reference numeral 401 when the gate-source voltage applied to the junction field-effect transistor 21 is 0 V, and changes into loci denoted by reference numerals 402, 403, and 404 when the gate-source voltage decreases from 0 V. That is, in the first embodiment, it becomes possible to change the relationship between the value of the drain-source voltage and the value of the drain current, that is, the value of the drain-source resistance, within the range of the resistance range also by changing the value of the gate-source voltage applied to the junction field-effect transistor 21. It should be noted that a Fig. 4, reference numeral 405 denotes the value of the drain-source resistance when the characteristic curve of the drain current is a locus denoted by reference numeral 404.

[0035] In addition, for example, the electrical properties of the Fig. 4 can be adjusted by changing at least one of the following factors: the type of the junction field-effect transistor 21 used, the number of turns of the first wound wire 11 and the second wound wire 12, the types of the first wound wire 11 and the second wound wire 12, the type of the electrical wire 26 included in the peripheral circuit 22, and the constant of the resistance 25 included in the peripheral circuit 22 (hereinafter, these factors are also referred to as "characteristic adjustment factors"). In addition, for example, the range of changes made when the value of the drain-source resistance is changed as described above can be appropriately designed by the above-described adjustment depending on the intended use of the filter circuit 1.

[0036] Therefore, in the filter circuit 1 according to the first embodiment, using these functions, the drain-source resistance value of the junction field-effect transistor 21 can be changed depending on a voltage applied to the first coiled wire 11 and the second coiled wire 12 at the time a voltage or current related to a transmission signal or electromagnetic noise flowing through the cable has increased or decreased. Thus, in the filter circuit 1 according to the first embodiment, in a case where the voltage or current related to the transmission signal and the electromagnetic noise described above increases or decreases, the resistance value between the two coiled wire terminals 23 in the variable impedance unit 20 can be changed.

[0037] For example, an operator sets the above-mentioned characteristic adjustment factors in advance so that the drain-source resistance of the junction field-effect transistor 21 decreases as much as possible when the voltage or current related to the transmission signal flowing through the cable has increased, or when the voltage or current related to the electromagnetic noise flowing through the cable has decreased. On the other hand, for example, the operator sets the above-mentioned characteristic adjustment factors in advance so that the drain-source resistance of the junction field-effect transistor 21 increases as much as possible when the voltage or current related to the transmission signal flowing through the cable has decreased, or when the voltage or current related to the electromagnetic noise flowing through the cable has increased.

[0038] Thereby, in the filter circuit 1 according to the first embodiment, the occurrence of waveform distortion or ringing of the transmission signal can be reduced because the drain-source resistance of the junction field-effect transistor 21 decreases and the impedance of the cable electromagnetically coupled by the magnetic material core 10 decreases in a case where the voltage or current with respect to the transmission signal flowing through the cable has increased, or in a case where the voltage or current with respect to the electromagnetic noise flowing through the cable has decreased.

[0039] Furthermore, in the filter circuit 1 according to the first embodiment, in addition to the advantageous effects described above, the propagation of electromagnetic noise having a level (intensity) lower than the level of the transmission signal through the cable can be reduced because the drain-source resistance of the junction field-effect transistor 21 increases and the impedance of the cable electromagnetically coupled by the magnetic material core 10 increases in a case where the voltage or current related to the transmission signal flowing through the cable has decreased or in a case where the voltage or current related to the electromagnetic noise flowing through the cable has increased.

[0040] Accordingly, in the first embodiment, by attaching the thus configured filter circuit 1 to a cable through which a transmission signal such as a digital communication signal or a clock signal flows, it is possible not only to reduce the occurrence of waveform distortion or ringing of the transmission signal, but also to reduce the propagation of electromagnetic noise through the cable. As a result, in the first embodiment, it is possible to enhance the effect of reducing the propagation of electromagnetic noise in a system using the cable to which the filter circuit 1 is attached, and also to improve the transmission quality of the transmission signal in the system.

[0041] Note that, in the example described above, the winding wire terminals 23 and 24 are provided at the corners of the peripheral circuit 22. However, the locations where the winding wire terminals 23 and 24 are provided are not limited to the corners of the peripheral circuit 22. For example, the winding wire terminals 23 and 24 can be provided at any location on the peripheral circuit 22, depending on the method of winding the first wound wire 11 and the second wound wire 12 around the magnetic material core 10, or the like.

[0042] Furthermore, the filter circuit 1 includes two coiled wires (the first coiled wire 11 and the second coiled wire 12) in the example explained in the above explanation. However, the filter circuit 1 is not limited to this case, but may also include three or more coiled wires. In this case, it is sufficient if the number of coiled wire terminals in the peripheral circuit 22 corresponds to the number of coiled wires.

[0043] For example, if the filter circuit 1 includes a third coiled wire in addition to the above-described first coiled wire 11 and second coiled wire 12, the peripheral circuit 22 includes a total of six coiled wire terminals. Furthermore, each of the six coiled wire terminals is connected to one of the drain terminals, the gate terminal, and the source terminal of the junction field-effect transistor 21 via the electrical line 26 and the resistor 25 as described above. Therefore, in the first embodiment, it is possible to adjust the magnitudes of the drain-source voltage and the gate-source voltage applied to the junction field-effect transistor 21; consequently, it is possible to adjust the value of the drain-source resistance.

[0044] Furthermore, element 21 of variable impedance unit 20 includes an N-channel field-effect transistor, as described above. However, element 21 of variable impedance unit 20 does not necessarily have to include a field-effect transistor with an N-channel junction, but may also include a field-effect transistor with a P-channel junction, for example. In this case, the drain current in the Fig. 4 when the gate-source voltage applied to element 21 is increased from 0 V. That is, the drain current characteristic in the Fig. 4 changes to the locations designated by reference numerals 402, 403 and 404 when the gate-source voltage applied to element 21 is increased from 0 V.

[0045] Furthermore, element 21 of variable impedance unit 20 includes a junction field-effect transistor in the example described above. However, element 21 of variable impedance unit 20 does not necessarily have to include a junction field-effect transistor, but may, for example, comprise a bipolar transistor, a metal-oxide-semiconductor field-effect transistor (MOSFET), or an insulated-gate bipolar transistor (IGBT).

[0046] Furthermore, the element 21 of the variable impedance unit 20 includes an N-channel field-effect transistor, and the filter circuit 1 includes two coiled wires (the first coiled wire 11 and the second coiled wire 12) in the example explained above. However, if the element 21 of the variable impedance unit 20 includes an N-channel or P-channel field-effect transistor, the second coiled wire 12 may be omitted from the filter circuit 1, but the filter circuit 1 may include one coiled wire (the first coiled wire 11).

[0047] Since the gate-source voltage is not applied to element 21 (junction field-effect transistor) in this case, it is sufficient for an operator to change the value of the drain-source resistance according to the drain current characteristic curve shown in Fig. 4 is marked with the reference number 401.

[0048] For example, it is sufficient for an operator to set the above-mentioned characteristic adjustment factors, except for the factors related to the second wound wire 12, in advance so that the drain-source resistance of the junction field-effect transistor 21 decreases as much as possible when a voltage or current related to a transmission signal flowing through the cable has increased, or when a voltage or current related to electromagnetic noise flowing through the cable has decreased.On the other hand, for example, it is sufficient if the operator sets the above-mentioned characteristic adjustment factors except for the factors related to the second wound wire 12 in advance so that the drain-source resistance of the junction field-effect transistor 21 increases as much as possible in a case where the voltage or current related to the transmission signal flowing through the cable has decreased, or in a case where the voltage or current related to the electromagnetic noise flowing through the cable has increased.

[0049] Also in this case, as explained above, it is convenient if the filter circuit 1 comprises two wound wires (the first wound wire 11 and the second wound wire 12) and the gate-source voltage is applied to the element 21, since this allows the user to change the value of the drain-source resistance more flexibly.

[0050] On the other hand, in a case where the element 21 of the variable impedance unit 20 includes an element other than a junction field-effect transistor (for example, in a case where the application of the gate-source voltage or gate-source current to the element 21 is required for the operation of the element 21), it is sufficient for the filter circuit 1 to include two coiled wires (the first coiled wire 11 and the second coiled wire 12). In this case, it is sufficient if the magnitude of the drain current flowing to the element 21 is controlled by controlling the magnitude of a current or voltage applied between the gate and source of the element 21 by introducing a current or voltage into the second coiled wire 12.

[0051] As mentioned above, the filter circuit 1 according to the first embodiment includes: the magnetic material core 10 having a through-hole through which a connecting wire can be inserted; the coiled wires 11 and 12 wound around the magnetic material core 10 through the through-hole of the magnetic material core 10; and the variable impedance unit 20 connected to the coiled wires 11 and 12 and having an impedance that varies depending on a current or voltage applied from the connecting wire to the coiled wires. Thus, the filter circuit 1 according to the first embodiment enables both reducing the propagation of electromagnetic noise through a cable and reducing the deterioration of signal transmission quality.

[0052] Furthermore, the variable impedance unit 20 comprises: the element 21 having characteristics in which the impedance of the element 21 is adaptively variable depending on the current or voltage introduced from the power line to the coiled wires 11 and 12; the electrical line 26 connecting the element 21 and the two ends of the coiled wires 11 and 12; and the resistor 25 connected to the electrical line 26, and the impedance of the variable impedance unit 20 changes depending on at least one of the following: the type of the element 21, the number of turns of the coiled wires 11 and 12, the types of the coiled wires 11 and 12, the type of the electrical line 26, and the constant of the resistor 25.Thus, the filter circuit 1 according to the first embodiment makes it possible to adjust the impedance of a cable accordingly and to easily reduce both the propagation of electromagnetic noise through the cable and the deterioration of the signal transmission quality.

[0053] Furthermore, the element 21 is a junction field-effect transistor, the wound wires comprise the first wound wire 11, the first end 11a of the first wound wire 11 is connected to the drain terminal of the junction field-effect transistor, the second end 11b of the first wound wire 11 is connected to the source terminal of the junction field-effect transistor, and the impedance of the variable impedance unit 20 corresponds to the ratio between the drain-source voltage of the junction field-effect transistor generated when the current or voltage is introduced from the connecting line to the first wound wire 11 and the drain-source current of the junction field-effect transistor flowing when the current or voltage is introduced from the connecting line to the first wound wire 11.Therefore, the filter circuit 1 according to the first embodiment enables both reducing the propagation of electromagnetic noise through a cable and reducing the deterioration of signal transmission quality with a simple configuration.

[0054] Furthermore, the coiled wires include the second coiled wire 12, the first end 12a of the second coiled wire 12 is connected to the gate terminal of the junction field-effect transistor, the second end 12b of the second coiled wire 12 is connected to the source terminal of the junction field-effect transistor, and the impedance of the variable impedance unit 20 changes depending on the gate-source voltage of the junction field-effect transistor generated when the current or voltage is applied from the conductive line to the second coiled wire 12. Thus, the filter circuit 1 according to the first embodiment enables flexible adjustment of the impedance of a cable and makes it possible to easily reduce both the propagation of electromagnetic noise through the cable and the deterioration of signal transmission quality.

[0055] It should be noted that in the present disclosure, all components of the embodiment may be changed or omitted. INDUSTRIAL APPLICABILITY

[0056] The present disclosure can reduce both the propagation of electromagnetic noise through a cable and the deterioration of signal transmission quality and is suitable for use in a filter circuit. LIST OF REFERENCE SYMBOLS

[0057] 1: Filter circuit; 10: Magnetic core; 11: First wound wire; 11a: End; 11b: End; 12: Second wound wire; 12a: End; 12b: End; 20: Variable impedance unit; 21: Element (junction field-effect transistor); 22: Peripheral circuit; 23: Winding wire terminal; 24: Winding wire terminal; 25: Resistor; 26: Electrical wire; 401 to 404: Drain current characteristic curve; 405: Drain-source resistance values

Claims

[1] Filter circuit (1), comprising: a core (10) made of magnetic material with a through-opening through which a connecting line can be inserted; a wound wire (11, 12) wound around the core (10) of magnetic material through the through-hole of the core (10) of magnetic material; and a variable impedance unit (20) connected to the wound wire (11, 12) and having an impedance that is variable depending on a current or a voltage introduced into the wound wire (11, 12) by the connecting line, wherein the variable impedance unit (20) comprises: an element (21) having properties in which the impedance of the element (21) is adaptively variable depending on the current or voltage introduced into the wound wire (11, 12) by the connecting line; an electrical line (26) connecting the element and the two ends of the wound wire (11, 12); and a resistor (25) connected to the electrical line (26), and the impedance of the variable impedance unit (20) changes depending on at least one of the following factors: type of element (21), number of turns of the wound wire (11, 12), type of wound wire (11, 12), type of electrical line (26) and constant of the resistance (25), and wherein the element (21) is a junction field effect transistor (21), the wound wire (11, 12) comprises a first wound wire (11), a first end (11a) of the first wound wire is connected to a drain terminal of the junction field effect transistor (21), a second end (11b) of the first wound wire (11) is connected to a source terminal of the junction field effect transistor (21), and the impedance of the variable impedance unit (20) is equivalent to a ratio between a drain-source voltage of the junction field effect transistor (21) generated when a current or voltage is introduced from the connecting line to the first wound wire (11) and a drain-source current of the junction field effect transistor (21) flowing when the current or voltage is introduced from the connecting line to the first wound wire (11). [2] Filter circuit (1) according to claim 1, wherein the wound wire (11, 12) contains a second wound wire (12), a first end (12a) of the second wound wire (12) is connected to a gate terminal of the junction field effect transistor (21), a second end (12b) of the second wound wire (12) is connected to the source terminal of the junction field effect transistor (21), and the impedance of the variable impedance unit (20) changes depending on a gate-source voltage of the junction field effect transistor (21) which is generated when a current or a voltage is introduced from the conductor track to the second wound wire (12).

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