TRAP CIRCUIT WITH TWO VARIABLE INDUCTORS

Abstract: In a bridge T type trap circuit having CR delta connection circuit and a varible LC tank circuit, the CR delta connection circuit is connected to the LC tank circuit via a variable inductor (19; 31, 33; 40). In one embodiment, a capacitor (18) is coupled with the variable inductor (19; 31, 33) to form another LC tank circuit. In another embodiment, a variable inductor (39) used in the first-mentioned LC tank circuit has a tap which is grounded. Impedance values of the first-mentioned LC tank circuit and the variable inductor (40) are ajdusted so that absolute values of real and imaginary parts thereof have given relationship. A resistor (22)of the CR delta connection circuit is used in common in a double-trap structure having two CR delta conection circuits and two series circuits each having two tank circuits.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to trap circuits used for removing interference signals in various signal processing circuits such as a television receiver, video tuner, satellite communication system or the like. 2. Prior Art Trap circuits of bridge T type used for the above-mentioned various apparatus handling radio frequecies are well known in VHF/UHF/SHF/microwave art. In conventional trap circuits of the above-mentioned type, an inductor of variable inductance type is used so as to obtain a desired frequency, while a variable resistor is used for obtaining maximum attenuation. However, with such a coventional trap circuit the capacitance of a capacitor coupled with the variable inductor in parallel must be extremely small when it is necessary to obtain a very sharp trapping characteristic at a high frequency such as several hundreds MHz, while the resistance of the variable resistor must be very large. As a result, insertion loss of such a trap circuit is very large. When it is intended to obtain a further sharper trapping characteristic, the value of the capacitors must be further reduced to be smaller than 1 pF which is difficult to actualize. In the case two traps are needed, when the conventional trap circuit is adjusted so that the attenuation of one trap is maximum, then the attenuation of the other trap does not equal maximum. Furthermore, FR-A-1 300 951 discloses a trap circuit with two variable capacitors. This trap circuit comprises a delta connection circuit having a resistor and first and second capacitors connected to said resistor, wherein the junction between one terminal of said resistor and said first capacitor constitutes the input terminal of said trap circuit, and wherein the junction between the other terminal of said resistor and said second capacitor constitutes the output terminal of said trap circuit. Between ground and the junction between said first and and said second capacitors two LC tank circuits arranged in parallel are provided. Each of said LC tank circuits comprises an inductor and a variable capacitor arranged in series. SUMMARY OF THE INVENTION It is the object of the invention to provide a trap circuit by which maximum attenuation can be obtained at two different frequencies which can be very high. In accordance with the present invention, there is provided a trap circuit comprising a delta connection circuit having a resistor and first and second capacitors connected to said resistor, wherein the junction between one terminal of said resistor and said first capacitor constitutes the input terminal of said trap circuit, and wherein the junction between the other terminal of said resistor and said second capacitor constitutes the output terminal of said trap circuit, and an LC tank circuit having a first inductor and a third capacitor, said trap circuit being characterized in that one junction between said first inductor and said third capacitor is grounded, and a second inductor is interposed between the junction between said first and said second capacitors and the other junction between said first inductor and said third capacitor. In accordance with the present invention, there is alternatively provided a trap circuit comprising a delta connection circuit having a resistor and first and second capacitors connected to said resistor, wherein the junction between one terminal of said resistor and said first capacitor constitutes the input terminal of said trap circuit, and wherein the junction between the other terminal of said resistor and said second capacitor constitutes the output terminal of said trap circuit, and an LC tank circuit having a first inductor and a third capacitor, said trap circuit being characterized in that said first inductor is connected in parallel to said third capacitor, said first inductor has a tap which is grounded, and a second inductor is interposed between the junction between said first and said second capacitors and a junction between said first inductor and said third capacitor. BRIEF DESCRIPTION OF THE DRAWINGS The object and features of the present invention will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: Fig. 1 is a circuit diagram showing a conventional trap circuit; Fig. 2 is a circuit diagram showing another conventional trap circuit; Fig. 3 is a circuit diagram showing a first embodiment trap circuit according to the present invention; Fig. 4 is an equivalent circuit diagram of the trap circuit of Fig. 3; Figs. 5A, 5B and 5C are impedance characteristic diagrams for describing the operation of the first embodiment trap circuit; Fig. 6 is a circuit diagram showing a second embodiment trap circuit according to the present invention; Fig. 7 is a diagram showing attenuation characteristic of the second embodiment trap circuit; Fig. 8 is a circuit diagram showing a third embodiment trap circuit according to the present invention; Fig. 9 is an equivalent circuit diagram of the trap circuit of Fig. 8; and Figs. 10, 11 and 12 are impedance characteristic diagrams describing the operation of the third embodiment trap circuit. The same or corresponding elements and parts are designated at like reference numerals throughout the drawings. DETAILED DESCRIPTION OF THE INVENTION Prior to describing preferred embodiments of the present invention, the above-mentioned conventional trap circuits will be described for a better understanding of the present invention. Fig. 1 shows the above-mentioned conventional trap circuit having capacitors 1, 2 and 3, a variable resistor 4, and an inductor 5. The inductor 5 is of variable inductance type so as to tune to a desired frequency. The variable resistor 4 may be adjusted to obtain maximum attenuation. Fig. 2 shows another conventional trap circuit used for obtaining two traps at different frequencies. In order to reduce insertion loss, two bridge T traps are formed of capacitors 6, 7, 8, 9, 10, and 11, two inductors 13 and 14 and a variable resistor 12 such that the variable resistor 12 is used in common in the double-trap structure. The inductance of the inductors 13 and 14 are varied so as to tune to two different desired frequencies while the variable resistor 12 is adjusted to obtain maximum attenuation. In the above-described conventional trap circuits, when it is intended to obtain an extremely sharp trapping characteristic, or when it is necessary to obtain a sharp trapping characteristic at a frequency higher than several hundreds of MHz, it is necessary to reduce the capacitance of the capacitors 1 and 2 to a very small value while the resistance of the resistor 4 used for obtaining maximum attenuation becomes very large resulting in large insertion loss of the trap circuit. Moreover, when it is intended to further sharpen the trapping characteristic, the capacitance of the capacitors 1 and 2 must be further reduced to a value smaller than 1 pF. Thus it is difficult to actualize such a circuit configuration. Furthermore, when two traps of different frequencies are required, in the conventional arrangement of Fig. 2, in order to reduce insertion loss a variable resistor 12 is used in common so that the resistance thereof is varied to obtain a maximum attenuation at one of the two frequencies. However, it is difficult to obtain a maximum attenuation at the other frequency due to variation of the capacitances of the capacitors 6, 7, 9 and 10. Therefore, attenuation adjustment is made at the sacrifice of attenuation amount at one or both of the two traps. The present invention which removes the above-described drawbacks will be described hereinbelow. According to the present invention in a bridge T type trap circuit, a CR delta connection is connected to an LC tank circuit via an inductor or a circuit including an inductor. The inductor is of variable inductance type. Furthermore, in one embodiment another LC tank circuit may be used as the above-mentioned circuit including an inductor. In this way when two LC tank circuits are connected in series to the CR delta connection circuit such that each LC tank circuit comprises a variable inductor, the resonance frequency of each of the LC tank circuit can be separately adjusted. More specifically, the resonance frequency of one of the two LC tank circuit is set to a frequency lower than a desired trap frequency while the other LC tank circuit is adjusted so that its resonance frequency is higher than the desired trap frequency. As a result, within a frequency range between these two different resonance frequencies, the inductance values of the two LC tank circuits are adjusted so that real parts and imaginary parts of the imepedances of these two LC tank circuits are resepctively equal to each other. As a result, maximum attenuation can be obtained at a desired frequency within the frequency range between the two resonance frequencies of the two LC tank circuits even if a fixed resistor is used in the CR delta connection circuit of the bridge T type trap circuit. Consequently, even if the fixed resistor is used in common in a double-trap structure, maximum attenuation can be obtained at two different desired frequencies. Furthermore, with this arrangement impedance variation with respect to frequency is made larger than that in conventional arrangement, while a sharper trapping characteristic can be obtained. In another embodiment, the inductance of the LC tank circuit is varied to vary pure resistance component, while the inductance of the inductor interposed between the CR delta connection circuit and the LC tank circuit is varied to obtain matching in imaginary part. Thus even if a fixed resistor is used in the CR delta connection circuit of the bridge T type trap circuit, a maximum attenuation can be obtained at a desired frequency. As a result, even if the fixed resistor is used in common in double-trap structure, maximum attenuation can be obtained at two different desired frequencies. Now preferred embodiments of the present invention will be described hereinbelow. FIRST EMBODIMENT Referring now to Fig. 3, a first embodiment of the present invention is schematically shown. The illustrated trap circuit comprises capacitors 15, 16, 18 and 20, a fixed resistor 17, and two variable inductors 19 and 21. The capacitors 15 and 16 and the resistor 17 are connected in Δ connection fashion. A junction between one terminal of the resistor 17 and the capacitor 15 is used as an input terminal 40, while another junction between the other terminal of the resistor 17 and the capacitor 16 is used as an output terminal 42. The capacitor 18 and the variable inductor 19 are coupled in parallel, while the capacitor 20 and the variable inductor 21 are coupled in parallel. A junction between the capacitor 18 and the variable inductor 19 is connected to a junction between the capacitors 15 and 16. The other junction between the capacitor 18 and the variable inductor 19 is connected to a junction between the capacitor 20 and the inductor 21. The other junction between the capacitor 20 and the inductor 21 is grounded. The capacitances of the capacitors 15, 18 and 20 are respectively expressed in terms of C₁, C₂ and C₃, while the inductances of the variable inductors 19 and 21 are respectively expressed in terms of L₁ and L₂. The capacitance of the capacitor 16 is equal to that of the capacitor 15, and is thus also expressed by C₁. When we Δ-Y transform the Δ connection of the resistor 17 and the capacitors 15 and 16, we obtain an equivalent circuit of Fig. 4. In Fig. 4, the references Z₁, Z₂, Z₃ and Z₄ respectively indicate impedances. Impedances Z₃, Z₄ and Z₅ are given by the following equations: Figs. 5A, 5B and 5C are graphs showing impedance characteristics of the above Eqs. (1), (2) and (3) with real parts and imaginary parts being separated from each other. When the term L₁ in Eq. (1) and the term L₃ in Eq. (3) are varied, both of the following relationships are satisfied simultaneously at a desired frequency fo: R₁ + R₂ + R₃ = 0 (4) X₁ + X₂ + X₃ = 0 (5) Then resultant impedances have the following relationship: Z₁ + Z₂ + Z₃ = 0 (f₀) (6) As a result, maximum attenuation can be obtained at the desired frequency fo. SECOND EMBODIMENT Fig. 6 shows a second embodiment of the present invention. In this second embodiment, in order to reduce the insertion loss of the trap circuit a fixed resistor 22 is used in common to two delta connection circuits including capacitors 23, 24, 25 and 26. Two parallel circuits each including a capacitor and a variable inductor are connected in series so that a series circuit of the two parallel circuits is connected to each of the two delta connection circuits in the same manner as in the first embodiment. More specifically, resonance capacitors 27 and 28 are coupled with variable inductors 31 and 32 respectively to form two parallel circuits, while other resonance capacitors 29 and 30 are coupled with variable inductors 33 and 34 respectively to form two parallel circuits. The variable inductors 31 and 32 may be adjusted to obtain a maximum attenuation at a first desired frequency f₁, and then the variable inductors 33 and 34 may be adjusted to obtain a maximum attenuation at a second desired frequency f₂. Fig. 7 is a graph showing attenuation characteristic obtained in this way. THIRD EMBODIMENT Fig. 8 shows a third embodiment trap circuit according to the present invention. The third embodiment trap circuit comprises a Δ connection circuit including two capacitors 35 and 36 and a fixed resistor 37. The capacitance of the capacitors 35 and 36 is identical and is denoted at the reference C₁, while the resistance of the resistor 37 is denoted at the reference R₁. A capacitor 38 and a variable inductor 39 are coupled in parallel to form an LC tank circuit. The variable inductor 39 has a tap which is connected to groud. A junction between the capacitor 38 and the variable inductor 39 is connected via another variable inductor 40 to a junction between the capacitors 35 and 36. The capacitance of the capacitor 38 is denoted at the reference C₂, while the inductance of the inductor 40 is denoted at the reference L₁. Similarly, the inductance between a first terminal of the inductor 39 and its tap and the other inductance between a second terminal of the inductor 39 and the tap are respectively expressed in terms of L₂ and L₃. In the trap circuit of Fig. 8, when we Δ-Y transform the Δ connection of the resistor 37 and the capacitors 35 and 36, we obtain an equivalent circuit of Fig. 9. In Fig. 9, the references Z₁, Z₂, Z₃, Z₄ and Z₅ respectively indicate impedances. The impedances Z₃, Z4 and Z5 may be obtained in the same manner as in the first embodiment as follows: wherein r₂ is a resistance component of the inductor L₁. wherein r₃ is resistance component of the inductor L₂; r₄ is resistance component of the inductor L₃; and Fig. 10 shows the impedance characteristic of the sum Z3 + Z4 obtained by adding the above Eqs. (7) and (8) to each other. More specifically, Fig. 10 shows only a frequency range higher than a resonance frequency of the LC tank circuit. As the inductance L₁ is varied, the impedance characteristic change as curves a, b and c while real part of Z₃ and Z₄ does not vary. Fig. 11 shows the impedance characteristic of the impedance Z₅. When inductances L₂ and L₃ of the Eq. (9)are varied, the real part in Eq. (9) varies as curves d, e and f. In addition, imaginary part varies as curves g, h, and i. Since the real part of the sum Z₃ + Z₄ is of negative resistance, we can obtain the following relationship at a desired frequency f₀ when inductances L₂ and L₃ of Z₅ are varied so that an absolute value of the real part of Z₅ equals an absolute value of real part of the sum Z₃ + Z₄ and the inductance L₁ of Z₁ is varied so that absolute value of the imaginary part of Z₅ is equal to an absolute value of imaginary part of the sum Z₃ + Z₄. Z₃ + Z₄ + Z₅ = 0 (f₀) (10) As a result, we can obtain a maximum attenuation. Fig. 12 shows characteristic impedance obtained under such condition.

Claims: Trap circuit comprising a delta connection circuit having a resistor (17) and first and second capacitors (15, 16) connected to said resistor (17), wherein the junction between one terminal of said resistor (17) and said first capacitor (15) constitutes the input terminal of said trap circuit, and wherein the junction between the other terminal of said resistor (17) and said second capacitor (16) constitutes the output terminal of said trap circuit, and an LC tank circuit (20, 21) having a first inductor (21) and a third capacitor (20), said trap circuit being characterized in that one junction between said first inductor (21) and said third capacitor (20) is grounded, and a second inductor (19) is interposed between the junction between said first and said second capacitors (15, 16) and the other junction between said first inductor (21) and said third capacitor (20). Trap circuit according to claim 1, characterized in that said first inductor (21) is of variable inductance type. Trap circuit according to claim 1 or 2, characterized in that said second inductor (19) is of variable inductance type. Trap circuit according to one of the preceeding claims, characterized by a fourth capacitor (18) connected in parallel to said second inductor (19) to form a second LC tank circuit (18, 19). Trap circuit according to claim 4, characterized in that one of said first and second LC tank circuits (18, 19, 20, 21) is arranged so that its resonance frequency is lower than a desired trap frequency, and the other of said first or second LC tank circuits is arranged so that its resonance frequency is higher than said desired trap frequency. Trap circuit comprising a delta connection circuit having a resistor (37) and first and second capacitors (35, 36) connected to said resistor (37), wherein the junction between one terminal of said resistor (37) and said first capacitor (35) constitutes the input terminal of said trap circuit, and wherein the junction between the other terminal of said resistor (37) and said second capacitor (36) constitutes the output terminal of said trap circuit, and an LC tank circuit (38, 39) having a first inductor (39) and a third capacitor (38), said trap circuit being characterized in that said first inductor (39) is connected in parallel to said third capacitor (38), said first inductor (39) has a tap which is grounded, and a second inductor (40) is interposed between the junction between said first and said second capacitors (35, 36) and a junction between said first inductor (39) and said third capacitor (38). Trap circuit according to claim 6, characterized in that said first inductor (39) is of variable inductance type. Trap circuit according to claim 5 or 6, characterized in that said second inductor (40) is of variable inductance type.

Inventors: ISHIKAWA, TAKAYOSHI, YOSHIDA, YASUHIKO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1986
Main classification: H03H7
IPC codes: H03H7