JP2012244216A - Tuning circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a tuning circuit that has an image trap circuit exhibiting a satisfactory attenuation characteristic over a wide frequency band.SOLUTION: The tuning circuit includes: a first inductor (L1); a second inductor (L2); a third inductor (L3); a first variable capacitive element (Cdi1) having one end connected to an end of the third inductor (L3) and having a variable capacitance depending on a desired tuning frequency; a first capacitor (C1) having one end connected to an input end and the other end connected to the end of the third inductor (L3); a second capacitor (C2) connected between the other end of the first variable capacitive element (Cdi1) and a ground; and a third capacitor (C3) connected between a middle point (A) dividing the second inductor (L2) into two inductors (L2a, L2b) and the junction of the first variable capacitive element (Cdi1) and the second capacitor (C2).

Description

  The present invention relates to a tuning circuit including an image trap circuit capable of suppressing an image frequency component of a desired channel.

  For example, in a superheterodyne receiver, a signal at a frequency higher than the local oscillation frequency by an intermediate frequency (this frequency is called an image frequency) is converted to the same intermediate frequency as the reception frequency by the frequency mixing circuit. Cause reception interference. Therefore, it is necessary to provide a countermeasure against image disturbance by removing a signal having an image frequency by providing a trap circuit in the input tuning circuit of the TV tuner (see, for example, Patent Document 1).

  FIG. 9 shows a configuration example of a tuning circuit for a TV tuner in which an image trap circuit for attenuating image frequency components is formed and arranged. The tuning circuit 11 shown in FIG. 9 is parallel to the tuning varactor diode 11a, the coils 11b to 11e that are connected in parallel to the varactor diode 11a in high frequency and are connected in series, and the coils 11c and 11d. And a connected diode 11f.

  The coil 11e is grounded at a high frequency by a capacitor 11g. A television signal is input to a connection point between the coils 11c and 11d, and a connection point between the cathode of the varactor diode 11a and the coil 11b is an output terminal of the tuning circuit 11. A connection point between the coil 11e and the capacitor 11g is grounded by a resistor 11h and is connected to the power supply terminal B by a resistor 11i. A predetermined voltage is applied to the power supply terminal B, and a bias voltage divided by the resistors 11h and 11i is applied to the anode of the diode 11f via the coils 11e, 11d, and 11c. The cathode of the diode 11f is connected to the band switching terminal Sw. A high-level or low-level switching voltage is applied to the band switching terminal Sw. The cathode of the varactor diode 11a is connected to the tuning voltage terminal Tu via the coil 11b. A tuning voltage is applied to the tuning voltage terminal Tu.

  The tuning circuit 11 and the subsequent RF amplifier circuit are coupled by a coupling varactor diode 12. The cathode of the varactor diode 12 is connected to the cathode of the varactor diode 11a, and the anode of the varactor diode 12 is connected to the connection point of the coils 11c and 11d via the resistor 13. That is, a bias voltage is also applied to the anode of the varactor diode 12. A capacitor 14 is connected between the connection point of the coils 11c and 11d and the anode of the varactor diode 12, and a capacitor 15 is connected between the cathode of the switch diode 11f and the anode of the varactor diode 12. As a result, the capacitor 15 is connected to the connection point of the coils 11d and 11e at a high frequency.

  When a high-band television signal in the VHF band is received, when a low-level switching voltage is applied to the band switching terminal Sw, the diode 11f is turned on, and the tuning circuit 11 is in the state shown by the equivalent circuit in FIG. . The coil 11j shows a configuration in which the coils 11c and 11d in FIG. 9 are connected in parallel. On the other hand, when receiving a low-band television signal in the VHF band, when a high-level switching voltage is applied to the band switching terminal Sw, the diode 11f is turned off, and the tuning circuit 11 is in the state shown by the equivalent circuit in FIG. Become. In the tuning circuit 11, a resonance circuit including the capacitor 14 and the capacitor 15 functions as an image trap circuit.

JP 2003-133914 A

  By the way, the tuning circuit 11 described above switches between two types of circuit configurations depending on whether the frequency band of the received television signal is the high band of the VHF band or the low band of the VHF band, and the resonance frequency of the image trap circuit. The image frequency component corresponding to each reception frequency band can be attenuated by changing. However, since the attenuation characteristics of the image trap circuit are generally fixed when receiving the high band of the VHF band and when receiving the low band of the VHF band, appropriate attenuation of the image frequency components over the entire wide frequency band. Cannot be realized.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a tuning circuit including an image trap circuit that exhibits good attenuation characteristics in a wide frequency band.

  The tuning circuit of the present invention includes a first inductor having one end connected to the input end, a second inductor having one end connected to the other end of the first inductor and the other end connected to the ground, and one end connected to the first inductor. A third inductor connected to the other end of the one inductor, a first variable capacitance element whose one end is connected to the other end of the third inductor and whose capacitance is switched according to a desired tuning frequency, and one end connected to the input end A first capacitor connected at the other end to the other end of the third inductor, a second capacitor connected between the other end of the first variable capacitance element and the ground, and the second inductor. And a third capacitor connected between a midpoint that is divided into two inductors and a connection point between the first variable capacitance element and the second capacitor.

  According to this configuration, since the third capacitor is connected to the other end of the first variable capacitance element, the density of coupling with other circuit elements of the third capacitor is reduced according to the impedance value of the first variable capacitance element. Can be changed. As a result, the trap frequency of the image trap circuit can be changed according to the reception frequency band, and a good attenuation characteristic of the image frequency component can be realized. Further, the amount of attenuation of the trap frequency at the time of reception on the high frequency side can be increased by the series-parallel resonance circuit including the second inductor and the second capacitor. Therefore, it is possible to provide a tuning circuit including an image trap circuit that exhibits good attenuation characteristics in a wide frequency band.

  The tuning circuit of the present invention includes a second variable capacitance element connected between a connection point between one end of the first variable capacitance element and the other end of the third inductor, and the other end of the first capacitor. May be.

  The tuning circuit of the present invention may be an input tuning circuit of a television tuner that receives a television signal in the UHF band.

  ADVANTAGE OF THE INVENTION According to this invention, the tuning circuit provided with the image trap circuit which shows a favorable attenuation | damping characteristic in a wide frequency band can be provided.

It is a circuit diagram which shows the tuning circuit which concerns on this Embodiment. It is a characteristic figure which shows the relationship between the tuning frequency of a tuning circuit, and the capacitance value of a varactor diode, and the relationship between the tuning frequency of a tuning circuit, and the capacitance value of a capacitor. It is a characteristic figure which shows the relationship between the tuning frequency of a tuning circuit, and the impedance of a varactor diode, and the relationship between the tuning frequency of a tuning circuit, and the impedance of a capacitor. It is a characteristic view which shows the characteristic simulation result of the tuning circuit at the time of the low frequency side reception of a UHF band. It is a characteristic view which shows the characteristic simulation result of the tuning circuit at the time of the mid-range reception of a UHF band. It is a characteristic view which shows the characteristic simulation result of the tuning circuit at the time of the high frequency side reception of a UHF band. It is a characteristic view which shows the frequency characteristic of the trap position of the tuning circuit which concerns on this Embodiment, a 1st comparative example, and a 2nd comparative example. It is a characteristic view which shows the attenuation characteristic of the image frequency of the tuning circuit which concerns on this Embodiment, a 1st comparative example, and a 2nd comparative example. It is a circuit diagram which shows the structural example of the tuning circuit by which the image trap circuit was formed and arranged. FIG. 5 is an equivalent circuit diagram of a tuning circuit when receiving a VHF band high-band television signal. It is an equivalent circuit diagram of a tuning circuit when receiving a VHF band low band television signal.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a circuit diagram showing a tuning circuit of a television tuner according to an embodiment. The tuning circuit 1 according to the present embodiment includes a plurality of inductors, capacitors, and the like, and is configured to be tuned to a signal frequency of a desired channel. In the present embodiment, the tuning circuit 1 is disposed in front of an RF amplifier circuit 3 that amplifies a television signal (RF signal) output from an antenna 2 that receives a television signal. Here, a tuning circuit 1 that can be tuned to a television signal in the UHF band (300 MHz to 3000 MHz, typically 470 MHz to 770 MHz) is shown, but the signal that is the target of the tuning circuit 1 is not limited to the television signal. The frequency band targeted for the circuit 1 is not limited to the UHF band.

  The tuning circuit 1 has one end connected to an input end P1 that is on the feeding point side of the antenna 2 via a capacitor C4, one end connected to the other end of the inductor L1, and the other end grounded (connected to ground). ) Inductor L2, inductor L3 having one end connected to the other end of inductor L1, inductor L4 grounding the other end of inductor L3, anode connected to the other end of inductor L3, and cathode connected to capacitor C2. And a varactor diode Cdi1 which is a variable capacitance element grounded in high frequency. The anode of the varactor diode Cdi2 that is a variable capacitance element is connected to the connection point between the other end of the inductor L3 and the anode of the varactor diode Cdi1. The cathode of the varactor diode Cdi2 is connected to the output terminal P2 via the capacitor C5.

  The cathode of the varactor diode Cdi1 is connected to the terminal T1 via the resistor R1, and the cathode of the other varactor diode Cdi2 is connected to the terminal T1 via the resistor R2. The tuning frequency can be selected by changing the capacitance value of the varactor diode Cdi1 according to the tuning voltage Vtu applied to the terminal T1.

  The tuning circuit 1 includes a capacitor C1 that connects between one end of the inductor L1 and the other end of the capacitor C5. The capacitor C1 forms a trap circuit together with other components (L1, L3, Cdi2, C5, etc.).

  In the tuning circuit 1, the cathode of the varactor diode Cdi1 and the midpoint of the inductor L2 are coupled via a capacitor C3. More specifically, the capacitor C3 is connected to a midpoint (node A) that divides the inductor L2 into two inductors L2a and L2b, and a connection point (node B) between the varactor diode Cdi1 and the capacitor C2. . The capacitor C3 is connected in parallel with C1 constituting the image trap circuit in the tuning circuit 1, and the degree of coupling with the signal line side / GND side changes according to the impedance of the varactor diode Cdi1. On the high band side, the coupling degree of the capacitor C3 to the signal line side is loose coupling, and on the low band side, the coupling degree of the capacitor C3 to the signal line side is tight coupling. The midpoint of the inductor L2 simply means a dividing point that divides the inductor L2 into two. That is, the midpoint does not have to be a point that divides the inductance into two equal parts. Further, the two inductors L2a and L2b may be independent separate inductors.

  When such a tuning circuit 1 is tuned to the high frequency side of the UHF band, a high voltage is applied to the cathode of the varactor diode Cdi1 through the terminal T1. As a result, the capacitance value of the varactor diode Cdi1 decreases and the resonance frequency (tuning frequency) of the tuning circuit 1 increases, so that the tuning circuit 1 is tuned to the high frequency side of the UHF band. When tuning circuit 1 is tuned to the low frequency side of the UHF band, a low voltage is applied to the cathode of varactor diode Cdi1 through terminal T1. As a result, the capacitance value of the varactor diode Cdi1 increases and the resonance frequency (tuning frequency) of the tuning circuit 1 decreases, so that the tuning circuit 1 is tuned to the low frequency side of the UHF band.

  Here, capacitance changes of the varactor diode Cdi1 and the capacitor C2 in the UHF band (reception band) will be described. FIG. 2 is a diagram showing capacitance changes of the varactor diode Cdi1 and the capacitor C2 in the tuning circuit 1. The horizontal axis indicates the tuning frequency (MHz), and the vertical axis indicates the capacitance value (pF). As shown in FIG. 2, the capacitor C2 has a fixed capacitance throughout the UHF band, but the varactor diode Cdi1 is controlled to have a large capacitance value during low-frequency reception and a small capacitance value during high-frequency reception. .

  The frequency characteristics of the impedances of the varactor diode Cdi1 and the capacitor C2 in the UHF band (reception band) will be described. FIG. 3 is a frequency characteristic diagram of the impedances of the varactor diode Cdi1 and the capacitor C2 in the tuning circuit 1. In FIG. 3, the horizontal axis indicates the tuning frequency (MHz), and the vertical axis indicates the impedance value (Ω). As shown in FIG. 3, the varactor diode Cdi1 has a characteristic that the impedance is large on the high frequency side of the UHF band and the impedance is small on the low frequency side. On the other hand, the capacitor C2 has a tendency that the impedance slightly decreases from the low frequency side to the high frequency side of the UHF band.

  In contrast to the image trap circuit formed mainly by the inductors L1, L3, the varactor diode Cdi2, the capacitor C1, and the like, the capacitor C3 is an additional capacitor connected in parallel via the varactor diode Cdi1. From the frequency characteristics of the impedance of the varactor diode Cdi1 and the capacitor C2, the coupling state between the capacitor C3 and the image trap circuit (signal line) changes according to the reception frequency.

  That is, on the low frequency side where the inter-terminal capacitance of the varactor diode Cdi1 is large, since the impedance of the varactor diode Cdi1 becomes small, the coupling state of the capacitor C3 with respect to the signal line of the image trap circuit becomes a tight coupling state. As a result, the influence of the capacitor C3 as an additional capacitor on the image trap circuit becomes large, and acts to shift the trap frequency to the low frequency side.

  On the other hand, since the impedance of the varactor diode Cdi1 is larger on the high frequency side where the inter-terminal capacitance of the varactor diode Cdi1 is small compared with the case where the varactor diode Cdi1 is tuned to the low frequency side, Become loosely coupled. As a result, the influence of the capacitor C3 on the image trap circuit is reduced, and the trap frequency is shifted to the high frequency side.

  Further, since the impedance of the capacitor C2 is smaller on the high frequency side than in the case of tuning to the low frequency side (FIG. 3), the grounding action via the capacitor C2 is strengthened. As a result, a series-parallel resonance circuit is formed by the capacitor C2 and the inductors L2a and L2b, and the amount of attenuation at the trap frequency increases due to the attenuation action due to the series resonance characteristics.

  Thus, according to the present embodiment, by connecting the capacitor C3 between the midpoint of the inductor L2 and the cathode of the varactor diode Cdi1, the signal line of the capacitor C3 via the varactor diode Cdi1 on the high frequency side. The trap frequency can be shifted to the high frequency side by reducing the degree of coupling to the low frequency side, and the impedance of the capacitor C2 arranged on the GND side is reduced, so that the amount of attenuation of the trap frequency can be increased. It becomes possible. Further, on the low frequency side, the trap frequency can be shifted to the low frequency side by increasing the degree of coupling of the capacitor C3 to the signal line via the varactor diode Cdi1 to achieve a tight coupling state.

  FIG. 4 is a characteristic diagram showing a characteristic simulation result of the tuning circuit 1 at the time of reception on the low frequency side of the UHF band. In FIG. 4, the horizontal axis indicates the tuning frequency (MHz) and the vertical axis indicates the attenuation (dB). The inductance of the inductor L1 is 5.1 nH, the capacitance value of the capacitor C1 is 0.8 pF, the capacitance value of the capacitor C3 is 2.4 pF, and the frequency characteristic at the time of channel reception of 367.25 MHz is simulated. Since the trap position is shifted to the low frequency side, a trap is formed at a close position with respect to the image frequency appearing on the high frequency side from the low frequency side reception frequency 367.25 MHz to 79 MHz.

  FIG. 5 is a characteristic diagram showing a result of a characteristic simulation of the tuning circuit 1 at the time of UHF band mid-range reception. The inductance of the inductor L1 is 5.1 nH, the capacitance value of the capacitor C1 is 0.8 pF, the capacitance value of the capacitor C3 is 2.4 pF, and a 599.25 MHz channel reception is simulated. A trap is formed at the same position with respect to the image frequency appearing on the high frequency side from the reception frequency of 599.25 MHz to 79 MHz.

  FIG. 6 is a characteristic diagram showing a characteristic simulation result of the tuning circuit 1 at the time of reception on the high frequency side of the UHF band. The inductor L1 has an inductance of 5.1 nH, the capacitance value of the capacitor C1 is 0.8 pF, and the capacitance value of the capacitor C3 is 2.4 pF, and the frequency characteristics at the time of channel reception at 855.25 MHz are simulated. Since the trap position is shifted to the high frequency side, the trap is formed at the same position with respect to the image frequency appearing on the high frequency side from the high frequency side reception frequency 855.25 MHz to 79 MHz. In addition, an attenuation pole due to the series resonance characteristic appears at a position slightly shifted from the trap position to the high frequency side.

  As shown in FIGS. 4 to 6, the tuning circuit 1 according to the present embodiment can sufficiently secure attenuation at an image frequency that is 79 MHz away from the reception channel. This is because the trap frequency of the image trap circuit is variable according to the reception frequency band according to the principle described above, and the attenuation at the trap frequency can be sufficiently increased by the series-parallel resonance circuit.

  Next, the frequency characteristic of the trap position and the attenuation characteristic of the image frequency of the tuning circuit 1 according to the present embodiment will be described in comparison with the first and second comparative examples. The first comparative example has a configuration in which the inductor L1 and the capacitor C3 are deleted from the circuit configuration of the tuning circuit 1 according to the present embodiment. In the second comparative example, the capacitor C3 is deleted from the circuit configuration of the tuning circuit 1 according to the present embodiment. The inductance of the inductor L1 was 5.1 nH, the capacitance value of the capacitor C1 was 0.8 pF, and the capacitance value of the capacitor C3 was 2.4 pF.

  FIG. 7 is a characteristic diagram showing frequency characteristics of the trap positions of the tuning circuit 1, the first comparative example, and the second comparative example according to the present embodiment. In FIG. 7, the horizontal axis indicates the reception frequency (MHz), and the vertical axis indicates the trap frequency (position from the reception frequency). In FIG. 7, the dotted line in the vicinity of the vertical axis of 79 MHz indicates the image frequency position. FIG. 8 is a characteristic diagram showing image frequency attenuation characteristics of the tuning circuit 1, the first comparative example, and the second comparative example according to the present embodiment. In FIG. 8, the horizontal axis indicates the reception frequency (MHz), and the vertical axis indicates the attenuation amount (dB) at the image frequency.

  As shown in FIG. 7, on the low frequency side of the UHF band, the trap frequency in the first comparative example is greatly shifted from the image frequency to the high frequency side. Further, in the second comparative example, the trap frequency is shifted in the image frequency direction compared to the first comparative example, but in the tuning circuit 1 according to the present embodiment, the trap frequency is further lower than the second comparative example. Shifted closer to the image frequency. On the high frequency side of the UHF band, in the first and second comparative examples, the trap frequency is greatly shifted from the image frequency to the low frequency side. However, in the tuning circuit 1 according to the present embodiment, the trap frequency is the image frequency. It matches.

  In the tuning circuit 1 according to the present embodiment, as shown in FIG. 7, the trap frequency of the image trap circuit is not greatly deviated from the image frequency in the entire UHF band. This is because the degree of coupling of the capacitor C3 to the signal line is changed according to the impedance value of the varactor diode Cdi1.

  Further, as shown in FIG. 8, the tuning circuit 1 according to the present embodiment realizes an extremely excellent attenuation characteristic of about −43 dB on the high frequency side of the UHF band. This is because the amount of attenuation at the trap frequency can be sufficiently increased by forming a series-parallel resonance circuit including the capacitor C2 of the tuning circuit 1 and the inductors L2a and L2b.

  As described above, in the tuning circuit according to the present invention, the third capacitor is connected to the cathode of the first variable capacitance element so that the third capacitor is connected to the signal line of the third capacitor according to the impedance value of the first variable capacitance element. The degree of coupling can be changed to loose coupling or tight coupling. Thereby, the trap frequency of the image trap circuit can be changed according to the reception frequency band. In addition, the series-parallel resonance circuit including the second capacitor and the second inductor can increase the attenuation particularly at the trap frequency at the time of reception on the high frequency side. Thus, according to the present invention, it is possible to provide a tuning circuit including an image trap circuit that exhibits good attenuation characteristics in a wide frequency band.

  In addition, this invention is not limited to description of the said embodiment, It can change suitably and implement, without deviating from the scope of the present invention. For example, other circuit elements can be provided between the circuit elements constituting the tuning circuit as long as the functions of the tuning circuit are not impaired. Each circuit element constituting the tuning circuit can be omitted as long as the tuning circuit can function.

  The tuning circuit of the present invention is useful for a tuning circuit of a television tuner, for example.

DESCRIPTION OF SYMBOLS 1 Tuning circuit 2 Antenna 3 RF amplifier circuit L1, L2, L2a, L2b, L3, L4 Inductor C1, C2, C3, C4, C5 Capacitor Cdi1, Cdi2 Varactor diode R1, R2 Resistance T1 terminal

Claims (3)

A first inductor having one end connected to the input end;
A second inductor having one end connected to the other end of the first inductor and the other end connected to the ground;
A third inductor having one end connected to the other end of the first inductor;
A first variable capacitance element having one end connected to the other end of the third inductor and having a capacitance switched according to a desired tuning frequency;
A first capacitor having one end connected to the input end and the other end connected to the other end of the third inductor;
A second capacitor connected between the other end of the first variable capacitance element and the ground;
A third capacitor connected between a midpoint of dividing the second inductor into two inductors and a connection point of the first variable capacitance element and the second capacitor;
A tuning circuit comprising:   The second variable capacitance element is connected between a connection point between one end of the first variable capacitance element and the other end of the third inductor, and the other end of the first capacitor. Item 2. The tuning circuit according to Item 1.   The tuning circuit according to claim 1, wherein the tuning circuit is an input tuning circuit of a television tuner that receives a television signal in the UHF band.

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