JP2001238143A - Intermediate frequency circuit for television tuner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve Q of a tuning characteristic and to reduce loss when receiving an FM broadcast even if the input impedance of an intermediate frequency amplifier is low. SOLUTION: A mixer 1, an intermediate frequency tuning circuit 2 which is connected to the mixer 1 and tuned between a video intermediate frequency and a sound intermediate frequency when receiving a television signal and an intermediate frequency amplifier 5 connected to the intermediate frequency turning circuit 2 through a coupling capacity means 3 are installed. The capacity value of the coupling capacity means 3 is switched to two ways of large and small. When receiving the television signal, the capacity value of the coupling capacity means 3 is enlarged and is reduced when receiving the FM broadcast signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an intermediate frequency circuit of a television tuner.

[0002]

2. Description of the Related Art An intermediate frequency circuit (hereinafter simply referred to as an intermediate frequency circuit) of a conventional television tuner has a mixer 31, an intermediate frequency tuning circuit 32 and an intermediate frequency amplifier 33, as shown in FIG. . The mixer 31 receives a television signal or an FM broadcast signal (represented by RF) and a local oscillation signal (represented by Lo). The mixer 31 outputs a television intermediate frequency signal when receiving a television signal, and outputs an FM broadcast signal frequency-converted to an audio intermediate frequency in the television intermediate frequency band when receiving an FM broadcast signal.

The intermediate frequency tuning circuit 32 includes a varactor diode 32a, a capacitor 32b having one end connected to the cathode of the varactor diode 32a and the other end grounded, and two inductance elements 32 connected in series with each other.
c, 32d, and a series connection circuit of the inductance elements 32c, 32d is connected in parallel to a series connection circuit of the varactor diode 32a and the capacitor 32b,
Construct a parallel tuning circuit. A voltage Vb is applied to a connection point between the inductance elements 32c and 32d, and this voltage is supplied to the cathode of the varactor diode 32a via the inductance element 32c.

Further, the anode of the varactor diode 32a is grounded through two resistors 34 and 35 connected in series. Switching means 3 is provided at the connection point between resistors 34 and 35.
6, the voltage Vb is applied or a voltage of 0 volt is applied. The connection point between the cathode of the varactor diode 32a and the inductance element 32c is connected to the output terminal of the mixer 31 and the coupling capacitor 37
To the input terminal of the intermediate frequency amplifier 33.

[0005] In the above configuration, at the time of receiving a television signal, the switching means 36 applies a voltage of 0 volt to the connection point between the resistors 34 and 35. Then, a reverse bias voltage is applied between the anode and the cathode of the varactor diode 32a. At this time, the tuning frequency of the intermediate frequency tuning circuit 32 is determined by the capacitance value of the varactor diode 32a, the capacitance value of the capacitor 32b, and the inductance values of the inductance elements 32c and 32d. The frequency is set between the frequency P and the audio intermediate frequency S (preferably between the video intermediate frequency P and the chrominance subcarrier frequency C), and the tuning characteristic is set as shown in FIG.
A of FIG.

On the other hand, at the time of receiving the FM broadcast signal, a voltage Vb is applied to the connection point between the resistors 34 and 35 by the cutting means 36. Then, the potential between the anode and the cathode of the varactor diode 32a becomes substantially the same, the capacitance value of the varactor diode 32a becomes extremely large, and the tuning frequency becomes low. The tuning frequency at this time is set to be the audio intermediate frequency S in the intermediate frequency band, and the tuning characteristics are shown in FIG.
B of FIG.

[0007]

However, in the FM broadcast signal reception, the tuning capacitance (series capacitance of the varactor diode 32a and the capacitor 32b) of the intermediate frequency tuning circuit 32 becomes large. The load Q of the entire tuning circuit, including the input impedance of the frequency amplifier 33, becomes larger as compared with the reception of the television signal, and the loss increases. Therefore, the peak point of the tuning characteristic when receiving the FM broadcast signal is 3d more than the peak point of the tuning characteristic when receiving the television signal.
B, which results in a shortage of the gain of the entire television tuner. As a result, there is a problem that the audio sensitivity at the time of receiving the FM broadcast signal is reduced.

Further, since the input impedance of the intermediate frequency amplifier 33 is low, the overall Q itself cannot be increased, the tuning characteristics become loose, and the broadcast frequency becomes lower at the adjacent frequency.
There has been a problem of causing interference with the M broadcast signal.

[0009] Therefore, the intermediate frequency circuit of the television tuner of the present invention is to increase the tuning characteristic Q and reduce the loss at the time of FM broadcast reception even when the input impedance of the intermediate frequency amplifier is low. With the goal.

[0010]

In order to solve the above problems, according to the present invention, when a television signal is received, an intermediate frequency signal of a television is output, and when an FM broadcast signal is received, audio in an intermediate frequency band of the television is output. A mixer for outputting an FM broadcast signal frequency-converted to an intermediate frequency, connected to the mixer, and tuned between a video intermediate frequency and an audio intermediate frequency in the intermediate frequency band when the television signal is received, An intermediate frequency tuning circuit that tunes to an audio intermediate frequency in the intermediate frequency band when receiving an FM broadcast signal; and an intermediate frequency amplifier coupled to the intermediate frequency tuning circuit via coupling capacitance means. The value is switched between a large value and a small value, and the capacitance value of the coupling capacitance means at the time of receiving the television signal. Greatly, and it reduces the time the FM broadcast signal reception.

Further, the intermediate frequency tuning circuit is constituted by a parallel tuning circuit, and the parallel tuning circuit is provided with a first varactor diode for changing the tuning frequency, and the coupling capacitance means includes a capacitor connected in series with each other. The second varactor diode is configured to apply a first reverse bias voltage to the first varactor diode at the time of receiving the television signal, and to make the second varactor diode conductive, at the time of receiving the FM broadcast signal. A second reverse bias voltage lower than the first reverse bias voltage is applied to the first varactor diode, and a third reverse bias voltage is applied to the second varactor diode.

In addition, an anode of the first varactor diode and a cathode of the second varactor diode are DC-connected, a first voltage is applied to the cathode of the first varactor diode, and A second voltage lower than the first voltage is applied to the anode of the second varactor diode, and a direct current between the anode of the first varactor diode and the cathode of the second varactor diode is received when the television signal is received. The voltage at the connection point is set to be equal to or lower than the second voltage, and the voltage at the DC connection point is set to a voltage between the first voltage and the second voltage when the FM broadcast signal is received.

A voltage dividing circuit for outputting a divided voltage from a connection point of two resistors connected in series with each other; connecting the connection point of the two resistors to the DC connection point;
At the time of M broadcast reception, a voltage larger than the second voltage is applied to the voltage dividing circuit, and the divided voltage is set to be a voltage between the first voltage and the second voltage. .

Further, a switching means for turning on when receiving FM broadcasting is provided, and the second voltage is applied to the voltage dividing circuit via the switching means when receiving FM broadcasting.

Further, the mixer is constituted by a balanced mixer having two output terminals, and the intermediate frequency amplifier is constituted by a balanced intermediate frequency amplifier having two input terminals. Wherein the two ends of the intermediate frequency tuning circuit are connected between two output terminals of the balanced mixer, and one input terminal of the balanced intermediate frequency amplifier is connected to the intermediate frequency tuning via one of the coupling capacitance means. Connect to one end of the circuit,
The other input terminal of the balanced intermediate frequency amplifier was connected to the other end of the intermediate frequency tuning circuit via the other of the coupling capacitance means.

The balanced intermediate frequency amplifier generates a DC bias voltage at the two input terminals according to the supplied power supply voltage, and uses the first voltage as the power supply voltage to supply the balanced intermediate frequency amplifier to the balanced intermediate frequency amplifier. And the respective anodes of the second varactor diode in the coupling capacitance means were respectively connected to two inputs of the balanced intermediate frequency amplifier.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An intermediate frequency circuit of a television tuner according to the present invention will be described with reference to FIGS. 1 is a circuit diagram showing a basic configuration of an intermediate frequency circuit of a television tuner of the present invention, FIG. 2 is an equivalent circuit diagram thereof, FIG. 3 is a tuning characteristic diagram, and FIG. FIG. 5 is a circuit diagram in the case of a balanced type, and FIG. 5 is an equivalent circuit diagram thereof.

In FIG. 1, a television signal or an FM broadcast signal (indicated by RF) and a local oscillation signal (indicated by Lo) are input to a mixer 1. The mixer 1 outputs a television intermediate frequency signal when receiving a television signal, and outputs an FM broadcast signal frequency-converted to an audio intermediate frequency in the television intermediate frequency band when receiving an FM broadcast signal.

The intermediate frequency tuning circuit 2 includes a first varactor diode 2a and a capacitor 2b having one end connected to the cathode of the first varactor diode 2a and the other end grounded.
And two inductance elements 2c and 2d connected in series to each other. A series connection circuit of the inductance elements 2c and 2d is connected in parallel to a series connection circuit of the first varactor diode 2a and the capacitor 2b. Thus, a parallel tuning circuit is formed. Inductance elements 2c and 2d
A first voltage Vb1 is applied to a connection point between the first varactor diode 2a and the first voltage Vb1, and this voltage is supplied to the cathode of the first varactor diode 2a via the inductance element 2c.

The connection point between the cathode of the first varactor diode 2a and the inductance element 2c is connected to the output terminal of the mixer 1 and to the coupling capacitance means 3. The coupling capacitance means 3 is connected to the input terminal of the intermediate frequency amplifier 5 via the DC cut capacitor 4. The coupling capacitance means 3 includes a coupling capacitor 3a and a second varactor diode 3b connected in series with each other. The anode of the second varactor diode 3b is connected to the DC cut capacitor 4. The impedance of the DC cut capacitor 4 is negligible.

The anode of the first varactor diode 2a and the second varactor diode 3b are DC-connected via two series-connected resistors 6 and 7 for blocking high frequency. Further, the anode of the second varactor diode 3b is connected to the connection point of the resistors 8 and 9 connected in series. The resistors 8 and 9 divide the first voltage. The first voltage is applied to one resistor 8 and the other resistor 9 is grounded. Then, the divided voltage appearing at the connection point between the resistors 8 and 9 becomes the second voltage Vb2.

A connection point between the two series-connected resistors 6 and 7 is connected to a connection point between two resistors 10 a and 10 b constituting the voltage dividing circuit 10. One point of the resistor 10b is grounded, and one end of the resistor 10a is connected to the switching means 11. The switching means 11 has at least two terminals 11a and 11b, and the two terminals 11a and 11b are switched on (conducting) or off (non-conducting), one end of the resistor 10b is connected to the terminal 10a, and the terminal 11b Is applied with a first voltage. In the drawing, there is another terminal 11c,
When terminals a and 11b are off, the terminals 11a and 11c
And are turned on. The terminal 11c is a resistor 12
Grounded. The terminal 11c and the resistor 12 are not always necessary. Then, the divided voltage (that is, the divided voltage in the voltage dividing circuit 10 when the terminals 11a and 11b are turned on)
The values of the resistors 8, 9, 10a, and 10b are set so that the voltage at the connection point between the resistors 10a and 10b) is higher than the second voltage at the connection point between the resistors 8 and 9.

In the above configuration, if the output impedance of the mixer 1 is Zo and the input impedance of the intermediate frequency amplifier 5 is Zi, the entire equivalent circuit is as shown in FIG. Then, when receiving the television signal, the connection between the terminals 11a and 11b of the switching means 11 is turned off. Then
A current flows in the second varactor diode 3b in the forward direction and is turned on, and a first reverse bias voltage is applied to the first varactor diode 2a. As a result, the capacitance value of the coupling capacitance means 3 becomes only the capacitance value of the coupling capacitor 3a and the capacitance value becomes large, and the tuning frequency of the intermediate frequency tuning circuit 2 is, for example, the And the audio intermediate frequency (preferably between the video intermediate frequency P and the color subcarrier frequency C). The tuning characteristics in this state are shown in FIG. The tuning characteristic is observed, for example, at the input end of the intermediate frequency amplifier 5.

On the other hand, when receiving an FM broadcast signal, the switching means 1
One terminal 11a and 11b are turned on. Then, the divided voltage of the voltage dividing circuit 10 is lower than the first voltage and higher than the second voltage. Then, a second reverse bias voltage is applied to the first varactor diode 2a, and a third reverse bias voltage is applied to the second varactor diode 3b. The second reverse bias voltage is lower than the first reverse bias voltage.

As a result, although the capacitance value of the first varactor diode increases and the tuning frequency of the intermediate frequency tuning circuit 2 decreases, the frequency coincides with the audio intermediate frequency in the television intermediate frequency band. ing.
On the other hand, when the reverse bias voltage is also applied to the second varactor diode 3b, the capacitance value of the entire coupling capacitance means 3 becomes smaller than the capacitance value of the coupling capacitor 3a alone.

The decrease in the capacitance value of the coupling capacitance means 3 means that the input impedance of the intermediate frequency amplifier 5 as a load connected to the intermediate frequency tuning circuit 2 has apparently increased. Therefore, the overall load Q including the output impedance of the mixer 1 and the input impedance of the intermediate frequency amplifier 5 is increased and the loss is reduced, and the tuning characteristic becomes steep as shown in FIG. The peak point becomes higher. Actually, it becomes higher by about 4 dB than the peak point of the tuning characteristic at the time of receiving the television signal.

The intermediate frequency circuit of the television tuner of the present invention can be constituted by a balanced type circuit as shown in FIG. In FIG. 4, the mixer 21 and the intermediate frequency amplifier 22 are each constituted by a balanced circuit. A first voltage is applied to the intermediate frequency amplifier 22 as a power supply voltage. Then, both ends of the intermediate frequency tuning circuit 2 are connected between two output terminals of the mixer 21. Both ends of the intermediate frequency tuning circuit 2 are connected to two input terminals of the intermediate frequency amplifier 22 by coupling capacitance means 3 and 3, respectively, and each anode of the second varactor diodes 3b and 3b is directly connected to the intermediate frequency amplifier 22. Connected to the two input terminals. Then, the cathodes of the second varactor diodes 3b, 3b are connected to a resistor 7 by high-frequency blocking resistors 6, 6, respectively. An equivalent circuit diagram of the above configuration is shown in FIG.

Since the intermediate frequency amplifier 22 is provided with a bias voltage for an amplifying element at its input terminal by the applied power supply voltage, the bias voltage is used as a second voltage as the second varactor diode 3b. , 3b. The operation of the intermediate frequency circuit of the above-described balanced television tuner is the same as the operation of the configuration shown in FIG. 1, and a description thereof will be omitted.

[0029]

As described above, the mixer and the intermediate unit connected to the mixer which tune between the video intermediate frequency and the audio intermediate frequency when receiving the television signal and tune to the audio intermediate frequency when receiving the FM broadcast signal. A frequency tuning circuit, and an intermediate frequency amplifier coupled to the intermediate frequency tuning circuit via the coupling capacitance means, wherein the capacitance value of the coupling capacitance means is switched between a large value and a small value, and the television signal reception is performed. Sometimes, the capacitance value of the coupling capacitance means is increased, and when the FM broadcast signal is received, the capacitance value is decreased, so that the input impedance of the intermediate frequency amplifier as a load connected to the intermediate frequency tuning circuit becomes apparently large, and therefore, the output impedance of the mixer is increased. The overall load Q including the input and the input impedance of the intermediate frequency amplifier increases, and the loss decreases.
The tuning characteristic becomes steep and the peak point becomes higher. As a result, when receiving an FM broadcast signal, an adjacent FM
There is no interference with the broadcast signal, and the receiving sensitivity is improved.

Further, the intermediate frequency tuning circuit is constituted by a parallel tuning circuit, a first varactor diode is provided in the parallel tuning circuit, and the coupling capacitance means is constituted by a capacitor and a second varactor diode connected in series to each other. At the time of receiving a television signal, a first reverse bias voltage is applied to the first varactor diode, and the second varactor diode is turned on. At the time of receiving an FM broadcast signal, the first varactor diode receives a first reverse bias voltage from the first varactor diode. Since the second reverse bias voltage is also low and the third reverse bias voltage is applied to the second varactor diode, switching between the tuning frequency and the capacitance value of the coupling capacitance means when the reception signal changes is easy. Become.

Further, the anode of the first varactor diode and the cathode of the second varactor diode are connected in a DC manner, a first voltage is applied to the cathode of the first varactor diode, and A second voltage lower than the first voltage is applied to the anode of the first varactor diode, and the voltage of the DC connection point between the anode of the first varactor diode and the cathode of the second varactor diode is changed to the second voltage when receiving a television signal. And the voltage at the DC connection point is set to a voltage between the first voltage and the second voltage when the FM broadcast signal is received, so that the voltage is applied to the first varactor diode and the second varactor diode. Switching can be linked to the reverse bias voltage. Therefore, the configuration is simple.

A voltage dividing circuit for outputting a divided voltage from a connection point between two resistors connected in series with each other is provided. The connection point between the two resistors is connected to the anode of the first varactor diode and the second varactor. Connected to the direct current connection point between the diode and the cathode, apply a voltage larger than the second voltage to the voltage dividing circuit at the time of FM broadcast reception, and divide the divided voltage into the first voltage and the second voltage. The voltage at the connection point is switched by setting the resistance of the voltage dividing circuit.

Further, since switching means for turning on the FM broadcast is provided, and the second voltage is applied to the voltage dividing circuit via the switching means at the time of receiving the FM broadcasting, there is no need to separately provide a voltage to be applied to the voltage dividing circuit. Is also good.

Further, a balanced mixer and a balanced intermediate frequency amplifier are used, both ends of the intermediate frequency tuning circuit are connected between two output terminals of the balanced mixer, and one input terminal of the balanced intermediate frequency amplifier is connected. Because one end of the intermediate frequency tuning circuit was connected to one end of the coupling capacitance means and the other input terminal of the balanced intermediate frequency amplifier was connected to the other end of the intermediate frequency tuning circuit through the other end of the coupling capacitance means. Type intermediate frequency circuit can be easily configured.

The balanced intermediate frequency amplifier generates a DC bias voltage at two input terminals according to the supplied power supply voltage, supplies the first voltage as the power supply voltage to the balanced intermediate frequency amplifier, and Since the respective anodes of the second varactor diode in the capacitance means are respectively connected to the two input terminals of the balanced intermediate frequency amplifier, the DC cut capacitor provided between the coupling capacitance means and the intermediate frequency amplifier can be eliminated.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a basic configuration of an intermediate frequency circuit of a television tuner of the present invention.

FIG. 2 is an equivalent circuit diagram of an intermediate frequency circuit of the television tuner shown in FIG.

FIG. 3 is a tuning characteristic diagram in the intermediate frequency circuit of the television tuner of the present invention.

FIG. 4 is a circuit diagram in a case where an intermediate frequency circuit of the television tuner of the present invention is configured as a balanced type.

5 is an equivalent circuit diagram of the intermediate frequency circuit of the television tuner shown in FIG.

FIG. 6 is a circuit diagram showing a configuration of an intermediate frequency circuit of a conventional television tuner.

FIG. 7 is a tuning characteristic diagram of an intermediate frequency circuit of a conventional television tuner.

[Explanation of symbols]

 1, 21 Mixer 2 Intermediate frequency tuning circuit 2a First varactor diode 2b Capacitor 2c, 2d Inductance element 3 Coupling capacitance means 3a Coupling capacitor 3b Second varactor diode 4 DC cut capacitor 5,22 Intermediate frequency amplifier 6,7 High frequency Blocking resistance 8, 9 Resistance 10 Voltage dividing circuit 10a, 10b Resistance 11 Switching means 11a, 11b, 11c Terminal 12 Resistance

Claims (7)

[Claims] A mixer for outputting an intermediate frequency signal of a television when receiving a television signal, and outputting an FM broadcast signal frequency-converted to an audio intermediate frequency in an intermediate frequency band of the television when receiving an FM broadcast signal; Connected to the mixer, tuned between the video intermediate frequency and the audio intermediate frequency in the intermediate frequency band when the television signal is received, and tuned to the audio intermediate frequency in the intermediate frequency band when the FM broadcast signal is received. An intermediate frequency tuning circuit, comprising an intermediate frequency amplifier coupled to the intermediate frequency tuning circuit via coupling capacitance means, configured to switch the capacitance value of the coupling capacitance means to two types of large or small, That the capacitance value of the coupling capacitance means is increased when receiving a television signal and reduced when receiving the FM broadcast signal. Characteristic intermediate frequency circuit of television tuner. 2. An intermediate frequency tuning circuit comprising a parallel tuning circuit, wherein the parallel tuning circuit is provided with a first varactor diode for changing the tuning frequency, wherein the coupling capacitance means includes a capacitor connected in series with a capacitor connected in series with each other. The second varactor diode is configured to apply a first reverse bias voltage to the first varactor diode at the time of receiving the television signal, and to make the second varactor diode conductive, at the time of receiving the FM broadcast signal. A second reverse bias voltage lower than the first reverse bias voltage is applied to the first varactor diode, and a third reverse bias voltage is applied to the second varactor diode. Item 2. An intermediate frequency circuit of the television tuner according to Item 1. 3. An anode of the first varactor diode and a cathode of the second varactor diode are DC-connected, a first voltage is applied to the cathode of the first varactor diode, and the A second voltage lower than the first voltage is applied to the anode of the second varactor diode, and a direct current between the anode of the first varactor diode and the cathode of the second varactor diode is received when the television signal is received. The voltage at the connection point is not more than the second voltage, and the voltage at the DC connection point is a voltage between the first voltage and the second voltage when the FM broadcast signal is received. The intermediate frequency circuit of a television tuner according to claim 2, wherein 4. A frequency dividing circuit for outputting a divided voltage from a connection point of two resistors connected in series with each other, connecting the connection point of the two resistors to the DC connection point, At the time of reception, a voltage larger than the second voltage is applied to the voltage dividing circuit, and the divided voltage is set to be a voltage between the first voltage and the second voltage. The intermediate frequency circuit of a television tuner according to claim 3, wherein: 5. The television according to claim 4, further comprising switching means for turning on when receiving FM broadcasting, wherein the second voltage is applied to the voltage dividing circuit via the switching means when receiving FM broadcasting. John Tuner's intermediate frequency circuit. 6. The mixer comprises a balanced mixer having two output terminals, the intermediate frequency amplifier comprises a balanced intermediate frequency amplifier having two input terminals, and comprises two coupling capacitance means. Wherein the two ends of the intermediate frequency tuning circuit are connected between two output terminals of the balanced mixer, and one input terminal of the balanced intermediate frequency amplifier is connected to the intermediate frequency tuning via one of the coupling capacitance means. 6. The circuit according to claim 2, wherein the other input terminal of the balanced intermediate frequency amplifier is connected to the other end of the intermediate frequency tuning circuit via the other one of the coupling capacitance means. An intermediate frequency circuit of the television tuner according to any one of the above. 7. The balanced intermediate frequency amplifier generates a DC bias voltage at the two input terminals according to a supplied power supply voltage, and uses the first voltage as the power supply voltage for the balanced intermediate frequency amplifier. 7. The intermediate frequency circuit of a television tuner according to claim 6, wherein each of the anodes of the second varactor diode in the coupling capacitance means is connected to two input terminals of the balanced intermediate frequency amplifier.