JP2002076966A - Television tuner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the gain of the whole when a television signal is at a low level, and spread the gain variable range. SOLUTION: A television tuner comprises: a variable gain amplifier 2 which amplifies a television signal and changes the gain by an AGC voltage in correspondence to a level of the television signal, a mixer 4 for converting the television signal amplified by the variable gain amplifier 2 to a signal of a intermediate frequency band, and an intermediate frequency output tuning circuit 7 which passes the signal of the intermediate frequency band and exchanges a tuning frequency. When the television signal to be input into the variable gain amplifier 2 is at a predetermined level or more, the intermediate frequency turning circuit 7 is tuned inside the band of the intermediate frequency band, and at the predetermined level or less, it is turned outside the band of the intermediate frequency band.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television tuner having a wide gain change range due to an AGC voltage and increasing the gain when the electric field strength of a television signal is weak.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram showing a schematic configuration of a conventional television tuner. The television signal (TV signal) input to the input terminal 21 is selected by a tuning circuit (not shown) provided on the way and then amplified by a variable gain amplifier 22 which is a high-frequency amplifier. On the other hand, an AGC voltage output from an AGC voltage generation circuit provided in a television receiver main body (not shown) is input to a variable gain amplifier 22 via an AGC terminal 23. The magnitude of the AGC voltage corresponds to the level of the television signal, and the gain of the variable gain amplifier 22 is controlled by the AGC voltage.

[0003] The television signal amplified by the variable gain amplifier 22 is input to a mixer 24 and frequency-converted into a signal in an intermediate frequency band. The signal in the intermediate frequency band passes through an intermediate frequency tuning circuit 25, is amplified by an intermediate frequency amplifier 26, and is further input to an intermediate frequency output tuning circuit 27.

The intermediate frequency output tuning circuit 27 includes a first capacitor 27a and an inductance element 27b inserted in series between the intermediate frequency amplifier 26 and an output terminal 28, and a connection point between the inductance element 27b and the output terminal 28. And a second capacitor 27c connected between the ground and the ground. The tuning frequency of this series tuning circuit is adjusted for the purpose of making the output level appropriate for the input signal level (for example, when the input level is 60 dBμV, the output level is 97 bdBμV).
Is set to be outside the band of the intermediate frequency band.

In the above configuration, when the level of the television signal is extremely low, for example, 40 A
When the voltage is B μV (microvolt), the voltage is approximately 5 volts, and the variable gain amplifier 22 operates at the maximum gain.
On the other hand, as the level of the television signal increases, the AGC voltage decreases to approximately 0.5 volt, and the variable gain amplifier 22 operates at the minimum gain (actually, a negative gain) at this time. Then, the gain change of the variable gain amplifier 22 becomes the entire gain change from the input terminal 21 to the output terminal 28 as it is. As shown in FIG. 4, the gain change characteristic is approximately 37 dB when the AGC voltage is 5 volts.
, And becomes approximately minus 16 dB at 0.5 volt. Therefore, the gain variable range by the AGC voltage is approximately 53
dB.

[0006]

However, in the conventional television tuner, the gain variable range is insufficient, and distortion when the level of the television signal is increased becomes a problem. In addition, when the level of the television signal is low, However, there is a problem that the sensitivity becomes insufficient because of the small gain.

Accordingly, it is an object of the television tuner of the present invention to increase the overall gain when the level of the television signal is low and to widen the variable gain range.

[0008]

In order to solve the above-mentioned problems, a television tuner according to the present invention amplifies a television signal and changes a gain by an AGC voltage corresponding to the level of the television signal. An amplifier, a mixer that converts the television signal amplified by the variable gain amplifier into a signal in an intermediate frequency band, and an intermediate frequency output tuning circuit that passes the signal in the intermediate frequency band and that is capable of switching a tuning frequency. With
When the television signal input to the variable gain amplifier is equal to or higher than a predetermined level, the intermediate frequency tuning circuit is tuned out of the intermediate frequency band, and when the television signal is equal to or lower than the predetermined level, it is tuned in the intermediate frequency band. .

The intermediate frequency output tuning circuit includes a varactor diode to which a signal in the intermediate frequency band is input, an inductance element connected between the other end of the varactor diode and an output terminal, the output terminal and ground. And a capacitor connected between the varactor diode, and tunes the series tuning circuit within the band of the intermediate frequency band when the varactor diode is on, and tunes the band outside the band of the intermediate frequency band when the varactor diode is off. I let it.

A bias transistor is applied to the anode of the varactor diode, the emitter is grounded, and the collector is connected via a resistor to a power supply having a voltage higher than the bias voltage via a resistor. The collector of the transistor was connected to the cathode of the varactor diode, and the switch transistor was turned on when the television signal was below the predetermined level, and turned off when it was above the predetermined level.

The AGC voltage is changed in a direction opposite to a direction in which the level of the television signal is increased or decreased, and
A voltage dividing circuit to which the C voltage is input is provided, and a divided voltage output from the voltage dividing circuit is input to a base of the switch transistor, and the divided voltage corresponds to the predetermined level of the television signal. The voltage was set to turn on the switch transistor at a predetermined AGC voltage.

Also, an intermediate frequency amplifier having an emitter follower transistor is provided at a stage preceding the intermediate frequency output tuning circuit, an emitter of the emitter follower transistor is connected to an anode of the varactor diode, and a collector of the switch transistor and the emitter follower are connected. The collector of the transistor was connected to a common voltage source.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A television tuner according to the present invention will be described below with reference to FIG. As for the television signal (TV signal) input to the input terminal 1, a television signal to be received is selected by a tuning circuit (not shown) provided on the way, and is input to a variable gain amplifier 2 which is a high frequency amplifier. Since the variable gain amplifier 2 uses a dual gate FET (not shown), the television signal is input to its first gate. On the other hand, the AGC terminal 3 is supplied with an AGC voltage output from an AGC voltage generation circuit provided in a television receiver body (not shown). This A
The GC voltage is input to the second gate of the FET. The magnitude of the AGC voltage corresponds to the level of the received television signal, and the gain of the variable gain amplifier 2 is controlled by the AGC voltage.

The television signal amplified by the variable gain amplifier 2 is input to a mixer 4 and frequency-converted into a signal in an intermediate frequency band. After passing through the intermediate frequency tuning circuit 5, the signal in the intermediate frequency band is amplified by the intermediate frequency amplifier 6 and further input to the intermediate frequency output tuning circuit 7.

The intermediate frequency amplifier 6 has a first-stage amplifier transistor 6a of a common-emitter type and a second-stage amplifier transistor 6b of an emitter-follower type. The collector of each transistor is connected to a voltage source.

The intermediate frequency output tuning circuit 7 includes a varactor diode 7a and an inductance element 7b inserted in series between the emitter 6 of the amplification transistor 6b and the output terminal 8.
And a series tuning circuit including a capacitor 7c connected between the connection point between the inductance element 7b and the output terminal 8 and the ground. The anode of the varactor diode 7a is connected to the emitter of the transistor 6b and a bias voltage is applied. The tuning frequency of the series tuning circuit is set so as to be within the band of the intermediate frequency band when the varactor diode is turned on.

On the other hand, the AGC voltage is connected to a resistor 9 connected in series.
a, 9b. Voltage divider 9
Outputs a divided voltage from a connection point between the resistors 9a and 9b, and this connection point is connected to the base of the switch transistor 10. The collector of the switch transistor 10 is connected to a voltage source via a voltage drop resistor 11, and the emitter is grounded. Then, the collector of the switch transistor 10 is connected to the cathode of the varactor diode 7a via the resistor 12.

On the other hand, the AGC voltage has a maximum voltage of about 5 volts when the level of the television signal is extremely low, for example, about 40 dBμV (microvolt), and becomes about 0.5 volts as the level of the television signal increases. To the minimum voltage. The value of the resistors 9a and 9b of the voltage dividing circuit 9 is determined by the AGC voltage (for example, 4 volts) when the level of the television signal falls below a certain predetermined level.
Is set to turn on.

In the above configuration, the variable gain amplifier 2
Is the maximum gain when the AGC voltage is maximum,
The minimum gain is obtained when the voltage is minimum. On the other hand, the switch transistor 10 has a predetermined value (approximately 4 volts here) corresponding to the predetermined level of the AGC voltage corresponding to the predetermined level of the television signal.
Turns on when the above is reached. Then, the varactor diode 7a is also turned on. As a result, the intermediate frequency output tuning circuit 7 tunes in the intermediate frequency band. When the AGC voltage is lower than 4 volts, the switch transistor 10 is turned off. Then, the collector voltage becomes higher than the emitter voltage of the emitter follower transistor 6b.
The varactor diode 7a is turned off, and the intermediate frequency output tuning circuit 7 tunes out of the intermediate frequency band.

Therefore, when the AGC voltage becomes approximately 4 volts or more, the loss of the intermediate frequency signal passing through the intermediate frequency output tuning circuit 7 is reduced, and the gain of the variable gain amplifier 2 is maximized, so that the overall gain is increased. I do. As shown in FIG. 2, when the AGC voltage is 4 volts or more, the gain increases due to the loss reduction of the intermediate frequency output tuning circuit 7 (A in FIG. 2).
Part), the overall gain change characteristic from the input terminal 1 to the output terminal 8 is approximately 48 dB. When the AGC voltage is 0.5
It is approximately minus 16 dB for bolts. Therefore, AG
The variable gain range by the C voltage is widened to approximately 64 dB.

[0021]

As described above, according to the television tuner of the present invention, the AGC corresponding to the level of the television signal is performed.
A variable gain amplifier whose gain changes according to a voltage, a mixer for converting a television signal amplified by the variable gain amplifier into an intermediate frequency band signal, and an intermediate frequency output tuning circuit for passing the intermediate frequency band signal. When the television signal is equal to or higher than a predetermined level, the intermediate frequency tuning circuit is tuned out of the intermediate frequency band, and when the television signal is equal to or lower than the predetermined level, the intermediate frequency tuning circuit is tuned within the intermediate frequency band. The gain is increased and the gain range is widened.

The intermediate frequency output tuning circuit is connected between a varactor diode to which an intermediate frequency band signal is input, an inductance element connected between the other end of the varactor diode and an output terminal, and an output terminal and the ground. Since the series tuning circuit is tuned within the band of the intermediate frequency band when the varactor diode is turned on and tuned out of the band of the intermediate frequency band when turned off, the varactor diode is turned on or off. The tuning frequency can be easily switched only by turning off.

In addition, a bias voltage is applied to the anode of the varactor diode, and a switch transistor is connected to a power supply whose emitter is grounded and whose collector has a higher voltage than the bias voltage via a resistor. Is connected to the cathode of the varactor diode, and the switch transistor is turned on when the television signal is lower than a predetermined level and turned off when the television signal is higher than the predetermined level, so that the varactor diode can be easily turned on or off.

Further, a voltage dividing circuit for changing the AGC voltage in a direction opposite to the direction of increasing and decreasing the level of the television signal and for inputting the AGC voltage is provided, and the divided voltage output from the voltage dividing circuit is switched by a switch transistor. Type in the base of
Since the divided voltage is set to a voltage for turning on the switch transistor at a predetermined AGC voltage corresponding to a predetermined level of the television signal, the transistor is automatically turned on or off according to the AGC voltage.

Further, an intermediate frequency amplifier having an emitter follower transistor is provided in the preceding stage of the intermediate frequency output tuning circuit, the emitter of the emitter follower transistor is connected to the anode of the varactor diode, and the collector of the switch transistor and the collector of the emitter follower transistor are connected. Since the varactor diode is connected to the common voltage source, the varactor diode is automatically turned on or off in conjunction with the operation of the switch transistor.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a television tuner of the present invention.

FIG. 2 is a diagram illustrating a gain change characteristic of the television tuner of the present invention.

FIG. 3 is a circuit diagram showing a configuration of a conventional television tuner.

FIG. 4 is a diagram illustrating gain change characteristics of a conventional television tuner.

[Explanation of symbols]

 Reference Signs List 1 input terminal 2 variable gain amplifier 3 AGC terminal 4 mixer 5 intermediate frequency tuning circuit 6 intermediate frequency amplifier 6a, 6b amplifying transistor 7 intermediate frequency output tuning circuit 7a varactor diode 7b inductance element 7c capacitor 8 output terminal 9 voltage dividing circuit 9a 9b resistance 10 switch transistor 11, 12 resistance

Claims (5)

[Claims] A variable gain amplifier that amplifies a television signal and changes a gain according to an AGC voltage corresponding to a level of the television signal; and converts the television signal amplified by the variable gain amplifier into an intermediate frequency band. And a middle frequency output tuning circuit for passing a signal of the middle frequency band and switching a tuning frequency, wherein the television signal input to the variable gain amplifier is equal to or higher than a predetermined level. In the television tuner, the intermediate frequency tuning circuit is tuned outside the band of the intermediate frequency band, and is tuned within the band of the intermediate frequency band below the predetermined level. 2. The intermediate frequency output tuning circuit includes a varactor diode to which a signal of the intermediate frequency band is input, an inductance element connected between the other end of the varactor diode and an output terminal, the output terminal and ground. It consists of a series tuning circuit consisting of a capacitor connected between
The television tuner according to claim 1, wherein the series tuning circuit is tuned within the band of the intermediate frequency band when the varactor diode is on, and tuned out of the band of the intermediate frequency band when the varactor diode is off. 3. A switch transistor which applies a bias voltage to the anode of the varactor diode, has an emitter grounded, and has a collector connected to a power supply having a voltage higher than the bias voltage via a resistor,
The collector of the switch transistor is connected to the cathode of the varactor diode, and the switch transistor is turned on when the television signal is lower than the predetermined level, and turned off when the television signal is higher than the predetermined level. Item 3. The television tuner according to Item 2. 4. A voltage dividing circuit for changing the AGC voltage in a direction opposite to a direction in which the level of the television signal increases and decreases, and a voltage dividing circuit for inputting the AGC voltage is provided. A voltage is input to a base of the switch transistor, and the divided voltage is set to a voltage for turning on the switch transistor when a predetermined AGC voltage corresponding to the predetermined level of the television signal is set. Item 4. The television tuner according to item 3. 5. An intermediate frequency amplifier having an emitter follower transistor is provided before the intermediate frequency output tuning circuit, an emitter of the emitter follower transistor is connected to an anode of the varactor diode, and a collector of the switch transistor and the emitter follower are connected. 5. The television tuner according to claim 3, wherein a collector of the transistor is connected to a common voltage source.