JP2002010154A - Television tuner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce any disturbing signal generated in a mixer, and to reduce the leakage of a local oscillation signal. SOLUTION: This device is provided with a differential mixer having two transistors to which a local oscillation signal and a received television signal are inputted, and an intermediate frequency amplifier for amplifying an intermediate frequency signal outputted from the mixer. A signal attenuating means is arranged at the output side of the intermediate frequency amplifier. Then, the local oscillation signal and the television signal are inputted to the base of one of those two transistors, and the base of the other transistor is grounded like high frequencies, and a fixed bias voltage is applied to the base of the other transistor, and the same voltage as the fixed bias voltage or a voltage different from the fixed bias voltage is applied to the base of one transistor. At the time of receiving the television signal at a specific channel, the different voltage is applied to the base of one transistor, and the attenuating frequencies of the signal attenuating means are made coincident with the frequencies of the local oscillation signal at the specific channel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television tuner in which distortion generated in a mixer is reduced and leakage of a local oscillation signal is reduced.

[0002]

2. Description of the Related Art As shown in FIG. 2, a conventional television tuner includes a mixer 31, a local oscillator 32, and an intermediate frequency amplifier 33. The mixer 31 includes various tuning circuits and high frequency amplifiers (both shown in FIG. 2). (Not shown), a desired television signal is input. Further, the local oscillation signal output from the local oscillator 32 is also input. And the mixer 3
The television signal input to 1 is mixed with the local oscillation signal, frequency-converted to an intermediate frequency signal, and output from the mixer 31. The intermediate frequency signal is amplified by the intermediate frequency amplifier 33.

[0005] A mixer 31 is usually a mixing transistor (not shown) having a fixed bias voltage applied to its base.
, And a television signal and a local oscillation signal are input to the base thereof. Then, the frequency of the television signal is converted by the nonlinear characteristic (secondary distortion) of the mixing transistor.

[0004]

The mixer originally utilizes the second-order distortion characteristic, but the mixing transistor has higher-order distortion in addition to the second-order distortion. Therefore, when other signals (not limited to television signals) are input to the mixer in addition to the television signal to be received, a plurality of unwanted signals other than the intermediate frequency signal required by the mixer are output. Occurs, and when these signals are generated in the frequency band of the intermediate frequency signal, they become interference signals.

For example, in a Chinese television system, the video carrier frequency of channel 3 is 65.75 MHz.
z (megahertz), local oscillation frequency is 103.75 MH
z, the audio intermediate frequency is 31.5 MHz, and the video intermediate frequency is 38.0 MHz.
When an undesired signal of 73.75 MH is input, an undesired signal of 35.75 MHz is generated due to the third-order distortion of the mixer. Since this signal is generated in the intermediate frequency band, it becomes an interference signal. The above is only an example,
Such an interference signal is generated when an unspecified channel is received due to a difference in the frequency of the undesired signal input to the mixer.

The magnitude of the generated interference signal is determined by the non-linear characteristics of the mixing transistor used and the operating point given to the mixing transistor. Unless changed, it was impossible to eliminate the interference signal or reduce its level.

[0007] Therefore, it is an object of the television tuner of the present invention to reduce the interference signal generated in the mixer and to reduce the leakage of the local oscillation signal.

[0008]

In order to solve the above-mentioned problems, a television tuner according to the present invention has two transistors, and has a differential type in which a local oscillation signal and a received television signal are inputted. A mixer, an intermediate frequency amplifier for amplifying an intermediate frequency signal output from the mixer, a signal attenuating means provided on an output side of the intermediate frequency amplifier, and a base for one of the two transistors. While inputting the local oscillation signal and the television signal, grounding the base of the other transistor at high frequency, applying a fixed bias voltage to the base of the other transistor, and applying the fixed bias voltage to the base of the one transistor. The same voltage as the fixed bias voltage or a voltage different from the fixed bias voltage can be applied, And the attenuation frequency of the signal attenuation means and applies the different voltages to the base of the one transistor to match the frequency of the local oscillation signal of the specific channel when receiving a television signal of the channel.

Further, a series circuit of a resistor and a switch is provided between the base of the one transistor and the ground, and the switch is turned on when receiving the television signal of the specific channel.

The signal attenuating means includes an inductance element, a capacitor, and a switch diode connected in series to the capacitor. The capacitor and the switch diode connected in series are connected in parallel to the inductance element. Connected, the switch diode is turned on when receiving the television signal of the specific channel, and the parallel resonance frequency of the inductance element and the capacitor is made to match the local oscillation frequency of the specific channel.

The collector of the NPN transistor is connected to the cathode of the switch diode.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A television tuner according to the present invention will be described below with reference to FIG. 1. A television signal received by an antenna (not shown) passes through a tuning circuit and a high-frequency amplifier (neither is shown). Then, a desired television signal is input to the mixer 1. The local oscillation signal output from the local oscillator 2 is also input to the mixer 1.

The mixer 1 includes two transistors 1a, 1
b, is configured as a differential type mixer. A bias voltage is applied to the base of the transistor 1a by bias resistors 1c and 1d, and a bias voltage is applied to the base of the transistor 1b by bias resistors 1e and 1f. . The bias voltage applied to each base is the same as each other. Then, the respective emitters are connected via the resistor 1g. A constant current source is connected to each emitter.

The base of one transistor is connected via a resistor 3 to an NPN transistor 4 serving as a switch means.
Connected to the collector. It is desirable that the resistance value of the resistor 3 can be changed. The emitter of NPN transistor 4 is grounded. The voltage of the external power supply 6 is applied to the base of the NPN transistor 4 by the changeover switch 5. Load resistors 1h and 1i are connected to the collectors of the transistors 1a and 1b, respectively.

A television signal and a local oscillation signal desired to be received are input to the base of one transistor 1a, and the base of the other transistor 1b is grounded at high frequency by a DC cut capacitor 1j. And N
The balanced intermediate frequency signals are output from the collectors of the PN transistors 1a and 1b. Accordingly, the mixer 1 operates as a mixer having an unbalanced input and a balanced output.

The intermediate frequency signal output from the mixer 1 is amplified by an intermediate frequency amplifier 7. Intermediate frequency amplifier 7
Has a main amplifier 7a and an emitter follower transistor 7b serving as a buffer amplifier. The emitter of the transistor 7b is DC grounded by a resistor 7c.

A trap circuit 8 functioning as signal attenuating means is connected to the emitter of the transistor 7b, and an intermediate frequency signal is output to an output terminal 9 via the trap circuit 8.
The trap circuit 8 includes an inductance element 8a connected between the emitter of the transistor 7b and the output terminal 9, a switch diode 8b having an anode connected to the emitter of the transistor 7b, a cathode of the switch diode 8b and the output terminal 9. And a capacitor 8c connected between them. Then, the cathode of the switch diode 8b is connected to the collector of the NPN transistor 4.

In the above configuration, normally, the external bias power supply 6 is not connected to the base of the NPN transistor 4 and is turned off, and the same bias voltage is applied to each base of the transistors 1a and 1b. Therefore, the voltage between the collector and the emitter of the transistor 1a and the voltage between the collector and the emitter of the transistor 1b are equal to each other. Also, the collector currents of the two are equal by the constant current source. Therefore, the DC operating point of the transistor 1a and the DC operating point of the transistor 1b are equal to each other, and a perfect balanced operation is possible. Then, the desired television signal input to the base of the transistor 1a is mixed with the local oscillation signal and frequency-converted into an intermediate frequency signal.

At this time, since the transistors 1a and 1b operate in a balanced manner, the input television signal and local oscillation signal are phase-cancelled, so that the mixer 1 outputs their second harmonics (accurately). Even harmonics appearing between the collectors of the two transistors 1a and 1b)
Is not output. Therefore, in the circuits subsequent to the next stage, the second harmonic does not interfere. Also, since the level of the third and higher harmonics is low, output from the tuner does not matter. Since the switch diode 8b is turned off, the intermediate frequency signal is output through the inductance element 8a.

By the way, in addition to the television signal of the channel to be received (desired television signal), there may be a case where the undesired television signal is input to the mixer 1 at all. In such a case, a desired television signal, an undesired television signal, and a local oscillation signal on a reception channel are input to the mixer 1. Then, a new signal due to these three signals may be generated due to the third- and subsequent-order distortion characteristics of the transistors 1a and 1b. Then, if a new signal is generated in the intermediate frequency band, this will be a hindrance.

On the other hand, in general, distortion due to non-linear characteristics of a transistor changes depending on a difference in a DC operating point of the transistor. That is, when the collector current or the voltage between the collector and the emitter changes, the distortion also changes.

Therefore, when the above-mentioned new signal (referred to as a disturbing signal) is generated, the voltage of the external power supply 6 is applied to the base of the NPN transistor 4 to turn it on. At this time, the switch diode 8b is also turned on. NPN
When the transistor 4g is turned on, the base of the transistor 1a is grounded via the resistor 3, so that its voltage decreases, and the collector current of the transistor 1a and the transistor 1a
The directions of increase and decrease with respect to the collector current b are opposite to each other. When the two collector currents have different values, the directions of the voltage increase and decrease between the respective collectors and the emitters are opposite, and the values are different. That is, the DC operating point is different.

Since the amount of change between the current and the voltage is adjusted by the resistor 3, the bias voltage of the transistor 1a is set so as to be an operating point at which the level of the disturbing signal in question becomes minimum.

In this way, the level of the interference signal output from the mixer 1 can be reduced, but the equilibrium state of the mixer 1 itself is broken. As a result, the input local oscillation signal is output from the mixer 1 without being completely phase canceled. Therefore, if the parallel resonance frequency of the inductance element 8 and the capacitor 8c of the trap circuit 8 is made to match the local oscillation frequency of the television channel to be received,
Since the local oscillation signal leaked from the intermediate frequency amplifier 7 is attenuated, it does not appear at the output terminal 9.

If the capacitor 8c is replaced with a varactor diode so that the voltage applied to both ends of the varactor diode can be changed, the local oscillation signal leaked to other channels where an interference signal occurs can be attenuated.

[0026]

As described above, the television tuner of the present invention includes a differential mixer having two transistors and an intermediate frequency amplifier for amplifying an intermediate frequency signal.
A signal attenuating means is provided on the output side of the intermediate frequency amplifier, a local oscillation signal and a television signal are input to the base of one of the two transistors, and the base of the other transistor is grounded at high frequency. A fixed bias voltage is applied to the base of one of the transistors, and the same or a different voltage as the fixed bias voltage can be applied to the base of one of the transistors. Since the voltage different from the fixed bias voltage is applied to the base of the transistor and the attenuation frequency of the signal attenuator is made to match the frequency of the local oscillation signal of the specific channel, the operating point of the two transistors of the mixer is changed. Can reduce the level of interfering signals generated by the mixer and By local oscillator signal can be attenuated.

A series circuit of a resistor and switch means is provided between the base of one transistor and the ground,
Since the switch is turned on when receiving a television signal of a specific channel, the operating point of the two transistors can be changed by changing the base voltage of one of the transistors.

Further, a capacitor and a switch diode connected in series in the signal attenuating means are connected in parallel to the inductance element, and the switch diode is turned on when receiving a television signal of a specific channel,
Since the parallel resonance frequency of the inductance element and the capacitor is matched with the local oscillation frequency of the specific channel, the local oscillation signal can be attenuated.

Further, since the collector of the NPN transistor is connected to the cathode of the switch diode, the change of the operating point of the two transistors of the mixer can be linked to the attenuation of the pole transmission signal by the signal attenuating means.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of a conventional television tuner.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mixer 1a, 1b Transistor 1c, 1d, 1e, 1f Bias resistance 1g Resistance 1h, 1i Load resistance 1j DC cut capacitor 2 Local oscillator 3 Resistance 4 NPN transistor (switch means) 5 Changeover switch 6 External power supply 7 Intermediate frequency amplifier 7a Main amplifier 7b Buffer amplifier 8 Signal attenuating means 8a Inductance element 8b Switch diode 8c Capacitor 9 Output terminal

Claims (4)

[Claims] 1. A differential mixer having two transistors and receiving a local oscillation signal and a received television signal, and an intermediate frequency amplifier amplifying an intermediate frequency signal output from the mixer. And a signal attenuating means is provided on the output side of the intermediate frequency amplifier, and the local oscillation signal and the television signal are input to a base of one of the two transistors,
The base of the other transistor is grounded at high frequency, a fixed bias voltage is applied to the base of the other transistor, and the same voltage as the fixed bias voltage or the fixed bias voltage is applied to the base of the one transistor. Are configured to be able to apply different voltages, and when receiving a television signal of a specific channel, apply the different voltage to the base of the one transistor and change the attenuation frequency of the signal attenuating means to the local oscillation of the specific channel. A television tuner characterized by matching the frequency of a signal. 2. A series circuit of a resistor and switch means is provided between a base of the one transistor and ground,
2. The television tuner according to claim 1, wherein the switch is turned on when receiving the television signal of the specific channel. 3. The signal attenuating means includes an inductance element, a capacitor, and a switch diode connected in series with the capacitor, and the capacitor and the switch diode connected in series are connected in parallel with the inductance element. Connected, the switch diode is turned on when receiving the television signal of the specific channel, and the parallel resonance frequency of the inductance element and the capacitor is matched with the local oscillation frequency of the specific channel. Claim 1
Or the television tuner according to 2. 4. The television tuner according to claim 3, wherein a collector of said NPN transistor is connected to a cathode of said switch diode.