JPS6121885Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention is a tuner device used in a television receiver, particularly a tuner device that uses a variable capacitance diode as a tuning element and uses a mixer in a VHF tuner section as an amplifier for the output of the UHF tuner section during UHF reception. It is related to.

FIG. 1 shows a conventional example of such a tuner device, in which 1 is a UHF tuner section, 2 is a VHF tuner section, 3 is a VHF RF circuit, 4 is an interstage tuning circuit, 5 is a local oscillation circuit, and 6 is a VHF tuner section. is a mixer. 7,
8 is a tuning coil, and 9 and 10 are variable tuning elements each consisting of a variable capacitance diode, which form the interstage tuning circuit 4. D is a switching diode installed between the output side of the UHF tuner section 1 and the VHF signal path 11, and when receiving UHF, the voltage UB applied from the UHF power supply terminal 12 via the resistor _R2 is applied. conduction [at that time, the conduction current flows to ground through resistor _R3 ]
Then, the IF output of the UHF tuner section 1 is supplied to the mixer 6 (at this time, the mixer 6 works as an amplifier).
At the same time, when receiving VHF, the voltage UB is not applied to the UHF current supply terminal 12, and the direct current flowing from the mixer 6 side to the ground through the resistors R ₁ and R ₃ turns off, and the UHF tuner section 1 side and the VHF tuner section are turned off. The section 2 side is cut off. In FIG. 1, C ₁ , C ₂ , C ₃ , C ₄ , and C ₆ are DC cut capacitors. _I1 is a coil for a low-pass filter, and forms a low-pass filter together with a capacitor provided in the output circuit of the UHF tuner section 1, an input capacitance of a transistor forming the mixer 6, and the like. 13 and 14 are VHF respectively
These are input terminals of the tuner section and the UHF tuner section, and 15 is an output terminal.

In such a conventional tuner device, switching diode D is cut off when receiving VHF, but since it has a reverse capacitance, it
There were disadvantages in that abnormalities occurred in the characteristics and that interference waves were transmitted from the UHF tuner section 1 side to the mixer 6 during VHF reception. In addition, the tuner device itself
Q of the tuning variable capacitance diode at the lower end of VHF
As a result of the decrease in the gain, the gain is significantly lowered as shown by curve A in FIG. 4, and the noise figure characteristics are lowered, so some kind of countermeasure is required. Such allowances are particularly important for special channel SCH tuners that have a wide receiving frequency range. SCH
The tuner has a wide receiving frequency range of about 50MHz to 300MHz, and it is necessary to apply a fairly low tuning voltage of 1V to 25V, so the Q of the variable capacitance diode is low for such low voltages. This is because it decreases significantly. Furthermore, if the mixer 6 is formed of a field effect transistor, normal operation cannot be achieved unless the input signal is large due to the characteristics of the field effect transistor. This is inconvenient because the mixer will not operate as desired in the area where the value is low. In order to prevent such gain reduction at the lower limit of the receiving frequency range, as shown in Fig. 2, there is a method of inserting a coil _I2 between the signal path 11 and the ground to achieve impedance matching at the lower limit of the receiving frequency range. However, in this case, a special coil _I2 for impedance matching is required, and the above-mentioned drawbacks during VHF reception due to the reverse capacitance of the switching diode still remain. Note that _C7 is a capacitor for DC cut.

The present invention proposes a tuner device devised to eliminate such drawbacks and problems.

In FIG _. 3 _, where the same parts as in FIG. ₃ , the connection point a between the coil _I1 ' and the switching diode D is connected to the output side of the UHF tuner section 1 via a DC cut capacitor _C4 , and the connection point a is connected to the output side of the UHF tuner section 1 through a resistor _R1 . voltage VB during VHF operation
The switching diode D is further connected to the power supply terminal 12 via a resistor R ₂ at the cathode side of the switching diode D. coil
I ₁ ' may be the same as the coil I ₁ for a conventional low-pass filter.

In Figure 3, when receiving VHF, voltage VB is applied to terminal 16, and voltage UB is not applied to terminal 12, so switching diode D becomes conductive, bypass capacitor _C5 is activated, and connection point a is grounded. Therefore, the coil I ₁ ' achieves suitable impedance matching for the lower end signal of the VHF low band. Therefore, as shown by curve B in FIG. 4, the gain of the lower end signal of the VHF low band does not drop undesirably and is improved by about 10 dB compared to the conventional one. In addition, if you choose R ₁ >> R ₃ for resistors R ₁ and R ₃ , terminal 12 will be connected due to conduction of diode D.
The voltage appearing at is small and therefore does not affect the UHF tuner section 1.

Next, during UHF reception, voltage VB is not applied to terminal 16, but voltage UB is applied to terminal 12, so the current flowing through the path of terminal 12 → resistor R ₂ → resistor R ₃ → ground causes switching diode D
A reverse bias voltage is generated on the cathode side of the switching diode D, and the switching diode D is turned off. Therefore UHF
The IF signal from the tuner section 1 is given to the mixer 6 through the coil I ₁ ', where it is amplified and sent to the output terminal 1.
5. In this way, the coil I ₁ ' is a low-pass filter that passes the IF signal from the UHF tuner section 1 (a low-pass filter is formed together with the capacitor provided in the output circuit of the UHF tuner section 1 and the input capacitance of the transistor of the mixer 6). It is configured to function as an impedance matching function for VHF low band lower end signals.

Therefore, the present invention has the advantage that only one coil is required. In addition, switching diode D is configured to turn on when receiving VHF, and has no reverse capacitance, so the VHF RF signal characteristics appearing on the output side of the interstage tuning circuit are not impaired, and the UHF tuner Since the interference signal from the section 1 side flows to the ground through the switching diode D, there is an advantage that it does not interfere with VHF reception.

The present invention is even more effective when used in SCH tuners, tuners using field effect transistors as mixers, and the like.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional tuner device.
FIG. 2 is a circuit diagram of another conventional example. FIG. 3 is a circuit diagram of a tuner device embodying the present invention, and FIG. 4 is an explanatory diagram thereof. 1...UHF tuner section, 2...VHF tuner section, 6...mixer, 9, 10...variable capacitance diode, 1 ₁ ...coil, D...switching diode.

Claims (1)

[Scope of utility model registration request] Using a variable capacitance diode as a tuning element and
In a tuner device that uses a VHF tuner mixer as an amplifier for the UHF tuner output during UHF reception, one end of the coil is connected to the VHF signal path to the VHF mixer, and the other end of the coil is grounded via a switching diode. and connect the connection point of the coil and switching diode to the UHF
A means is provided for connecting the switching diode to the output side of the tuner section and for turning the switching diode on when receiving VHF and turning it off when receiving UHF. A tuner device characterized in that it is designed to work as impedance matching for VHF low band signals.