JPS5827570Y2 - Tuner for television receivers - Google Patents

Description

[Detailed explanation of the invention] This invention operates as a mixing circuit when receiving VHF, and
In a television receiver tuner equipped with a cascode circuit that operates as a UHF intermediate frequency amplification circuit during VHF reception, the gain deviation between the low channel band and high channel band is reduced during VHF reception, and the cascode circuit operates as a UHF intermediate frequency amplification circuit during UHF reception. The present invention relates to a tuner for a television receiver that prevents abnormal oscillations occurring in a first transistor circuit.

Generally, in television receivers equipped with a VHF tuner and a UHF tuner, the mixing circuit of the vHF tuner is
Used as a UHF intermediate frequency amplification circuit during reception.

As a circuit that performs the two operations of a mixing circuit and an amplifying circuit, there is a circuit constructed by connecting two transistors in cascode.

In other words, this cascode type circuit has a first transistor of a common emitter type and a second transistor of a common base type connected in cascode, and when receiving VHF, a VHF high frequency signal and a VHF local oscillation signal are connected to the base of the first transistor. is applied, an intermediate frequency signal is taken out from the collector of the second transistor via a tuned circuit, and U
During HF reception, a UHF intermediate frequency signal is applied to the base of the first transistor, and the intermediate frequency signal is extracted from the collector of the second transistor via a tuning circuit.

Such a cascode circuit operates stably with little feedback from the output to the input, but there is a large gain deviation between the low channel band and the high channel band when receiving VHF, and abnormal oscillation in the first transistor when receiving UHF. There were disadvantages such as the occurrence of

This idea was made in response to the above points, and it connects an impedance element having a resistance component and a capacitance component in series between the connection point of the first transistor and the second transistor of the cascode type circuit and the reference potential point. To provide a tuner for a television receiver that reduces gain deviation between low channel band and high channel band during vHF reception and prevents abnormal oscillation from occurring during UHF reception.

Abnormal oscillation in a cascode type circuit is caused by stray capacitance between the collector reference potential point of the first transistor and this first transistor.
When the transistor is mounted on a printed circuit board, the frequency determined by the inductance component of the wiring pattern of the printed circuit board is approximately equal to the frequency of the interstage circuit provided on the input side of the first transistor. This occurs when the output is fed back to the input, and its frequency is approximately 500 to 6
00 MHz.

Experiments have revealed that the impedance element can prevent this abnormal oscillation and further reduce the gain deviation between the low channel band and the high channel band.

An embodiment of the tuner of this invention will be described in detail below with reference to the drawings.

In FIG. 1, a VHF broadcast signal input from a VHF antenna 1 is selected by a VHF high frequency amplification circuit 2, and an interstage circuit 3
and is led via a capacitor 4 to a cascode circuit 5 which acts as a mixing circuit.

This VHF high frequency signal is simultaneously mixed with a local oscillation signal of a predetermined frequency that is applied from a VHF local oscillation circuit 6 to a mixing circuit 5 via a capacitor 7, converted to an intermediate frequency, taken out by a tuning circuit 8, and then connected to a capacitor 9. It is led to the output terminal 10 via.

The cascode type circuit 5 has a base connected to the capacitor 4 and the capacitor 7 as well as a reference potential point via the resistor 12, and an emitter connected to the resistor 13 and the capacitor 14.
A first transistor 15 of a common emitter type connected to a reference potential point through a parallel circuit of
The collector is directly connected to the collector of the transistor 15, the collector is connected to the tuning circuit 8, and the base is connected to the capacitor 16.
A common-base type second transistor 18 is connected to the reference potential point via the transistor 18, and a second transistor 18 is connected in series between the connection point between the collector of the first transistor 15 and the emitter of the second transistor 18 and the reference potential point. It is composed of an impedance element 19 having a resistance component 19R and a capacitance component 19C.

Note that a resistor 17 and a resistor 20 are connected in series to the base of the first transistor 15, a resistor 17 is connected to the base of the second transistor 18, and a choke coil is connected to the collector of the second transistor 18. 21 and tuning circuit 8
A power supply E is supplied through the two in series.

Further, a capacitor 22 is connected to the power source E in parallel.

When receiving UHF, the UHF input from the UHF antenna 31
The broadcast signal is selected by the UHF high frequency amplification circuit 32 and supplied to the mixing circuit 33, and simultaneously mixed with a UHF local oscillation signal of a predetermined frequency that is supplied from the UHF local oscillation circuit 34 to the mixing circuit 33 to generate a UHF intermediate frequency signal. The signal is converted into a cascode circuit i which acts as a UHF intermediate frequency amplification circuit via a capacitor 35, is amplified, and is guided to an output terminal 10 via a tuning circuit 8 and a capacitor 9.

Note that when receiving UHF, the VHF high frequency amplifier circuit 2 and V
The HF local oscillation circuit 6 is inoperative.

Due to the connection of the impedance element, the resistance component 19R and the capacitance component 19C are connected in series between the connection point of the collector of the first transistor and the emitter of the second transistor and the reference potential point. Lowers the impedance of the transistor's collector.

This reduces the Q of the resonant circuit due to the stray capacitance between the collector reference potential points of the first transistor and the inductance component of the wiring pattern of the printed circuit board, thereby preventing abnormal oscillation.

According to experiments, when the capacitance value of the capacitive component 19C was set to 1000 PF, abnormal oscillation could be completely prevented by setting the resistance value of the resistive component 19R to 51 to 300 Ω.

Regarding the gain deviation between the low channel band and the high channel band during VHF reception, when the capacitance value was 1000 PF and the resistance value was set to approximately 180 Ω, it almost disappeared.

As described above, according to this invention, abnormal oscillations are prevented by appropriately selecting the resistance and capacitance values of the impedance element 19 connected in series between the connection point of the two transistors of the cascode type circuit and the reference potential point. In addition, it is possible to reduce the gain deviation between the low channel band and the high channel band.

Furthermore, since the capacitive component 19C blocks direct current, the direct current bias of the cascode circuit 1p can always be set to the optimum state.

Cross modulation characteristics (1%
Cross modulation) is also capable of increasing the interference level in the range A, where the normally used gain attenuation is O to 15 dB, as shown by the dotted line in FIG.

Note that in FIG. 2, the solid line indicates the case where the impedance element 19 is not added.

In addition, the relationship between the frequency and voltage level of the local oscillation signal leaking to the outside through the antenna terminal provided on the television receiver is as shown by the dotted line in Figure 3.
It can be seen that the harmonics in the range are also reduced.

The solid line indicates the case where "on the impedance element" is not added.

In this way, according to this invention, in addition to being able to prevent abnormal oscillations and reducing the gain deviation between the low and high channel bands, it is also possible to increase the interference level, making it resistant to interference waves, and to reduce unnecessary radiation. It also has the effect of being able to reduce the

Although it is of course possible to construct a single nanoimpedance element with a normal resistance element and a capacitor element, it can also be constructed with a single chip capacitor whose electrodes are made of a metal with a high specific resistance.

In other words, a chip capacitor using a metal with a high specific resistance as an electrode has a large value of its resistance component, and can lower the Q of the resonant circuit that causes abnormal oscillation.

Key metals with specific resistance include palladium, platinum, and nickel.

As described above, according to this invention, there is a resistance component and a capacitance component in series between the connection point of two cascode transistors that function as a mixing circuit when receiving VHF and as a UHF intermediate frequency amplification circuit when receiving UHF and the reference potential point. By connecting an impedance element, it is possible to reduce the gain deviation between the low channel band and the high channel band when receiving VHF, and prevent abnormal oscillations when receiving UHF.It is also resistant to interference waves and has less unnecessary radiation. It is possible to provide a tuner for a television receiver.

[Brief explanation of the drawing]

Fig. 1 is a circuit configuration diagram showing a partial block diagram of an embodiment of the television reception m-tuner of this invention, and Fig. 2 shows the relationship between the interference wave level and gain attenuation of the tuner of this invention and other tuners. Characteristic Diagram for Comparison FIG. 3 is a characteristic diagram for comparing the relationship between the local oscillation frequency and the local oscillation voltage level in the tuner according to this invention and other tuners. 2...VHF high frequency amplification circuit, 3...
Interstage circuit, 5...Cascode type circuit, 6...
... VHF local oscillation circuit, 19 ... impedance element, 33 ... UHF mixing circuit.

Claims (1)

[Claims for Utility Model Registration] 1. A common emitter type first transistor;
a common-base type second transistor connected in cascode to the transistor; and an impedance element having a resistance component and a capacitance component connected in series between the connection point of the first and second transistors and a reference potential point. Preparation, VH
When receiving F, VHF is applied to the input terminal of the first transistor.
A cascode that operates as a VHF mixing circuit by applying a high frequency signal and a VHF local oscillation signal, and operates as a UHF intermediate frequency amplifier circuit by applying a UHF intermediate frequency signal to the input terminal of the first transistor when receiving UHF. A tuner for a television receiver characterized by being equipped with a type circuit. 2. The tuner for a television receiver according to claim 1, wherein the impedance element is comprised of a chip capacitor whose electrode is made of a metal with a high specific resistance.