JPS6238323Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a tuner local oscillation circuit for a television receiver, and is particularly applicable to an electronic tuner.

In electronic tuners, the output of a conventional local oscillation circuit is applied to a tuning circuit using, for example, a PLL frequency synthesizer system, and a tuning voltage corresponding to the deviation between the frequency of the output of the local oscillation circuit and the frequency corresponding to the channel command output is applied to the PLL loop. A system is used in which the oscillation frequency of the local oscillation circuit is changed by the tuning voltage so that the oscillation frequency follows the receiving frequency according to the channel command.

However, in this type of local oscillator circuit, in order to prevent malfunction of the PLL loop in the next stage, it is required that the distortion of the output voltage is small (in other words, that the harmonic components are small).
The PLL loop has low sensitivity, and is prone to malfunction if there is a level difference in the local oscillator output voltage when the frequency changes. Furthermore, if the level of the harmonic components of the local oscillator output voltage is high, there is a risk that the trigger level of the counter provided in the PLL loop will be exceeded, causing the counter to malfunction and follow an undesired receiving frequency.

On the other hand, conventional local oscillation circuits of this type include a common-collector type transistor oscillation circuit as shown in Figure 1, and a common-base type transistor oscillation circuit (not shown), which can obtain an output voltage large enough to be practically satisfactory. It is often used as a circuit that can do a lot of things.

Figure 1 shows a VHF local oscillation circuit, which includes a variable capacitance diode 2 at the base of an oscillation transistor 1,
A tank circuit 9 is connected to a band switching diode 5 connected to a switching drive power source 4 or ground via a switch 3, and an oscillation coil 6, 7 and a capacitor 8 connected to this band switching diode 5. There is. The voltage across the emitter resistor 10 of the transistor 1 is fed back to the base via the capacitor 11, and thus the transistor 1 operates in oscillation. However, the voltage across the emitter resistor 10 is applied to the output resistor 13 via the DC blocking capacitor 12.
The oscillation output is sent to the output terminal 14 connected to the output resistor 13.

By the way, although the conventional local oscillation circuit shown in FIG. 1 can obtain a considerably large output voltage, it inevitably produces a considerably distorted waveform due to the large harmonic components. Therefore, there is a drawback that it is necessary to take measures such as providing a filter at the input section of the PLL loop connected to the subsequent stage of the output terminal 14 to reduce the voltage level of the harmonic component.

In addition, in a local oscillation circuit using a variable capacitance diode 2 as an oscillation frequency variable element as in the case of Fig. 1, when the tuning voltage V _T applied to the variable capacitance diode 2 decreases, the Q of the element itself decreases, which causes The voltage decreases, and as a result, when viewed as a whole of the local oscillation circuit, the oscillation output voltage fluctuates in response to changes in the tuning frequency, which is a drawback of exhibiting so-called frequency characteristics.

Furthermore, in the case of Fig. 1, the DC blocking capacitor 1
As for transistor 2, a relatively small capacitance is selected in order to avoid its influence on the oscillation circuit around transistor 1. However, even in this case, the bypass capacitor 15 where the capacitor 12 is connected to the emitter of the transistor 1
will be equivalently connected in parallel to
As a result, the oscillation level of the oscillation circuit as a whole increases, causing voltage drift in the local oscillation circuit,
This caused the output impedance to change. In addition, since the capacitance value of the DC blocking capacitor 12 must be selected to a relatively small value, conversely, the output voltage has frequency characteristics with respect to changes in the tuning voltage, in other words, changes in the local oscillation frequency. It was also the cause.

Taking the above points into consideration, the present invention has developed a local oscillation circuit that can reduce the change in oscillation output voltage with respect to the local oscillation frequency as much as possible, and that can easily obtain an output voltage of an appropriate size as needed. This is what we are trying to provide.

An example of the present invention will be described in detail with reference to the drawings below.
In FIG. 2, 21 is an unbalanced differential oscillation circuit main body in which the mutually connected emitters of a pair of transistors 22 and 23 are grounded via a constant current transistor 24 and its emitter resistor 25.

The collector of one transistor 23 is grounded through a bypass capacitor 26. This bypass capacitor 26 and the other transistor 22
Oscillation coils 27 and 28 are connected between the collector of
A tank circuit 31 consisting of a capacitor 29 and a variable capacitor 30 is connected. Note 3
Reference numeral 2 denotes a band switching diode connected to the switching drive power supply 34 or ground via a switch 33.

A bypass capacitor 35 is connected between the base of this transistor 22 and ground, and the base of the other transistor 23 is connected to an output impedance resistor 38 with a low resistance value via a DC blocking capacitor 36 and an output level control resistor 37. , the voltage across this output resistor 38 is sent to an output terminal 39 as an oscillation output. Further, the base of the transistor 23 on the output side is connected to the collector of the transistor 22 on the tank circuit side via a feedback capacitor 40.

In the above configuration, when the resonant current of the tank circuit 31 flows to the transistor 22, the transistor 23 operates differentially in response, and the output voltage is fed back from its base through the feedback capacitor 40, thereby creating an unbalanced differential oscillation circuit. The main body 21 operates in oscillation, and its oscillation output is sent from the base of the transistor 23 to the output terminal 39 via the DC blocking capacitor 36 and the output level control resistor 37.

According to the configuration shown in FIG. 2, the oscillation circuit main body 21 is configured as an unbalanced differential oscillator, and the transistor 23 on the output side operates as a feedback element, so that the transistor 23 becomes a tank circuit when viewed from the output side. Provides a buffering effect for the side transistor 22. As a result, the tank circuit side transistor 2 is formed by the circuit elements connected to the base of the output side transistor 23, that is, the DC blocking capacitor 36, the output level control resistor 37, and the output resistor 38.
2 can be reduced to a negligible extent, and therefore a much more stable local oscillation circuit can be realized compared to the conventional configuration shown in FIG. Incidentally, in the conventional configuration shown in Figure 1, the output obtained at the emitter of transistor 1 is fed back to the base to perform the generation operation, and the point at which the oscillation output is extracted from this oscillation loop is the transistor as shown in the figure. 1
It can be either an emitter electrode or a base electrode. However, since both of these electrodes have high impedance, the influence of the DC blocking capacitor 12 cannot be ignored, but the configuration shown in FIG. 2 does not have this problem.

In this way, according to the configuration shown in FIG. 2, there is no risk that the influence of the circuit elements on the output circuit side will occur on the tank circuit side, so it is necessary to select the DC blocking capacitor 36 that is most suitable for its original DC blocking operation. In addition, the output level control resistor 37 and the output resistor 38 can be used to suppress the voltage deviation with respect to changes in the oscillation frequency that the oscillation power supply of the oscillation circuit main body 21 inherently has within a permissible range, and also to have an oscillation voltage value. The desired value can be easily selected depending on the situation.

According to experiments on this point, the maximum voltage of the unbalanced differential oscillation circuit main body 21 shown in FIG. 2 shows a large deviation of about -5 to +7 dBm depending on the fluctuation of the oscillation frequency. However, this level deviation could be suppressed to within 6 dB by changing the output level control resistor 37, and a sufficiently large value could be obtained as the oscillation output voltage.

Incidentally, in the configuration shown in FIG. 2, the output voltage Vo, the power supply voltage of the oscillation circuit body 21 is V, and the oscillation circuit body 2
If the internal resistance value of 1 is R _S , the output voltage V ₀ is Vo=V(Tu)・R ₃₈ /R ₃₈ +R ₃₇ +R _S ...(1
) becomes. Here, R ₃₇ is the resistance value of the output level control resistor 37, and R ₃₈ is the resistance value of the output resistor 38. Further, V(Tu) indicates that the power supply voltage V is a function of the tuning voltage V _T .

According to equation (1), it is clear that the voltage V ₀ is a function of the tuning voltage V _T , but the deviation can be reduced by changing the value of the output level control resistor 37 .

That is, in FIG. 3, the curves V ₀₂ , V ₀₂ and
V ₀₃ is the tuning voltage (therefore, the oscillation frequency) when the value R ₃₈ of the output resistor 38 is kept constant and the value R ₃₇ of the output level control resistor 37 is gradually increased from a small value.
This is an experimental measurement of the level change (i.e., frequency characteristic) of the output voltage V ₀ with respect to the change in . According to the experimental results shown in FIG. 3, when the value R ₃₇ of the output level control resistor 37 is increased, the curve V ₀₁
From V ₀₂ to V ₀₃ , the output voltage V ₀ decreases, but the deviation becomes smaller. Conversely, if you decrease the value R ₃₇ of the output level control resistor 37, the curve V ₀₃ will change.
V ₀₂ and V ₀₁ change, and the output voltage V ₀ increases, but its deviation also increases.

Therefore, by selecting the value R ₃₇ of the output level control resistor 37 as necessary, it is possible to obtain an output voltage that is sufficiently large for practical use and whose deviation value is suppressed within an allowable range.

As described above, according to the present invention, by making the oscillation circuit main body into an unbalanced differential circuit configuration, it is possible to effectively reduce the influence of the output side circuit elements on the oscillation operation of the oscillation circuit main body, thereby improving the frequency characteristics. It is possible to realize a local oscillation circuit that can easily obtain a small oscillation output voltage and a sufficient magnitude as needed.

[Brief explanation of the drawing]

Fig. 1 is a connection diagram showing a conventional local oscillation circuit, Fig. 2 is a connection diagram showing an example of a tuner local oscillation circuit for a television receiver according to the present invention, and Fig. 3 is a curve diagram showing its frequency characteristics. be. 1... Oscillation transistor, 9... Tank circuit, 12... DC element capacitor, 13... Output resistor, 21... Unbalanced differential oscillation circuit body, 2
2, 23...first and second transistors, 31...
Tank circuit, 36...DC blocking capacitor, 3
7...Output level control resistance, 38...Output resistance,
39...Output terminal.

Claims (1)

[Scope of utility model registration request] The first one is configured as a differential type by connecting the emitters together.
and a second transistor, a tank circuit connected to the collector of the first transistor, a feedback circuit connecting the collector side of the first transistor and the base side of the second transistor, and the feedback circuit A series circuit including a DC blocking capacitor, an output level control resistor, and an output resistor connected between the base of a second transistor to be connected and a reference potential point, and an output from both ends of the output resistor of the series circuit. A tuner local oscillation circuit for a television receiver.