JPH0349482Y2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to an electronic tuner used in television receivers and the like.

(b) Conventional technology Television receivers generally use high-frequency AGC.
2. Description of the Related Art An electronic tuner having an amplifying circuit and having a configuration as shown in FIG. 2, which is called an input tuning type, has been used in the past. That is, in Figure 2, 1 is an input terminal, 2 is an input filter circuit, 3 is an input tuning circuit using a variable capacitance diode D1 as a tuning element, and 4 is a dual gate FET (T) used as an amplification element. AGC
(Automatic gain control) amplifier circuit, 5 is an interstage tuning circuit using variable capacitance diodes D2 and D3, 6 is a local oscillation circuit, 7 is a mixing circuit, 8 is IF (intermediate frequency)
Variable inductance L and capacitor C4 for tuning
and an output tuning circuit consisting of capacitors C5 and C6 and a resistor R6 for matching to the VIF (video intermediate frequency) stage in the receiver; 9 is an output terminal.

The AGC amplifier circuit 4 is the first gate of FET (T)
The received signal selected by the input tuning circuit 5 is applied to G1, the AGC control voltage given to the control terminal 9 is applied to the second gate G2 via the resistor R3, and the output signal from the drain D is applied to the coupling capacitor.
The signal is applied to the interstage tuning circuit 5 via C3. Note that R1 and R2 are the first gate G1
bias resistor, C1 is the input capacitance correction capacitor for the first gate G1, C2 is the bypass capacitor for the second gate G2, R4 is the drain load resistor, R5, C8
is the source resistance and its bypass capacitor, 10
is the power supply (+VCC) terminal.

Here, AGC to the second gate G2 of FET (T)
Control voltage VG is usually about 0.5V (when receiving strong electric field)
It is designed to vary in the range of ~7V (when receiving a weak electric field), but as is generally known, the input capacitance of the FET changes depending on the voltage applied to the gate, so the circuit shown in Figure 2 Now, the first gate G1 above.
The input capacitance Co equivalently shown between and the ground point will change. That is, according to actual measurements by the present inventor, the value of the capacitance Co was about 4PF when the AGC control voltage VG was 7V, but about 3.2PF when the AGC control voltage VG was 0.5V. When the input capacitance Co changes in this way, this capacitance changes to the variable capacitance diode of the input tuning circuit 3.
Since it is connected in parallel to D1 (variation range of about 2 to 12 PF), the frequency characteristics of the tuning circuit 3 will be greatly changed.

For example, when receiving a weak electric field (when VG = 7V), the frequency characteristics of the input tuning circuit 3 become as shown in Figure 3a, and when compared with the normal case in the medium electric field receiving state shown in Figure 3b, The gain at the video carrier frequency point P is considerably larger than that at the audio carrier frequency point S and the color subcarrier frequency point C.

On the other hand, when receiving a strong electric field (when VG = 0.5V), the same figure c
The magnitude relationship of the gains at the PS point is as follows (the ratio of the gains at the P and S points is hereinafter referred to as the P/S ratio)
is reversed. Then, the signal having the characteristics shown in FIG. If it is given, the following problems will arise. That is,
Since general TV receivers use intercarrier audio demodulation, in the case shown in Figure 3a, the influence of the video signal becomes large and buzz is likely to occur in the audio signal, and conversely, in the case shown in Figure 3c In this case, the influence of the audio carrier signal becomes large, causing beat interference such as audio beats and color beats to the video signal.

Therefore, as shown in Japanese Utility Model Application Publication No. 48-35252, conventional methods have been used to equivalently reduce the change in input capacitance Co by providing the aforementioned input capacitance correction capacitor c1 (approximately 5PF). I was doing it. At this time, the larger the capacitance value of the correction capacitor C1, the greater the correction effect, but in that case there was a drawback that the variable tuning range of the input tuning circuit 3 was significantly narrowed.

(c) Problems to be solved by the invention The present invention was made in consideration of the above points, and can reliably correct the P/S ratio change of the IF output signal caused by the tuner's AGC amplifier circuit. Moreover,
The purpose is to prevent the variable tuning range of the input tuning circuit from being changed in any way.

(d) Means for solving the problem In this invention, a variable capacitance element is used as the tuning element of the output tuning circuit of the tuner that derives the IF output signal, and the capacitance of this element is applied to the AGC amplifier circuit in the tuner. It is designed to change according to the AGC control signal.

(E) Effect In the above configuration, the tuner is controlled by changing the capacitance of the variable capacitance element so that the frequency characteristics of the output tuning circuit change in the opposite direction to the changing direction of the frequency characteristics of the input tuning circuit. The output P/S ratio is controlled to be substantially constant regardless of the received electric field strength.

(F) Embodiment FIG. 1 shows an embodiment of the tuner of the present invention, and the same parts as the conventional example shown in FIG. In Figure 2, remove the input capacitance correction capacitor C1 and
Variable capacitance diode D4 and DC blocking capacitor for tuning capacitor C4 in output tuning circuit 8
C7 is connected as shown in the figure, and the AGC control voltage VG applied to the terminal 9 is inverted by a transistor Q and applied to the midpoint between the two connections through a current limiting resistor R7.

In such an embodiment, when receiving a weak electric field, that is, AGC
When the control voltage is VG = 7V, the voltage applied to the variable capacitance diode D4 is lower than when receiving a medium electric field, so its capacitance value increases and the capacitor
The combined capacitance value with C7 and C4 also increases. Therefore,
At this time, the tuning frequency of the output tuning circuit 8 will deviate slightly from the normal state in the medium electric field reception state shown in FIG. 4 in the direction of lower frequencies as shown by the broken line.
This means that the P/S ratio in the output tuning circuit 8 becomes smaller, and therefore, the increase in the P/S ratio in the frequency characteristics (Fig. 3a) of the input tuning circuit 3 at this time is It will cancel out.

Furthermore, in the case of strong electric field reception, that is, VG = 0.5V, the voltage applied to the variable capacitance diode D4 increases and the capacitance value decreases, and therefore the above-mentioned combined capacitance value also decreases. The tuning frequency of the output tuning circuit 8 deviates slightly from the state shown by the solid line in Fig. 4 to the higher frequency direction as shown by the dashed line P/S.
The ratio becomes larger. Therefore, in this case as well, the frequency characteristics of the input tuning circuit 3 at this time (third
The decrease in the P/S ratio in Figure c) is offset.
Therefore, capacitor C4 in the output tuning circuit 8,
By appropriately selecting C7, the variable inductance L, and the variable capacitance diode D4, the changes in the P/S ratios in the input tuning circuit 3 and the output tuning circuit 8 can always be canceled out without excess or deficiency. , the P/S ratio of the IF output signal derived to the output terminal 9 can always be maintained at a predetermined value regardless of the received electric field strength.

(G) Effects of the invention According to the electronic tuner of the invention, it is possible to correct the P/S ratio change of the IF output signal due to the AGC amplifier circuit, so that the P/S ratio changes when receiving a weak electric field due to the above amplifier circuit. It is possible to prevent beat disturbances such as color beats and audio beats caused by the increase in electric field intensity, and it is also possible to prevent buzz caused by a decrease in the P/S ratio when receiving a strong electric field. Also,
This does not change the variable range of the input tuning circuit in any way.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the electronic tuner of the present invention, Fig. 2 is a circuit diagram showing a conventional electronic tuner, and Fig. 3 a, b, and c are characteristic diagrams showing the frequency characteristics of the input tuning circuit. , FIG. 4 is a characteristic diagram showing the frequency characteristics of the output tuning circuit. 3: Input tuning circuit, 4: AGC amplifier circuit, 8:
Output tuning circuit, D4: variable capacitance diode.

Claims (1)

[Claims for Utility Model Registration] 1. An input tuning circuit 3, and a dual gate FET (within which the received signal from the input tuning circuit 3 is applied to the first gate and the AGC control voltage (VG) is applied to the second gate). In an electronic tuner comprising an AGC amplification circuit 4 using the AGC amplifier circuit 4 using the AGC amplifier T) and an output tuning circuit 8 that outputs the signal from the AGC amplifier circuit 4 converted into an intermediate frequency signal, the AGC control voltage (VG) is The output tuning circuit 8 is arranged in a direction that cancels out fluctuations in the frequency characteristics of the input tuning circuit 3 caused by changes in the input capacitance (Co) of the dual gate FET (T).
is used as a tuning element of the output tuning circuit 8 in order to change the frequency characteristics of the electronic tuner and reduce fluctuations in the relative gain ratio of the video carrier frequency to the audio carrier frequency of this electronic tuner caused by changes in the received electric field strength. An electronic tuner comprising: a variable capacitance element (D ₄ ) whose capacitance is variably controlled according to the AGC control voltage (VG).