JPS6233389Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a so-called voltage synthesizer type tuning circuit.

The voltage synthesizer type tuning circuit applies a tuning DC voltage to the tuning variable capacitance diode of the UHF/VHF tuner 1 shown in Fig. 1 by selecting one of the channel contents stored in advance as a digital value in the memory 2. from the logic control circuit 3.
A signal in the form of PWM (Pulse Width Modulation) is outputted, amplified by a voltage conversion circuit 4, smoothed by a low-pass filter 5, and applied to the variable capacitance diode of the tuner 1, where 6 is a tuner. This is a switching circuit for switching bands.

Incidentally, the use of radio waves as a means of communication has increased significantly in recent years, and radio wave problems have been raised as a social issue. Television receivers are no exception to this influence, but most of it is due to passive interference, such as the negative effect on the screen and sound caused by jamming radio waves when receiving television signals. However, considering the future use of radio waves, public
As their private use continues to increase, the issue of privacy rights in communications by private users has come into focus, and measures are required to make it impossible to receive signals for purposes other than the intended purpose of the device. However, conventional television receivers not only do not have any such measures taken, but also have a tuner tuning voltage variable range as wide as possible (for example, 0.2V).
The design goal was to be able to vary the frequency range (up to 30V), and to be able to receive channels in all reception bands without fail in the receivable frequency range of the tuner. Therefore, as shown in Fig. 2, the emitter of the transistor Tr of the voltage conversion circuit 4 provided at the final output stage of the voltage synthesizer type tuning circuit is directly grounded, and the collector is directly connected to the output point. The residual voltage of the transistor Tr (that is, the voltage at the output point after it is turned on) is small, so when the PWM signal shown in Figure 3A is applied to the base from terminal 7, the potential at the output point becomes low due to the on state of the transistor Tr. The lowest value in this case becomes small, and the tuning voltage supplied to the tuner 1 through the low-pass filter 5 is given to a considerably low level. The circuit in Figure 2 is
At the low level part of the PWM signal, the transistor Tr is turned off and the potential at the conversion output point a is high, and at the high level part, the transistor Tr is turned on and the potential at the conversion output point a becomes low, and the result is just reversed. It becomes a signal of shape [see Figure 3 B]. This signal is connected to the +120V power supply and resistor R3 in addition to the transistor Tr.
However, by the following low-pass filter 5, it is smoothed at point b as shown in Fig. 3C, and furthermore, the ripple component is completely removed at point d as shown in Fig. 3D. It is a direct current voltage. In FIG. 2, the upper limit of the output voltage when the transistor Tr is off is limited by the constant voltage diode _Dz .

In the conventional circuit shown in Fig. 2, as mentioned above, the tuning voltage is applied to the tuner to a fairly low level, so that the receivable range of the tuner exceeds the lower limit of the original reception band (solid line) as shown by the dotted lines in Fig. 4. Further below, there is a frequency equivalent to about 1 channel or more for VHF low band V _L , and about 3 for VHF high band V _H.
Frequency equal to or higher than the channel, UHF band U
This means that you can receive an extra frequency equivalent to about 10 channels. In order to effectively utilize radio waves and to deal with the problem of communication secrecy mentioned above, there is a movement in various countries to legally restrict the receivable range of television receivers only for commercial television broadcasting. For example, FTZ in West Germany
(Fern melde Technisches Zent ralamt) standard, DOC (Depar tment of
These standards strictly regulate the upper and lower limits of frequency bands that can be received by television receivers. west german
Taking the FTZ standard as an example, the upper end of the UHF band and the lower end of the VHF band each allow a margin of one channel, that is, about 7 to 8 MHz.

Therefore, the purpose of the present invention is to suitably limit the lower limit voltage in a voltage synthesizer type tuning circuit, and more specifically, to provide a voltage synthesizer in which the converted voltage is extracted from the collector or emitter of a transistor in the final output stage. In a voltage synthesizer-type tuning circuit that has a conversion circuit and a low-pass filter that smoothes the converted voltage, and provides the output to the tuning variable capacitance diode of the tuner, it is possible to limit the minimum value of the tuning voltage with an extremely simple configuration. This limits the lower limit of the tuner's receivable frequency.

The present invention will be described in detail below according to embodiments shown in the drawings. FIG. 5 shows an embodiment of the present invention,
The difference from FIG. 2 is that a resistor _R7 is connected between the emitter of the transistor Tr and the ground.
By inserting such a resistor _R7 , the voltage _E1 at the output point a when the transistor Tr is turned on becomes _E1 = (120 - Vc _E ) x R, where the collector-emitter saturation voltage when the Tr is conductive is Vc _{E. 7} /R ₃ +R ₇ +Vc _E , and compared to the case without resistor R ₇ , the conversion output E ₁
is increased by (120−Vc _E )×R ₇ /R ₃ +R ₇ .

Furthermore, FIG. 6 shows an embodiment in which diodes D ₁ ...D _o are inserted in place of the resistor R ₇ , and the minimum voltage E ₁ ' in this case is the forward drop of each diode D ₁ ...D _o . Letting the voltage be Vak, E ₁ '=Vc _E +nVak, and the voltage has increased by the voltage drop nVak caused by the diode.

The embodiment shown in FIG. 7 shows a case where an impedance element Z consisting of a resistor or a diode is connected between the output point a and the collector, but even when connected in this way, the embodiment shown in FIGS. It is easy to understand that this will give similar results.

Further, FIG. 8 shows a case where an impedance element Z is connected between the source and the ground when a field effect transistor is used as the transistor Tr.

According to this invention, the minimum value _E1 of the tuning voltage can be raised to, for example, 1.25V, so the lower limit of VHF low band Vl by FTZ can be set to 7M below the receiving band.
It can be kept within Hz and easily meets the FTZ standard. In addition, in the case of the conventional example shown in Fig. 1 mentioned earlier, the tuning voltage output as the tuning circuit is
In order to limit the lower receivable frequency of VHF low band _VL as much as possible since it extends below 0.3V,
The tuner side has to be designed so that it can receive the lowest two channels (in the case of West Germany) by applying a tuning voltage of about 0.5V, but this degrades the performance of the tuner when receiving the lowest two channels. (The Q of the variable capacitance diode deteriorates when the applied tuning voltage is low), which is undesirable.However, in this invention, the minimum value of the tuning voltage applied to the tuner is as high as 1.25V, so the tuner side is connected to a relatively high tuning voltage. The lowest channel (2
It also has the effect of improving the performance of the tuner since it can be designed to accept the channel).

Furthermore, according to the present invention, all the complicated adjustments of the receivable range can be made during the design and production of the tuner, so that no adjustment is required during the production of the television set, which is highly effective.

In the above explanation, we mainly explained the measures taken regarding FTZ standards, but Canada's
Of course, it is also possible to comply with the DOC standard.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a tuning circuit, FIG. 2 is a circuit diagram showing the configuration of the output stage of a conventional tuning circuit, FIG. 3 is an explanatory waveform diagram of FIG. 2, and FIG. FIG. 2 is a drawing showing the characteristics of a tuner when a tuning voltage is applied using the circuit shown in FIG. 2; FIG. 5 is a circuit diagram showing the configuration of the output stage of the tuning circuit of the present invention, and FIGS. 6, 7, and 8 are circuit diagrams showing other different embodiments of the present invention. FIG. 9 is a drawing showing the characteristics of the tuner when a tuning voltage is applied by the tuning circuit of the present invention. 1...Tuner, _R7 ...Resistor, 4...Voltage conversion circuit, 5...Low pass filter, Tr...Transistor, _D1 to D _o ...Diode, Z...Impedance.

Claims (1)

[Scope of utility model registration request] The final output stage has a voltage conversion circuit that extracts the converted voltage from the output electrode of the transistor, a low-pass filter that smooths the converted voltage, and the voltage synthesizer type selector that supplies the output to the tuning variable capacitance diode of the tuner. In the station circuit, a resistor or a diode is inserted in the collector-emitter path of the transistor to raise the minimum value of the converted voltage when the transistor is conductive, thereby limiting the lower limit of the reception frequency of the tuner. Tuning circuit.