JPH0138992Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to an electronic tuner, and particularly to a local oscillation circuit for an electronic tuner that switches bands between a low band and a high band using a switching diode.

In a general electronic tuner, a switching diode is used to selectively switch the tuning coil between the high band and the low band. This band switching operation is performed by supplying a forward or reverse bias voltage to the switching diode, and there are two voltage application methods: a voltage switching method and a terminal switching method. In the former voltage switching method, voltage is applied to the switching diode using a single voltage terminal (hereinafter referred to as the switching voltage terminal), and the polarity of the voltage is switched for band selection, for example, whereas in the latter terminal switching method, voltage is applied to the switching diode using a single voltage terminal (hereinafter referred to as the switching voltage terminal). Two low band voltage terminals (hereinafter referred to as band selection voltage terminals) are prepared in advance, and the voltage from the selected voltage terminal applies a positive or reverse bias voltage to the switching diode. Although it is desirable to arbitrarily select between these two switching methods, the common method requires the addition of another switching diode. On the other hand, it is required to stabilize the oscillation of the local oscillation circuit over a wide band.

Therefore, the present invention has been proposed in view of the above, and aims to provide an electronic tuner circuit that achieves stable oscillation operation over a wide band while sharing the band switching method.

According to the present invention, the switching diode for band switching is connected in series with the DC blocking capacitor and inserted between the operating voltage supply side of the oscillating active element and the high channel coil-low channel coil connection point of the tuning coil. be done. A forward bias voltage is applied from the first voltage terminal to the connection point between the switching diode and the DC blocking capacitor to make the switching diode conductive via the choke coil. This first voltage terminal functions as a switching voltage terminal in the voltage switching method, and as a high band voltage terminal in the terminal switching method. On the other hand, the operating voltage supply side of the oscillation active element is connected to the second
In the voltage switching method, the drive voltage for the active element is constantly supplied to this second voltage terminal, and in the terminal switching method, it functions as a low band voltage terminal.

According to the above circuit configuration, when the low band is selected, the tuning coil is directly grounded without being affected by the switching diode, thereby increasing the Q of the tuning circuit.
As a result, a high Q local oscillation circuit is formed, ensuring stable oscillation over a wide range. This kind of stable low-band oscillation operation is especially important for 133ch.
This is advantageous when the low band of the VHF band tuner section is relatively wide, such as in a CATV tuner. Therefore, in the present invention, the band switching method can be applied to either the voltage switching method or the terminal switching method, and can also provide practical effects such as stabilizing the oscillation characteristics during low band reception.

Embodiments of the present invention will be described in detail below in comparison with conventional circuits. FIG. 1 shows a local oscillation circuit for an electronic tuner that switches bands using a conventional voltage switching method. This circuit is composed of an oscillating transistor 1 and a tuning circuit 2, and the transistor 1 is driven by a voltage from a power supply terminal 3. Tuned circuit 2
includes a variable capacitance diode 4, a high-channel coil 5 and a low-channel coil 6 that are tangential in series, and a switching diode 8 connected to a connection point 7 between both coils. In addition, a second variable capacitance diode 9 is added between the oscillation transistor 1 and the band characteristics of the tuned circuit are improved. The tuning voltage is applied from the tuning terminal 10 to each variable capacitance diode 4, 9.
A switching voltage from a band switching switching voltage terminal 11 is applied to the switching diode 8, thereby forming a two-band switching type local oscillation circuit.

FIG. 2 shows a local oscillation circuit for an electronic tuner that selects a band using a conventional terminal switching method. This circuit is constructed by adding a second switching diode 12 to the circuit shown in FIG. In this case, terminals 3 and 11 in Figure 1 function as the low band voltage terminal 13 and high band voltage terminal in Figure 2, and the other circuit configurations are almost the same, and the same parts are designated by the same symbols. The individual explanations will be omitted.

In the conventional circuits shown in FIGS. 1 and 2, different circuit configurations are required depending on the switching method, voltage switching or terminal switching, to perform band switching. In addition, in both circuits, a switching diode 8 is inserted in parallel with the tuning coil during low band reception, and the circuit Q
As a result, stable oscillation operation over a wide band could not be ensured.

FIG. 3 shows an electronic tuner circuit according to the present invention, which provides a novel local oscillation circuit that can solve the problems associated with the local oscillation circuits of FIGS. 1 and 2. This local oscillator circuit has a second circuit, excluding the band switching circuit part, in order to facilitate comparison with the conventional circuit.
The same circuit configuration as in the figure is shown, and the same parts are shown using the same symbols. That is, this local oscillation circuit is composed of an oscillation transistor 1 and a tuning circuit 2, and the tuning circuit 2 is coupled to variable capacitance diodes 4 and 9, tuning coils 5 and 6 for high and low channels, and a connection point 7 between these two coils. It includes a band switching circuit 15 of the present invention. Here, a tuning terminal 10, a low band reception power supply terminal 16, and a high band reception power supply terminal 17 are used for power supply voltage supply.
This enables band selection using a terminal switching method in which a voltage is applied to one of the power supply terminals 16 and 17. At the same time, in this circuit, if the power supply terminal 16 is used as an operating voltage terminal and the power supply terminal 17 is used as a switching voltage terminal, it is possible to select a band using a voltage switching method.

The band switching circuit 15, which is a feature of the present invention, includes a series circuit of a DC blocking capacitor 21 and a switching diode 22, and a connection point 23 of this series circuit.
and the power supply terminal 17.
Equipped with. And the DC blocking capacitor 21
and a switching diode 22 is inserted between the connection point 7 between the high channel coil 5 and the low channel coil 6 and the collector lead 18 on the operating voltage supply side of the oscillation transistor 1. Collector lead 8 is connected to second DC blocking capacitor 19
It is grounded at high frequency through the resistor 20 and coupled to the power supply terminal 16 through the resistor 20 .

In the above configuration, the band switching circuit 15 uses one switching diode 22 to perform both functions of band switching and transistor operating voltage switching. If band selection is performed using a terminal switching method, the oscillation voltage can be adjusted by changing the operating voltage for each band, and the difference in injection level to the mixer between bands can be eliminated. An important advantage of the circuit according to the present invention is that it has a high Q effect on the tuning circuit of the band switching circuit.
Stable operation can be obtained over a wide band against oscillations during low band reception. This is advantageous in a 133ch CATV tuner with a wide low band frequency range, and has great practical effects.

[Brief explanation of drawings]

1 and 2 are local oscillation circuit diagrams for a conventional electronic tuner, and FIG. 3 is a local oscillation circuit diagram for an electronic tuner according to the present invention. 1... Active element for oscillation (transistor), 2...
...tuned circuit, 4...variable capacitance diode, 5, 6
... Tuning coil, 10 ... Tuning terminal, 1
5... Band switching circuit, 16, 17... Voltage terminal, 19, 21... DC blocking capacitor, 20
...Resistor, 22 ...Switching diode, 2
4...Chiyoke coil.

Claims (1)

[Scope of utility model registration request] For electronic tuners configured by coupling an oscillating active element to a tuned circuit that includes a variable capacitance diode, a high channel coil and a low channel coil connected in series, and a band switching diode coupled at high frequency to the connection point of these two coils. In the local oscillation circuit, the switching diode is inserted in series with a DC blocking capacitor between the operating voltage supply side electrode of the active element and the connection point between the two coils, and the switching diode and the capacitor are connected in series. 1. An electronic tuner circuit characterized in that a first voltage terminal is provided at the point via a chiyoke coil, and a second voltage terminal is provided at the electrode via a resistor.