JPH0139003Y2 - - Google Patents

Description

[Detailed Description of the Invention] Technical Field This invention relates to an input tuning circuit for an electronic tuner, and particularly to a low channel image trap circuit in the input stage.

BACKGROUND ART An electronic tuner that divides the VHF band into two bands, high channel and low channel, is equipped with an image trap for high channel and low channel at its input stage to improve the image rejiggering of each. . However, in European channel and Chinese channel systems where the difference between the Haitian channel and the low channel is large, when receiving the Haitian channel, the components of the image trap circuit for the low channel may form a trap in the high-end band of the Haitian channel. Cuts off some bands and causes reception interference.

Figures 1 to 3 show the conventional tuner input stage tuning circuit and the equivalent circuit for receiving each band.
This is to explain the interference based on the trap circuit for the low channel when receiving the high channel. That is, as shown in Figure 1, the input stage of this tuner converts the received signal from input terminal 1 into IF trap 2.
The tuning signal is supplied to the input tuning circuit 3 via the input tuning circuit 3, and the tuning signal is coupled to the amplifier transistor T _r . The tuned circuit 3 includes a tuned coil device in which a high channel main coil 4, a low channel main coil 5, a low channel sub coil 6, and a high channel sub coil 7 are connected in series at high frequency via high pass capacitors 8 and 9, and this coil device and a capacitor. A tuning variable capacitance diode 11 connected in parallel at high frequency via 10, and two low channel coils 5 and 6.
is equipped with a band switching device including a switching diode 13 connected in parallel at high frequency via bypass capacitors 9 and 12, and a variable capacitance diode 1
A tuning voltage is applied to the switching diode 1 from the tuning terminal 14, and a switching voltage is applied to the switching diode 13 from the high channel power supply terminal 15 or the low channel power supply terminal 16, thereby performing band switching and channel selection. For such a tuning circuit 3, an image trap circuit is usually provided corresponding to each reception band, and the low channel image trap circuit 21 is constructed by connecting a capacitor 17 in parallel to the low channel sub coil 6. However, the high channel image trap circuit 22 is constructed by connecting a capacitor 18 to the high channel sub coil 7 in parallel. Further sub-coil 6,
7 are tracking correction coils.

The input tuning circuit 3 having the above configuration forms equivalent circuits shown in FIGS. 2 and 3 when each band is received. In each equivalent circuit diagram during low channel and high channel reception, the inductance values of coils 4 to 7 and capacitors 17 and 18 are L ₁
〜L ₄ and the capacitance values C ₁ and C ₂ are also shown, and the trap circuits 21 and 2 of the respective image frequencies are shown.
form 2. When receiving the high channel in FIG. 2, the high channel image trap circuit 22 sets the trap frequency _H using the following general formula.

However, a parallel circuit consisting of the coils 5, 6 and the capacitor 17 is inserted between the input terminal 1 and the high channel tuning circuit due to the conduction of the switching diode, and a trap circuit with an undesired frequency _u is formed.

Here, in a channel system where the low channel and high channel are far apart, _the frequency u is located on the high end side of the high channel and causes reception interference for that channel. When receiving a low channel, the equivalent circuit is as shown in FIG. 3. The trap frequency _L of the low channel image trap circuit 21 is set by the following general formula, but There will be no interference of any kind.

DISCLOSURE OF THE INVENTION The present invention was proposed in view of the above drawbacks, and provides a new and improved input tuning circuit for an electronic tuner that eliminates reception interference caused by the low channel image trap circuit component during high channel reception. The purpose is to

According to the present invention, an image trap circuit for each receiving band is installed in a two-band electronic tuner that divides the VHF band into two, high channel and low channel, according to the selection of each band of high channel and low channel. In the input circuit to be provided, the image trap circuit for the low channel is configured to obtain its trap capacitance with a variable capacitance diode, so that it takes a predetermined capacitance when receiving the low channel, and its capacitance is made sufficiently small when receiving the high channel. This disclosure discloses that the trap frequency with the low channel coil portion is increased to eliminate reception interference in the high-end channel when receiving the high channel.

The specific configuration includes a tuning coil device in which a high channel main coil, a low channel main coil, a low channel sub coil, and a high channel sub coil are connected in series at high frequency, and a tuning variable capacitor connected in parallel at high frequency to this tuning coil device. In the input circuit of an electronic tuner, which is equipped with a band switching diode and a band switching diode connected in parallel to the low channel main and sub coils at high frequency, a trap variable capacitance diode is attached to the low channel side tuning coil. A low channel image trap circuit is formed, and the reverse bias voltage to the trap variable capacitance diode is changed according to channel band switching to eliminate interference caused by the low channel image trap circuit during high channel reception. . In particular, such a circuit configuration is effective in channel systems in which the low-end frequency band of the low channel is low and the high-end frequency band of the high channel is high, such as the European channel, the Australian channel, and the Chinese channel.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 4 is a circuit diagram showing the input stage of the VHF electronic tuner according to the present invention. When compared with the conventional circuit shown in FIG. using this variable capacitance diode 30
The circuit is substantially the same except that a reverse bias voltage applying circuit 31 is added to the circuit. Therefore, circuit components corresponding to those in FIG. 1 are indicated using the same reference numerals, and detailed description thereof will be omitted. That is, the low channel image trap circuit 21 includes a trap variable capacitance diode 3 instead of a fixed capacitor.
0 was used, and the configuration was such that different reverse bias voltages were applied when receiving the low channel and when receiving the high channel. Here, the reverse bias voltage application circuit 31 includes a resistor 32 inserted between the high channel power supply terminal 15 and the ground, and resistors 33 and 34 and diodes 35 and 36 between the high channel power supply terminal 15 and the low channel power supply terminal 16. Insert
Further, a resistor 37 is inserted between the roach channel power supply terminal 16 and ground.

In the above configuration, when receiving a high channel, the voltage from the high channel power supply terminal 15 is forward biased to make the switching diode 13 conductive, and at the same time, a reverse bias is applied to the trap variable capacitance diode 30 with a voltage of about 12V. generates a small capacity. However, such a capacitance value does not interfere with Haitian channel reception. On the other hand, when receiving a low channel, the voltage from the low channel power supply terminal 16 is divided by resistance, and a voltage lower than 12V corresponding to the division ratio is reverse biased to the variable capacitance diode 31, and the lower channel image trap is circuit 21
Generates the relatively large capacity needed to function as a Here, the division ratio by resistance division is resistance 32, 3
The resistance values of 3 and 34 can be arbitrarily determined, and a desired capacitance can be obtained by changing the reverse bias voltage.
For example, the resistance values of resistors 32 to 34 are R ₁ ,
If R ₂ and R ₃ are used, the division ratio R can be determined by the following formula.

R=R ₁ /R ₁ +R ₂ +R ₃ Therefore, when a voltage of 12V is supplied from the power supply terminal, the reverse bias voltage V _R to the trap variable capacitance diode 30 has the following value.

V _R = R ₁ / R ₁ + R ₂ + R ₃ ×12 [V] This voltage V _R provides a reverse bias state for the switching diode 13 and maintains the cut-off state, so the low channel reception state is maintained. ,
At the same time, image trap circuit 2 for roach channel
1 to operate normally.

[Brief explanation of the drawing]

Figure 1 is the input stage circuit diagram of a conventional tuner, Figure 2 is the input stage circuit diagram of a conventional tuner.
1 is an equivalent circuit diagram when receiving a high channel, FIG. 3 is an equivalent circuit diagram when receiving a low channel, and FIG. 4 is an input tuning circuit diagram of an electronic tuner according to the present invention. 4, 5, 6, 7... Tuning coil device, 11...
Tuning variable capacitance diode, 13... Switching diode, 15... High channel power supply terminal, 16... Low channel power supply terminal, 21...
...Image trap circuit for low channel, 22
...Image trap circuit for Haiti channel, 3
0...Variable capacitance diode for trap.

Claims (1)

[Scope of utility model registration request] A tuning coil device in which a high channel main coil, a low channel main coil, a low channel sub coil, and a high channel sub coil are connected in series at high frequency, a tuning variable capacitance diode connected in parallel at high frequency to this tuning coil device, and the low channel In a tuner input circuit comprising a switching diode for band switching connected in parallel at high frequency to a series portion of a main coil and a low channel sub coil, and an image trap circuit for a high channel and a low channel, the image trap for the low channel is provided. The circuit includes a trap variable capacitance diode, and reverse bias is applied to the trap variable capacitance diode with different switching voltages depending on band switching, thereby eliminating trap interference during high channel reception by the low channel image trap circuit. An input tuning circuit for an electronic tuner characterized by the following.