JPH0339961Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a tuner, and particularly to an input circuit that selectively branches signals supplied to the same input terminal into a UHF band, a CATV band, and a VHF band.

2. Description of the Related Art FIG. 2 shows an example of a tuner input circuit that selectively branches signals supplied from the same input terminal into a desired frequency band. In the figure, 1 is a coaxial cable input terminal, which is connected to a high-pass filter 2 and a low-pass filter 3. A series circuit of a first capacitor 4 and a first coil 5 is connected to the output of the high-pass filter 2, and the other end of the first coil 5 is grounded. A UHF input inductor 6 is electromagnetically coupled to the first coil 6, and a capacitor 7 and a variable capacitance element 8 are connected to both ends thereof. 9 is an IF trap,
It is connected to the low pass filter 3, and its output is the second
It is grounded through a series circuit of a capacitor 10 and a resistor 11. A series circuit of a first diode 12 and a third capacitor 13, a second diode, and a fourth
A series circuit of capacitors 15 is connected to each other,
Although the other ends of the third and fourth capacitors 13 and 15 are not shown, they are connected to the Hyper Band input tuning section.
Connected to the VL/VH Band input circuit.

16 and 17 are first and second diodes 12 and 14
This shows the resistance that provides the bias to control opening and closing.

This operation is performed by the first and second diodes 12 and 14.
When both are made non-conductive and the UHF tuner body (not shown) is put into operation, the first coil 5 and the input inductor 6 are electromagnetically coupled, and the signal input from the input terminal 1 is transferred to the input inductor 6. communicated.

Furthermore, when the operation of the UHF tuner body is stopped and the first diode 12 is made conductive and the second diode 14 is made non-conductive, the signal input from the input terminal 1 is passed through the low-pass filter 3 and the IF trap 9.
It is transmitted to the HyperBand input tuning section.

Similarly, when the operation of the UHF tuner body is stopped and the first diode 12 is made non-conductive and the second diode 13 is made conductive, VL and VH are output from input terminal 1.
A signal is transmitted to the Band input tuning section.

In this way, the operation of the UHF tuner body,
By controlling the conduction of the first and second diodes, signals input from the same input terminal are branched to each tuner body.

Problems that the invention attempts to solve By the way, in the circuit shown in Figure 2, the high-pass filter 2
The circuit from to input inductor 6 is connected in the same state regardless of the operating state of the UHF tuner, so when receiving CATV or VHF,
There was a problem in that the UHF input circuit became a load and was susceptible to characteristic deterioration, specifically deterioration of the chatter index characteristic. In addition, there was a problem of deterioration of IF rigidity characteristics during UHF reception.

Means for solving the problems This invention was proposed in view of the above problems.
A second coil 23 is connected to a first capacitor to which an input signal is supplied, and a first coil 22 is electromagnetically coupled to a second capacitor and a UHF band input inductor.
are connected in parallel, the first capacitor, the second coil, and the second capacitor constitute a high-pass filter circuit, and the connection point of the first and second coils is grounded at high frequency via the first switching element 24. The second and third switching elements are connected together, and when the first switching element is conductive, the first coil and
The UHF band input inductor is electromagnetically coupled, and at this time, the fourth switching element 38 is opened, and when the first switching element is non-conducting, the second switching element 38 is
selectively conducting the third switching element;
It is characterized by being able to obtain signals in the CATV band and VHF band.

Embodiment A specific embodiment of the present invention will be described below with reference to FIG. In the figure, 18 is the coaxial cable input terminal, 19 is the IF trap, 20 and 21 are the first,
The second capacitor, 22 and 23 are the first and second coils, 24, 25 and 26 are the first to third switching diodes, and 27, 28, 29 and 39 are the first to third switching diodes.
4th coupling capacitor, 30 is a bypass capacitor, 31 to 36, 40 is a bias resistor that supplies bias voltage for opening and closing each switching diode 24 to 26, 38, 37 is the first coil 2
Figure 2 shows a UHF input inductor coupled to Figure 2.

from the input terminal 18 through the IF filter 19
The other end of the capacitor 20 is connected to a series-parallel circuit in which a second coil 23 is connected in parallel to a series circuit of a second capacitor 21 and a first coil 22. The other end of this series circuit is grounded via a first switching diode 24 and a bypass capacitor 30. Both ends of this diode 24 are grounded via resistors 31 and 33, and a connection point between the resistor 31 and the diode 24 is connected via a resistor 32 to a power source BU that controls conduction of the diode 24. A first coupling capacitor 27 is connected to the connection point between the series-parallel circuit and the diode 24, and a resistor 34 is connected to the connection point between the diode 24 and the bypass capacitor 30.
The other ends of the first coupling capacitor 27 and the resistor 24 are commonly connected, and the second switching diode 2
A series circuit of 5 and the second coupling capacitor 28 and a series circuit of the third switching diode 26 and the third coupling capacitor 29 are connected, respectively, and a resistor 35 is connected to the connection point between the diode of each series circuit and the coupling capacitor. Power source BH/L is connected through the resistor 36, and power source BHYP is connected through the resistor 36. A series circuit of a fourth switching diode 38 and a fourth coupling capacitor 39 is connected in parallel to the first capacitor 20.
A resistor 40 is connected to a common connection point of the second and third switching diodes 25 and 26.

This operation will be explained below. First, turn on the power supply BU
ON, power supply BH/L, and BHYP are both OFF.
As a result, the diode 24 becomes conductive, and the first and second
A high-pass filter composed of capacitors 20 and 21 and a second coil 23 and a first coil 22 for UHF inductive coupling are grounded via a capacitor 30.
Therefore, the first coupling capacitor 27 is also grounded at high frequencies, and the second and third coupling capacitors 28 and 29
No high frequency signals appear. At this time, the first and second capacitors 20 and 21 and the second coil 23 are
It constitutes a high pass filter that passes the UHF band.
A signal only in the UHF band is supplied to the first coil 22. The first coil 22 and the input inductor 37 are electromagnetically coupled and a UHF signal is extracted.

Next, turn off the power supplies BU, BH/L and turn off the power.
Turn on BHYP. As a result, a current path consisting of the resistor 35, the diode 25, and the resistors 34 and 33 is formed, and the second switching diode 25 becomes conductive. At the same time, a current path is formed between the resistor 40 and the diode 38, and the diode 38 becomes conductive. on the other hand,
The first switching diode 24 becomes reverse biased, and the first and second coils 2 of the series-parallel circuit
The connection point between 2 and 23 is disconnected from the bypass capacitor 30.

At this time, the first capacitor 20 is high-frequency bypassed by the diode 38 and the fourth coupling capacitor 39, and the first coil 22 is bypassed by the second coil 23.
Since the inductance is small compared to the inductance, it is ignored in the low frequency range, and can be regarded as a parallel circuit of the second capacitor 21 and the second coil 23. Since the first diode 24 is open, the first
Coil 22 and input inductor 37 are not coupled.
A CATV band signal appears at the output terminal via the first and second coupling capacitors 27 and 28.

Also, turn off the current BU and BHYP and turn off the power.
When BH/L is turned on, a VHF band signal is obtained at the output of the third coupling capacitor 29 in the same way as when the power supply BHYP is turned on.

In this way, each band can be selected.

As described above, according to the present invention, it is possible to switch bands while maintaining the best matching condition for each band, and in particular, when receiving all channels, an IF trap can be provided at the tip of the input circuit, which is different from the conventional method. Compared to UHF
It is possible to prevent the deterioration of the IF direction characteristics of the V channel, and improve the rejection ratio of CB radio interference of the V channel.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional example. 18... Same input terminal, 20... First capacitor, 21... Second capacitor, 22... First coil, 23... Second coil, 24... First switching element (first switching diode) ),
25... Second switching element (second switching diode), 26... Third switching element (first switching diode), 37
...Input inductor, 38...Fourth switching element (first switching diode).

Claims (1)

[Scope of utility model registration request] Signals input to the same input terminal in the UHF band,
The input circuit selectively branches into the CATV band and VHF band, and the first capacitor to which the input signal is supplied,
A series-parallel circuit in which the second coil is connected in parallel is connected to a series circuit of the first coil which is electromagnetically coupled to the second capacitor and the UHF band input inductor, and the connection point of the first and second coils is connected to the first switching element. The first coil and the UHF band inductor are electromagnetically coupled to each other when the first switching element is conductive, and the first coil and the UHF band inductor are connected electromagnetically when the first switching element is non-conductive. 2. A tuner input circuit characterized in that the third switching element is selectively made conductive to obtain signals in the CATV band and VHF band.