JPS6276914A - Input filter circuit - Google Patents

Abstract

PURPOSE:To vary the resonance frequency to obtain a trap over a wide range and to prevent the generation of distortion by connecting a coil and two varactor diodes connected in anti-parallel in series between a common and a connecting point between two coils connected in series between input and output terminals. CONSTITUTION:An input terminal 30a is connected to an output terminal 30b via coils L11, L12 constituting an input filter 30 in series. A series circuit comprising a coil L13 and varactor diode D11, D12 connected in anti-parallel is connected between a common and a connecting point of the coils L11, L12. A mixer 13 mixes a signal from the input filter 30 and an oscillation output from a local oscillator 14 to output an intermediate frequency signal. The oscillation frequency of the local oscillator 14 is varied by a channel selection voltage from a channel selection circuit 15 in response to the reception frequency. Thus, a high frequency voltage applied to the varactor diodes is less in level and the generation of distortion is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an input filter circuit, and is used, for example, in a high frequency input section of a cable television receiver.

[Technical background of the invention]

An up-down tuner as shown in FIG. 5 is used in a cable television receiver.

This tuner selects the desired channel signal from among the high frequency television signals transmitted from the main line.
It converts the frequency of a channel (for example, a second channel) that can be received by the tuner of a television receiver. The signal supplied to the input terminal 11 is supplied to the first mixer 13 via the input circuit 12. The first mixer 13 is
The oscillation output from the first oscillator 14 and the input signal are mixed to create an intermediate frequency signal. The first oscillator 14 is a local oscillator for tuning, and its oscillation frequency is varied by a control signal from the tuning circuit 15. The intermediate frequency signal from the first mixer 13 is passed through a pantone filter 16, an intermediate frequency amplification 5
J 7 is supplied to the second mixer 19 via the pantonous filter 18 . The second mixer 19 mixes the intermediate frequency signal and the oscillation outputs of the second oscillators 20 to generate, for example, the frequency signal of the second channel. The frequency signal of the second channel produced in this way is supplied to the tuner input terminal of the television receiver via the output circuit 2.

By the way, the configuration of the input circuit 12 is as follows. That is, the i signal from the input terminal 11 is supplied to the mixer 13 via a high nose filter 121, a low nose filter 122, and a variable filter 123.

The variable filter 123 is configured as shown in FIG. 6, and its frequency band is varied depending on the selected channel. That is, the coils Ll and L2 are connected in series between the input and output terminals 23a and 23b, and between the connection point of the coils Ll and L2 and the ground,
Coil L3゜Capacitor C, Variable capacitance diode D1
series circuit is connected. Here, a node switching signal vT is supplied from the terminal 23C via the resistor R1 to the connection point between the capacitor C1 and the cathode of the variable capacitance diode D1.

The band characteristic of this variable filter 123 is 50 MHz to 450 MHz as shown in FIG. 7 (4) when the number of filters is large.
ME (spread in z, if VT is small.

As shown in FIG. 7(B), a trough point Tf is generated.

Therefore, when the reception channel frequency is high, the characteristics are switched to those shown in FIG. 7(B), and when it is low, the characteristics are switched to those shown in FIG. 7(B).

In this way, the variable filter 123 is used to reduce unnecessary channel components that cause cross-modulation when input to the mixer 13, and to reduce this when receiving a low frequency channel. The purpose is to reduce leakage of the high frequency channel signal to the input side (referred to as pack talk phenomenon) due to frequency modulation of the high frequency channel signal by the first oscillator.

[Problems with background technology]

When designing the variable filter 123, since the variable capacitance diode DJ has nonlinear characteristics, it is necessary to set the value of the coil L1 large and the value of the capacitor C1 small in order to suppress the terminal voltage from increasing as much as possible. be. This is because it is possible to reduce the high frequency voltage applied to both ends of the variable capacitance diode DJ and prevent the generation of distorted waves.

However, 1. There is a growing demand for up-down tuners with wider reception frequency bands. This is because the number of p-channels is increasing due to the diversification of various broadcast contents. Therefore, the variable filter 123 is required to have a wide variable band range. In order to realize such a variable filter, it is necessary to reduce the value of the coil L1 so that the resonance point can be set at a high frequency. but,
When the value of coil L1 is decreased, variable capacitance diode D
The high frequency voltage applied to I becomes sharp, and its nonlinear characteristics result in second-order distortion.

Furthermore, as the number of input channels increases, the number of distortion combinations also increases, making it difficult to take measures to suppress distortion.

[Purpose of the invention]

This invention was made in view of the above circumstances, and is an input filter circuit that can easily vary the resonant frequency for obtaining a trap from a high frequency to a low frequency, and that does not cause distortion due to the nonlinear characteristics of a variable capacitance diode. The purpose is to provide

[Summary of the invention]

For example, as shown in FIG. 1, a coil L13 and variable capacitance diodes Dll and D12 are connected in series between the connection point of the coils Lll and L12 connected in series between the input and output terminals and the ground. With this configuration, even if the coil L13 is made smaller and the variable range of the resonance point is expanded,
The high frequency voltage applied to the variable capacitance diodes D11 and D12 can be made small, and wide band variation can be made without generating distorted waves.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which the input terminal 30h has a frequency of 50 MHz to 550 MHz or 50 MHz.
A large number of channel signals transmitted in the range of MHz to 650 MHz are transmitted from the CATV main line. The input terminal 30m is connected to the coil L that constitutes the input filter 30.
ll and L12 in series to the output terminal 3θb. In the input filter 30, a series circuit of a coil L13 and variable capacitance diodes Dll and D12 is connected between the connection point of the coils Lll and L12 and the ground. The variable capacitance diodes Dll and D12 are in opposite directions,
A node control signal vt is applied from a terminal 30C to the connection point of the mutual cathodes via a resistor R11.

The output terminal 30b of the input filter 30 is connected to the next stage mixer 1.
3. Mixer 13 mixes the signal from input filter 30 and the oscillation output from local oscillator 14, and outputs an intermediate frequency signal. The oscillation frequency of the local oscillator 14 can be varied by the tuning voltage from the tuning circuit 15 depending on the receiving frequency.

According to the input filter 30 described above, the trap frequency of this circuit is fo, and the variable capacitance diodes Dll, D1
Let the capacity of 2 be CD I =CD 2. Temporarily, add one filter with trap frequency fo similar to this one.
If you try to realize this with one variable capacitance diode, the result will be fo"2πE". Therefore, Lx-'-Lx1, so a large coil must be used.In order to increase the resonant frequency, LX must be made small. Then, the high frequency voltage applied to the variable capacitance diode becomes sharper and distorted waves are generated.

However, according to the present invention, the coil Lll may be small, and the high frequency voltage applied to the variable capacitance diode is approximately 1 compared to the conventional one.

This improves second-order distortion due to the nonlinear characteristics of the filter. Further, since the variable capacitance diodes Dll and D12 are connected with opposite polarities, the distortion components generated from the diodes themselves are canceled out, and the generation of distortion components is further reduced.

Furthermore, when two variable capacitance diodes are used, a sharper characteristic near the trap frequency is obtained, as shown by the solid line in FIG. 2, than when one is used. Therefore, when the voltage of the control signal VT is increased to shift the trap frequency to a higher direction, it is effective in suppressing the tendency for high frequency components to be reflected back to the input side and leaked. .

The present invention is not limited to the above-described embodiment, but may be configured as shown in FIG. 3. That is, coils L 11 , L12 . L14 is connected in series, and the coil L1 is connected between the connection point of coil Lll and L12 and the ground.
3. Connect variable capacitance diodes Dll and D12 in series. In addition, coil L12. Between the connection point of L14 and ground,
Coil L15, capacitor C11, and variable capacitance diode D13 are connected in series. A control signal V4 is applied from the terminal SOC through a resistor R12 between the capacitor C1l and the variable capacitance diode D13.

According to this embodiment, a trap circuit is added to the circuit shown in FIG. 1 by the coil L15, the capacitor C1, the low capacitance variable capacitance diode itself 3, etc. Therefore, when the voltage of the control signal Vt is high, it becomes a wide band pass filter as shown in FIG. 4 (4), but when the voltage of the control signal Vt is low, it becomes a wide band pass filter as shown in FIG. K. Multiple trap frequency locations occur, making it possible to widen the blocking frequency band.

〔Effect of the invention〕

As explained above, in order to vary the trap frequency over a wide frequency range, the present invention requires only a small high-frequency voltage to be applied to the variable capacitance voltage, which reduces the occurrence of distortion. It is possible to provide an input filter circuit that can obtain unique characteristics.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, FIG. 2 is an explanatory diagram of characteristics near the trap frequency of the circuit in FIG. 1, and FIG. 3 is a circuit diagram showing another embodiment of the invention. Fig. 4 is a characteristic diagram of the circuit shown in Fig. 3, Fig. 5 is a configuration diagram of a CATV receiver, Fig. 6 is a diagram showing a conventional variable filter circuit, and Fig. 7 is a diagram showing a conventional variable filter circuit.
The figure is a characteristic explanatory diagram of the circuit of FIG. 6. L11 to L15... Coil, DIl to D13... Variable capacitance diode. Applicant's representative Patent attorney Takehiko Suzue Figure 1 Figure 2

Claims (1)

[Claims] A first coil and a second coil are connected in series between the input and output parts, and the first
, a series circuit including a third coil and first and second variable capacitance diodes connected in series in opposite directions is connected between the connection point of the second coil and the ground, and the first and second An input filter circuit comprising a terminal for applying a voltage for varying frequency band characteristics to the commonly connected cathodes of variable capacitance diodes.