JP3276377B2 - CATV converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a converter for each terminal device in a CATV system.

[0002]

2. Description of the Related Art With the development of new media in recent years, C
The amount of information in ATVs has also increased, and the demand for multi-channel transmission using coaxial cables or optical cables has become extremely large. In accordance with this demand, there is a strong demand for widening the reception frequency band of a CATV converter or a CATV tuner for converting a high-frequency carrier signal into an intermediate frequency signal in a terminal device of a CATV system.

[0003] In response to such a demand, a CATV converter shown in FIG. 3 has been used as a conventional technique. Hereinafter, description will be given based on the drawings. Terminal 1 is CATV
A receiving end of a high-frequency signal transmitted from the center of the system, for example, a signal of 55 MHZ to 550 MHZ, a variable low-pass filter (hereinafter referred to as an LPF) 6 whose cutoff frequency changes according to a control voltage Vt, and a variable LP 7.
This is a mixer that outputs a frequency signal having a difference between the output frequency of F and the output frequency of the local oscillator 8.

A control voltage generator 10 generates a predetermined control voltage Vt based on data from a microcomputer (hereinafter referred to as a CPU) 11 which designates a tuning frequency based on tuning information of a user of the CATV converter. The output terminal 12 is a terminal for outputting an output signal of the mixer 7, for example, an intermediate frequency component of the video signal carrier 612.175 MHZ. In the subsequent stage, the intermediate frequency signal passes through a band-pass filter and an intermediate frequency amplifier circuit. It is mixed with two local oscillation frequencies and output as a second intermediate frequency signal.

[0005] The CATV converter of FIG. 3 performs the following operations and operations. The broadband high-frequency signal transmitted from the CATV center is supplied to the input terminal 1 and the variable LP
Input to F6. When the user selects a channel desired to be viewed, the CPU 11 sends predetermined data according to a channel selection frequency corresponding to the selected channel to the control voltage generator 10. The control voltage generator 10 generates a control voltage Vt in accordance with predetermined data and a control voltage Vt which changes from a low voltage to a high voltage in accordance with a high frequency from a low tuning frequency by using, for example, a digital / analog conversion circuit.

The variable LPF 6 has a control voltage Vt as described later.
Has a frequency characteristic in which the cutoff frequency increases with the rise of
It is also provided to remove frequency signal components higher than the tuning frequency and to prevent the oscillation frequency signal of the local oscillator 8 from flowing backward or leaking into the transmission system of the CATV system. On the other hand, the local oscillator 8 is composed of an oscillation circuit including a varicap or a varistor of a variable capacitance element whose capacitance changes according to the control voltage Vt, and oscillates as an upper local at a frequency higher than the tuning frequency. This is because, in the lower local, the signal leaks into the transmission system of the system or the like, and becomes a disturbing signal to other terminal devices, making it impossible to receive a high-quality signal.

The mixer 7 generally employs a double balanced mixer circuit having good conversion efficiency and low generation of harmonic signals, and a variable LPF 6 from which high frequency components are removed.
And an intermediate frequency signal representing the difference between the output signal of the local oscillator 8 and the output signal of the local oscillator 8 oscillating at a higher frequency.
Output to

The variable LPF 6 is constituted by, for example, a T-type LPF circuit shown in FIG. Here, a T-type LPF is formed by the inductors L1 and L2, the capacitor C1, and the varicap VC1, and the control voltage Vt is supplied to the varicap VC1 via the choke coil L3, and the LPF is cut off as the control voltage Vt increases. A variable frequency characteristic in which the frequency increases is obtained. Here, generally, the cutoff frequency is set to a frequency 50 MHz to 100 MHZ higher than the tuning frequency, and does not disturb the high frequency signal of the tuning frequency component that passes through the influence of the phase shift characteristic and the amplitude characteristic at the shoulder portion of the frequency characteristic. Like that. The variable LPF 6 can cut off the high frequency components of the wide receiving frequency band, thereby reducing the generation of harmonics, cross-modulation, intermodulation interference, and the like in frequency conversion by the mixer 7.

[0008]

However, the above C
In the ATV converter, the level is graded according to the transmission frequency in order to supply a high quality video signal to all the terminal devices of the CATV system as much as possible in consideration of the attenuation characteristic which is one of the transmission characteristics of the CATV system by the coaxial cable or the like. Tilt characteristics. However, cross-modulation in the mixer 7 so In the near field of the terminal device are lowered low-frequency component of the high band in the tilt characteristic is up (hereinafter, referred to as XM.) And cross modulation (hereinafter, However, in the case of a long-distance terminal device having a tilt characteristic in which the high-frequency range is reduced, the component passing through the variable LPF 6 is large and the
XM and IM will occur.

Therefore, a first object of the present invention is to provide such an X
An object of the present invention is to realize a CATV converter that does not generate interference signals such as M and IM. In the CATV converter shown in FIG. 3, the variable LPF 6, the mixer 7, and the local oscillator 8
Are provided as input terminals 1, output terminals 12, supply terminals 13 for the control voltage Vt, and input / output terminals only for the terminals of the power supply input (not shown).
For example, by providing a level difference such as a high or low reception frequency,
Another object of the present invention is to realize a CATV converter that can easily replace the differentiated module as needed.

A further object of the present invention is to make the output level of an AGC circuit, which may be provided to make the input level of the mixer 7 constant, more constant. That is, the AGC circuit detects the signal level of the intermediate frequency amplifying stage subsequent to the mixer 7, and if the signal level is small, the AGC circuit
The gain of the GC circuit is increased to keep the input voltage of the mixer 6 constant. However, in the terminal device in which the tilt characteristic due to the attenuation characteristic of the transmission cable or the like is low in the high frequency range, the high frequency channel is selected. In this case, since the intermediate frequency signal level is low, the gain of the AGC circuit increases, the low frequency component further increases, and eventually, the absolute amount of the input level of the mixer 7 increases, and the XM, An interference wave of the IM is easily generated, and it is intended to prevent deterioration of the image quality due to this kind of distortion.

It is also an object of the present invention to provide a CATV converter that sufficiently matches a terminal device of a CATV system that operates in a wider band due to the increase in information in recent years.

[0012]

Means for Solving the Problems] To achieve the above object, CATV converter of the present invention receives the RF signal from the center of the CATV system, the frequency characteristic by a control voltage corresponding to the channel selection frequency by the station selection A CATV converter for obtaining an intermediate frequency signal by a mixer from the high-frequency signal through a variable LPF that changes and an output signal of a local oscillator whose oscillation frequency changes by the control voltage, wherein the high-frequency signal is provided before the mixer. A switching circuit for selecting a circuit that passes through the circuit and an HPF circuit that cuts off the low frequency band, and switches the switching circuit to the HPF circuit when the control voltage is equal to or higher than a predetermined voltage.

Further, the CATV converter of the present invention has a C
A high frequency signal is received from the center of the ATV system, and the output of a local oscillator whose oscillation frequency changes according to the high frequency signal via a variable LPF whose frequency characteristic changes according to a control voltage corresponding to the tuning frequency by tuning and the control voltage.
CATV for obtaining an intermediate frequency signal from a signal and a mixer
A switching circuit for selecting, by a switching signal, a circuit that passes through the high-frequency signal and a HPF circuit that cuts off the low-frequency signal at a stage preceding the mixer; and the switching by comparing the control voltage with a predetermined voltage. A comparator that outputs a signal, and controls the switching circuit by the switching signal;
When the control voltage is equal to or higher than a predetermined voltage, the switching circuit
Wherein the switching Rukoto the HPF circuit side.

[0014]

In the CATV converter according to the present invention, when the control voltage becomes higher than a predetermined voltage, the band of the high-frequency signal input to the mixer does not include a low-frequency component. When the low frequency component of the tilt characteristics for the optimization of the high frequency component is high, the high frequency component of the low frequency component is cut off, the generation of distortion in the mixer is reduced, and the generation of harmonic signals and interference of XM, IM, etc. Wave generation is reduced, high-quality images are obtained, and these interfering signals generated by the mixer do not leak to the center side of the CATV system, so that the reception quality of other terminal devices may deteriorate. Absent.

In the case where the differentiated module is replaced, the switching signal of the switching circuit is generated by one signal from the control voltage terminal, and the variable LPF and the local oscillator are controlled according to the control voltage. Therefore, only the terminals for the control voltage, the input terminal, the output terminal, and the power supply terminal are provided, and the number of these terminals is small, and the terminal can be easily replaced as required.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. Note that the same parts as those in FIG. 3 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 1, reference numeral 1 denotes an input terminal of a high-frequency signal whose frequency is 55.25 MHZ to 871.25.
MHZ and broadband signals, 2 is a through circuit for passing high-frequency signals, 3 is a high-pass filter (HPF) circuit for passing high-frequency signals described later, and 4 and 5 are through circuits 2 and HPF A switching circuit that switches between the circuit 3 and the switching signal by a switching signal, and 9 is a comparator that compares the control voltage Vt with a predetermined voltage Vref to generate a switching signal.

Next, the operation will be described. First, when the selected channel has a low tuning frequency, the control voltage Vt is low, so that the switching voltage output from the comparator 9 compared with the predetermined voltage Vref is low, and the switching circuits 4, 5
Is set to the terminal a side, and the broadband high-frequency signal from the center is directly input to the variable LPF 6 through the through circuit 2.

The high-frequency signal of the high-frequency signal is cut off by the LPF having a cut-off frequency corresponding to the control voltage Vt by the variable LPF 6 and input to the mixer 7. This mixer 7
Outputs a frequency signal of a difference between an output signal of the local oscillator 8 oscillating at a frequency corresponding to the control voltage Vt and an output signal of the variable LPF 6 and outputs the signal to the output terminal 12 as an intermediate frequency signal. At this time, the oscillation frequency of the local oscillator 8 is higher than the desired reception frequency by an intermediate frequency, for example, 943.75 MHZ.

Next, when the tuning frequency is high, for example, 400
A case of a frequency higher than HZ or 500 MHZ will be described. Since the control voltage Vt by the control voltage generator 10 instructed by the data from the CPU 11 according to the selected channel is high, the switching voltage output of the comparator 9 compared with the predetermined voltage Vref is high, and the switching circuit 4,
5 is switched to the terminal b side. The switch switching frequencies of the switching circuits 4 and 5 are set to approximately half the transmission frequency from the center, taking into account the influence of the tilt of the tilt characteristic. A high-frequency signal from the center is input to the HPF circuit 3, and a frequency component of 300 MHZ or 400 MHZ or more is output.

The low-frequency cut high-frequency signal is input to the variable LPF 6 via the terminal C of the switching circuit 5 and cuts off a frequency component about several tens of MHZ higher than the tuning frequency. The output of the variable LPF 6 is input to the mixer 7 and mixed with the output of the local oscillator 8 which oscillates at an intermediate frequency higher than the tuning frequency by, for example, 943.75 MHZ, that is, 999 MHZ to 1.815 GHZ. Is output as

Thereafter, although not shown, only the desired signal component of the intermediate frequency signal is selected by a dielectric element filter or the like and transmitted to the second mixing stage. The signal is output from the second mixing stage as a second intermediate frequency signal of, for example, 45.75 MHZ, and supplied to a second intermediate frequency amplifier circuit or a descrambling circuit that restores the scrambled video signal to the original video signal.

The high-frequency signal in the above embodiment has a wide band of 55 MHz to 872 MHZ, and may be expanded to 1 GHZ or more in the future. Further, in the above embodiment, C is selected according to the channel selected.
An example in which the control voltage Vt is generated from the control voltage generator 10 based on the data from the PU 11 has been described. However, a PLL circuit is configured by a prescaler, a programmable divider, a phase comparator of a reference frequency, and a low-pass filter from the output of the local oscillator 8. Then, the output of the low-pass filter is set to the control voltage V
t, the division number of the programmable divider
Even if the control is performed based on the data from 11, the same effect as the present invention can be obtained.

Next, a specific example of the peripheral circuit of the HPF 3 will be described with reference to FIG. The capacitors T2 and C3 and the inductor L4 form a basic T-type high-pass filter. A diode D1 whose resistance changes according to an applied current consisting of a switching diode or a PIN diode;
D2, bias resistors R1, R2, and current limiting resistors R3, R
4, R5 and the bypass capacitor C4, the circuit 2, the switching circuits 4, 5, the comparator 9, and the predetermined voltage Vr.
ef.

[0024] The peripheral circuit includes resistors R4, R5 is a supplied control voltage Vt to the connection point, if the forward voltage of more low bias resistors R1, R2 by the divided voltage from the diode D1 of the control voltage Vt supply Vcc Is the diode D2
The circuit 2 is turned off and the diode D1 is turned on to form a through circuit 2. The high-frequency signal is output through the diode D1. Next, when the control voltage Vt is higher than the divided voltage by the diode,
When the voltage is higher than the forward voltage of D2 , the diode D1 is turned off and the diode D2 is turned on to turn on the T-type high-pass filter.
Becomes data, the high frequency signal is low frequency components is blocked through this T-type high-pass filter, only the high frequency component is output. With such a simple configuration, the peripheral circuit of the HPF 3 is configured, and a desired object is achieved.

In the present invention, a case where an AGC circuit is provided before the mixer 7 will be described. The peripheral circuit of the HPF 3 and the variable LPF 6 operate as described above.
Even if the desired frequency component selected is small and the gain of the AGC circuit is high, the tilt characteristic makes it possible to perform the operation even if the high frequency component is at a short distance from the center and the high frequency component is at a high level. Even if the distance is low and the low-frequency component is at a high level, the level of the input signal to the mixer 7 is suppressed to a substantially constant level, and no interfering waves such as XM and IM are generated, and noise components are generated. Does not occur, so that a video signal with good S / N can be obtained.

[0026]

Since the CATV converter of the present invention is configured as described above, the generation of XM or IM interference is reduced with a simple configuration, the generation of harmonic signals is reduced, and the noise mixed into the video signal is reduced. Since the number is reduced, the S / N is improved, and a high-quality video signal can be obtained. Further, even when the mixer and its peripheral portion are modularized, the number of terminals is small, and the differentiated module can be easily replaced.

Furthermore, since the local oscillation frequency signal component and the intermediate frequency signal component do not flow backward or leak to the cable trunk system, interference with other terminal devices and the like is reduced, and C
The transmission efficiency and reliability of the entire ATV system can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a CATV converter according to the present invention.

FIG. 2 is a diagram showing a specific example of a peripheral circuit of the HPF circuit 3 shown in FIG. 1 of the present invention.

FIG. 3 is a circuit diagram of an embodiment in the related art.

FIG. 4 is a view showing a specific example of the variable LPF 6 of FIG. 3 according to the embodiment of the prior art.

[Explanation of symbols]

 Reference Signs List 3 HPF circuit 4, 5 Switching circuit 6 Variable LPF 7 Mixer 8 Local oscillator 9 Comparator 10 Control voltage generator 11 CPU

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masao Hayakawa 2-26-10 Tenjin, Nagaokakyo-shi, Kyoto Inside Murata Manufacturing Co., Ltd. (72) Hidenobu Muranaka 2-26-10 Tenjin, Nagaokakyo-shi, Kyoto In Murata Manufacturing Co., Ltd. (72) Wataru Takekawa Inventor 2-26-10 Tenjin, Nagaokakyo-shi, Kyoto Inspector Murata Manufacturing Co., Ltd. Michiharu Sugawara (56) References JP-A-62-2787 (JP, A) JP-A-48-32425 (JP, A) JP-A-3-177117 (JP, A) JP-A-2-88380 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 7/10 H04N 7/14-7/173

Claims (2)

(57) [Claims] 1. A high frequency signal from a center of a CATV system is received, and an oscillation frequency is changed by the control voltage and the high frequency signal via a variable LPF whose frequency characteristic is changed by a control voltage corresponding to a tuning frequency by tuning. A CATV converter for obtaining an intermediate frequency signal from a local oscillator output signal by a mixer, wherein a switching circuit for selecting a circuit that passes through the high-frequency signal and an HPF circuit that cuts off a low frequency band is provided at a stage preceding the mixer. ,
CAT wherein the switching circuit is switched to the HPF circuit side when the control voltage is equal to or higher than a predetermined voltage.
V converter. 2. A high-frequency signal is received from a center of a CATV system, and an oscillation frequency is changed by the control voltage and the high-frequency signal through a variable LPF whose frequency characteristic is changed by a control voltage corresponding to a tuning frequency by tuning. A CATV converter for obtaining an intermediate frequency signal from a local oscillator output signal from a local oscillator by a mixer, wherein a switching circuit selects a circuit that passes through the high-frequency signal and a HPF circuit that cuts off the low frequency band by a switching signal before the mixer. A circuit for comparing the control voltage with a predetermined voltage and outputting the switching signal;
Signal to control the switching circuit, and the control voltage
When the voltage is higher than the voltage, the switching circuit is switched to the HPF circuit side.
CATV converter characterized by Toggles Rukoto.