JPH06245198A - Output frequency variable modulator for catv - Google Patents

Abstract

PURPOSE:To secure high C/N (carrier against noise) even in a CATV terminal by eliminating the influence of the noise even when plural modulators are used. CONSTITUTION:In this output frequency variable modulator for the CATV for which a synthesizer oscillator 36 is used as a local oscillator for a frequency conversion circuit 35 and a microcomputer 38 decides the oscillation frequency of the synthesizer oscillator 36 based on the set information of an output frequency setting switch 37, a variable tuning band pass filter 42 is provided between the wide-band amplifier 40 of a final stage and an output terminal 41. When an output frequency is set by the output frequency setting switch 37, the microcomputer 38 supplies information for converting the output frequency into a DC voltage to a D/A converter 43. The D/A converter 43, based on the supplied information, supplies voltages for respective output frequencies to the variable tuning band pass filter 42 to be tuned to the output frequencies and noise components generated in other frequency bands are interrupted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable output frequency modulator used in, for example, CATV (cable television).

[0002]

2. Description of the Related Art Conventionally, in a CATV system, a plurality of modulators are used as shown in FIG.

FIG. 4 is a block diagram showing a schematic configuration of a CATV system. In FIG. 4, reference numeral 10 denotes a head end device, and the head end device 10 includes, for example, a CD.
Audio equipments 1 and 2 such as a player and a tape recorder, and video equipments 3 and 4 such as a video tape recorder and a video disc player are connected, and a broadcast receiving antenna 5 for receiving a general broadcast wave is connected. The head-end device 10 is composed of FM modulators 11 and 12, independent broadcast television modulators 13 and 14, a retransmission receiver 15, and a mixer 16.

The FM modulators 11 and 12 in these headend devices 10 modulate the acoustic signals input from the acoustic equipments 1 and 2 so that they can be received by a commercially available FM receiver and input them to the mixer 16.

[0005] In addition, TV modulators 13 and 14 for independent broadcasting
Converts the video signals input from the video equipment 3 and 4 into signals that can be received by a commercially available television receiver, and inputs the signals to the mixer 16. Further, the retransmitting receiver 15 amplifies a general broadcast wave received by the broadcast receiving antenna 5 to a required output level by the retransmitting receiver 10 and inputs it to the mixer 16.

The mixer 16 mixes these signals in a hybrid circuit or the like and outputs them via the CATV transmission cable 17. Since the signal transmitted by the cable 17 is attenuated on the way, it is amplified by the amount attenuated by the relay amplifier 18 and supplied to the transmission cable 17 again.

The transmission cable 17 is provided with a branching device 19 for supplying a signal to the terminal.
The terminal 9 is connected to the terminal 21 via the branch line 20 from the branch terminal 9.

The terminal 21 is provided with a two-divider 22 for dividing a CATV signal into two, one distribution terminal is connected to an audio device such as an FM receiver 23, and the other distribution terminal is a CATV terminal. TV receiver 2 via 24
Connected to 5.

The CATV terminal 24 is
In CATV transmission, the frequency band of general terrestrial broadcast waves (low channel: 90 to 102 MHz, high channel: 170 to 222 MHz, UHF: 470 to 770 M)
Hz), the TV transmission band is 90 to 550 MH
Since there are up to z, it cannot be received by a general commercially available television receiver, so a generic term is used for frequency conversion devices, receivers, etc. that enable even a commercially available television receiver to receive.

With the above-described structure, the FM receiver 23 can receive the signals FM-modulated by the FM modulators 11 and 12 of the head-end device 10, and the head-end device 10 has an independent broadcast television modulator. The signals modulated by 13 and 14 can be received by the television receiver 25. Next, the configuration of the modulator used in the above ATV system will be described. FIG. 5 is a block diagram of a conventional output frequency variable type television modulator.

In FIG. 5, 30 is a video input terminal, 31
Is an audio input terminal. The video signal supplied to the video input terminal 30 is amplitude-modulated by the video IF modulator 32 into a video IF signal, which is supplied to the video / audio mixer 34. Also, the audio signal supplied to the audio input terminal 31 is
The frequency is modulated by the FM modulator 33 into an audio IF signal, which is supplied to the video / audio mixer 34. The video / audio mixer 34 mixes the video IF signal and the audio IF signal and outputs them to the frequency conversion circuit 35.

A local signal is supplied to the frequency conversion circuit 35 from a synthesizer oscillator 36 which is a local oscillator, and the sum or difference with each IF signal is obtained to obtain a required output frequency. Here, 38 is a microcomputer, which operates to determine the oscillation frequency of the synthesizer oscillator 36 based on the setting information of the output frequency setting switch 37.

The video / audio signal converted to the required output frequency by the frequency conversion circuit 35 is a wide band filter 39 (within the maximum variable frequency, for example, 90 MHz to 550 MHz).
90 MHz to 550 MHz when variable up to MHz
After the unnecessary components are removed by the bandpass filter (1), the signal is amplified to a required output level by the broadband amplifier 40, and the output terminal 4
It is output from 1. Next, the conventional output frequency variable type FM
A block diagram of the modulator is shown in FIG. This FM modulator is the same as the television modulator in that only the video IF modulator 32 and the video / audio mixer 34 of the television modulator in FIG. 5 are not provided.

[0014]

In the conventional CATV modulator, a wide band amplifier is required for all the amplifiers to obtain a required output level. Also, this amplifier is usually 30
It has an amplification degree of -40 dB and naturally not only generates thermal noise but also amplifies noise. Moreover, CAT
Uniform noise is generated in the entire V transmission band (for example, from 90 MHz to 550 MHz). Therefore, when a plurality of modulators are used as shown in FIG. 4, noise is also proportional to the number of used units, and the C / N (carrier to noise) of the output of the mixer 16 deteriorates. For example, the C / N of the modulator alone is AdB
If the same modulator is used for 10 units, noise component N0
Is represented by "N0 = 10log n, where n is the number of units used",
"N0 = 10 log 10 = 10 dB".

Therefore, the C of a modulator having a C / N of AdB
/ N deteriorates as "C / N = AdB-10 dB". Therefore, even if the performance of the modulator alone is designed well,
If multiple units are used, the performance will be the same and the high quality image cannot be supplied to the terminal.

The present invention has been made in view of the above circumstances, and provides a variable output frequency modulator capable of supplying a high quality image to a CATV terminal without deterioration of C / N even when a plurality of units are used. The purpose is to

[0017]

DISCLOSURE OF THE INVENTION The present invention is a CATV modulator which uses a synthesizer oscillator and whose output frequency can be varied by an output frequency setting switch. It is provided with a variable tuning bandpass filter provided and means for variably setting the tuning frequency of the variable tuning bandpass filter in accordance with the frequency set by the output frequency setting switch.

[0018]

When the output frequency is set by the output frequency setting switch, the voltage for each output frequency is supplied to the variable tuning band pass filter, and the band pass filter is tuned in conjunction with the output frequency. The band of this variable tuning bandpass filter is TV or F
Since it is set to the band corresponding to M, it blocks noise components generated in other frequency bands, and prevents deterioration of C / N even when using multiple modulators, thus producing high quality images. Supply to the terminal.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a variable output frequency type television modulator according to the present invention. Codes 30 to 4
Since each circuit section indicated by 1 has the same configuration as the conventional modulator shown in FIG. 4, the same reference numerals as those in FIG. 4 are given and detailed description thereof is omitted.

Therefore, in the present invention, the wide band amplifier 40
A variable tuning bandpass filter (tracking filter) 42 that tunes in synchronism with the output frequency is provided between the output terminal 41 and the output terminal 41 to block noise outside the output frequency band. That is, the microcomputer 38 has a D / A
By connecting a converter 43 and supplying a tuning voltage from the D / A converter 43 to the variable tuning bandpass filter 42, the bandpass filter 42 is tuned in synchronization with the output frequency.

When the output frequency is set by the output frequency setting switch 37, the microcomputer 38 instructs the synthesizer oscillator 36 to set the output frequency, and at the same time D /
Information to convert the output frequency into a DC voltage is given to the A converter 43. As a result, from the D / A converter 43,
The voltage for each output frequency is supplied to the variable tuning bandpass filter 42, and the bandpass filter 42 is tuned in conjunction with the output frequency.

The frequency band of the variable tuning bandpass filter 42 is set to 6 MHz when the television broadcast is the NTSC system. When the wideband amplifier 40 is provided in multiple stages, the variable tuning bandpass filter 42 may be connected between the stages.

Further, FIG. 2 shows an example in which it is applied to an FM modulator, which is different from the television modulator shown in FIG. 1 in that the video IF modulator 32 and the video / audio mixer 34 are not provided.
Only the point that the band of the variable tuning bandpass filter 42 is set to 300 KHz is different, and other configurations and operations are the same as those of the television modulator. Next, details of the variable tuning bandpass filter 42 will be described with reference to FIG. The variable tuning bandpass filter 42 shown in FIG. 3 is an example of a two tuning type.

The variable tuning bandpass filter 42 has a signal input terminal 51, a signal output terminal 52, and a tuning voltage input terminal 53. Then, the signal input terminal 51 and the signal output terminal 52
A series circuit of a coupling capacitor 54, a variable capacitance diode 55, and a capacitor 56 is provided between and. The tuning voltage supplied to the tuning voltage input terminal 53 is applied between the variable capacitance diode 55 and the coupling capacitor 56 via a choke coil 57 for blocking high frequency components.

The first portion is provided between the signal input terminal 51 and the ground.
The resonator (tuning circuit) 58 of is connected to the signal output terminal 5
A second resonator (tuning circuit) 59 is connected between 2 and the ground. The first resonator 58 includes a coil 61 in a series circuit of a capacitor 61, a variable capacitance diode 62, and a capacitor 63.
4 are connected in parallel, and the tuning voltage supplied to the tuning voltage input terminal 53 is applied between the capacitor 61 and the variable capacitance diode 62 via the choke coil 65.

The second resonator 59 is formed by connecting a coil 69 in parallel to a series circuit of a capacitor 66, a variable capacitance diode 67 and a capacitor 68. A tuning voltage input terminal is provided between the capacitor 66 and the variable capacitance diode 67. The tuning voltage supplied to 53 is provided via the choke coil 70.

The above-mentioned first resonator 58 and second resonator 5
9 is a tuning voltage input terminal 53 from the D / A converter 43.
The tuning voltage applied via changes the capacitances of the varactor diodes 62 and 67 and changes the tuning point.

By predetermining the DC voltage corresponding to the output frequency and tabulating the DC voltage data in the memory of the microcomputer 38, the data for the output frequency can be easily given to the D / A converter 43 and the tuning DC voltage can be obtained. Obtainable.

Further, the variable capacitance diode 55 is used as the coupling capacitor together with the fixed capacitors 54 and 56. This is done every time the frequency of the tuning circuit is changed.
If the capacitance of the coupling capacitor is not changed, the bandwidth of the bandpass filter 42 changes, so a variable element is used to minimize the amount of change.

By using the variable tuning bandpass filter 42 configured as described above, the output frequency setting switch 37
When the output frequency is set with, the microcomputer 38
Indicates the output frequency to the synthesizer oscillator 36 and gives the D / A converter 43 information for converting the output frequency into a DC voltage. This allows the D / A converter 4
From 3, the voltage for each output frequency is variable tuning bandpass filter 4
2 and the bandpass filter 42 tunes in conjunction with the output frequency.

The variable tuning bandpass filter 42 blocks noise components generated in other frequency bands, and prevents deterioration of C / N even when a plurality of modulators are used, thereby providing a high quality image. Is supplied to the terminal.

[0033]

As described above in detail, according to the present invention, C
Since the variable tuning bandpass filter is provided between the final stage amplifier and the output terminal of the ATV modulator or between the stages of the final stage amplifier, it is possible to prevent noise components generated in other frequency bands, and a plurality of modulators are provided. Even when is used, C / N is not deteriorated and high-quality video can be supplied to the terminal.

Further, by making the bandpass filter a variable tuning type, the bandpass filter can be automatically tuned even if the output frequency is switched, and the modulator can be mass-produced.
That is, even with the same type of modulator, the frequency of the synthesizer oscillator can be set by the output frequency setting switch, and the frequency of the variable tuning bandpass filter can be automatically tuned to the output frequency. Can be mass-produced and production efficiency can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a variable output frequency type television modulator according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a variable output frequency type FM modulator in the embodiment.

FIG. 3 is a circuit configuration diagram showing details of a variable tuning bandpass filter in the embodiment.

FIG. 4 is a block diagram showing a configuration of a conventional CATV system.

FIG. 5 is a block diagram showing the configuration of a conventional output frequency variable television modulator.

FIG. 6 is a block diagram showing a configuration of a conventional output frequency variable FM modulator.

[Explanation of symbols]

1, 2 ... Audio equipment, 3, 4 ... Image equipment, 5 ... Broadcast receiving antenna, 10 ... Headend device, 11, 12 ... FM
Modulator, 13, 14 ... Independent broadcast TV modulator, 15 ...
Retransmission receiver, 16 ... Mixer, 17 ... Transmission cable, 1
8 ... Repeater amplifier, 19 ... Brancher, 20 ... Service line, 2
1 ... Terminal, 22 ... 2 distributor, 23 ... FM receiver, 24 ...
CATV terminal, 25 ... Television receiver, 30 ... Video input terminal, 31 ... Audio input terminal, 32 ... Video IF modulator, 33 ... FM modulator, 34 ... Video / audio mixer, 35 ...
Frequency conversion circuit, 36 ... Synthesizer oscillator, 37 ... Output frequency setting switch, 38 ... Microcomputer,
39 ... Broadband filter, 40 ... Broadband amplifier, 41 ... Output terminal, 42 ... Variable tuning bandpass filter, 43 ... D / A converter, 51 ... Signal input terminal, 52 ... Signal output terminal, 53
... Tuning voltage input terminal, 58 ... First resonator, 59 ... Second
Resonator.

Claims (1)

[Claims] 1. A CATV modulator which uses a synthesizer oscillator and whose output frequency can be varied by an output frequency setting switch. A variable output frequency modulator for CATV, comprising: and a means for variably setting the tuning frequency of the variable tuning bandpass filter in accordance with the frequency set by the output frequency setting switch.