JP2001086474A - Frequency converting device for catv optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a frequency converting device for accurately reading a data signal transmitted with frequency multiplex in a CATV optical transmission system. SOLUTION: Concerning the frequency converting device for accurately receiving an up band signal, which is sent onto an optical transmission line with frequency multiplex, on the side of a CATV station, an optical signal sent from the optical transmission line is converted to an electric signal and an up band signal is provided and branched by a brancher 231. A carrier wave fC1 is extracted from the branched output and the frequency is divided by a frequency divider 233a. Besides, the frequency of the output signal of a VCO 235 is divided as well and an output signal matched to the carrier wave frequency fC1 is outputted from the VCO 235 by synchronizing the phases of the both. With that output signal, the up band signal branched to the trunk output side of the brancher 231 is demodulated and the original up band signal (frequency fD) is provided. Such a demodulator having a different BPF 232 is parallel connected to an IN terminal so as to deal with frequency multiplex transmission.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency converter for upstream band signals used on the receiving side of an optical transmission line in a CATV optical transmission system. In particular, the present invention relates to a frequency converter that modulates a carrier light with a frequency-multiplexed upstream band signal, transmits the modulated optical signal as an optical signal, and converts the upstream band signal from the transmitted optical signal to the original band with high accuracy and extracts the same.
The present invention provides a data communication and video signal transmission by frequency multiplexing in the upstream band, which allows more consumers to connect.
Applied to ATV optical transmission systems.

[0002]

2. Description of the Related Art Conventionally, an optical signal is transmitted using an optical fiber from a head end of a central unit to a supply area, and the optical signal is converted into a frequency multiplexed electric signal in the supply area, and the electric signal is converted. 2. Description of the Related Art There is known a system for transmitting data to a plurality of distributed electric transmission lines. Also, in the system, an upstream band signal transmitted from each customer is propagated through each electric transmission line, electric signals on a plurality of electric transmission lines are mixed in the same band, and carrier light is modulated by the mixed electric signal. Then, a system for transmitting data to a head end side by an optical fiber is known. In particular, Japanese Patent Application Laid-Open
In -55868, as shown in FIG. 5, each of the upstream band signals s1, s2, s3, and s4 is frequency-converted by a frequency converter, frequency-multiplexed, and transmitted. That is,
The outputs of the respective frequency converters are mixed and frequency-multiplexed by the mixer 17, and the carrier light is modulated by the frequency-multiplexed electric signal and transmitted to the optical transmission line 22. That is, a transmission method has been proposed in which the upstream band is widened using an optical signal and multi-channel transmission is performed.

[0003]

However, in such a system, in the head end 10 of the CATV station,
After each carrier is removed from the frequency-multiplexed upstream signal by, for example, a trap filter (not shown), a carrier having the same frequency as the carrier used for frequency multiplexing is generated on the transmission side by a local oscillator, and demodulated by this carrier. Then, it is necessary to demodulate to the original upstream band of each electric transmission line by frequency conversion. That is, the above s1 to s4 (bandwidth: 10M
Hz to 55 MHz). But,
The trap filter method is not always capable of accurately removing the carrier wave because it is affected by a change in temperature or the like. Therefore, due to the problem of interference due to the carrier used on the transmission side and the problem of frequency error of the carrier on the demodulation side,
Accurate demodulation of the upstream band signal was difficult, and beats and the like occurred in the video signal.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to obtain a demodulated carrier phase-synchronized with a carrier of an upstream band signal on a receiving side of a CATV optical transmission system. It is to enable accurate demodulation of a band signal. This realizes a CATV optical transmission system in which the upstream band is widened, expands the line capacity, and provides high-quality data transmission and multi-channel video transmission using the upstream band to a larger number of subscribers. The purpose is to:

[0005]

Means for Solving the Problems and Action / Effect The invention according to claim 1 for solving the above-mentioned problems is a transmission side that performs electric / optical conversion in an optical transmission line to which a plurality of electric transmission lines are connected. In the above, frequency modulation is performed by modulating and band-converting each carrier with each upstream band signal of the electric transmission path, modulating the carrier light with the frequency-multiplexed electric signal, and transmitting the optical signal to the central unit. In a frequency conversion device used on the receiving side that performs optical / electrical conversion in a CATV optical transmission system, a demodulated carrier wave that is phase-synchronized with each carrier included in the optical / electrically converted electric signal is generated, and the demodulated carrier wave is used to generate an electric signal Is demodulated to obtain each uplink band signal.

According to the first aspect of the present invention, a demodulation carrier phase-synchronized with a carrier used on the transmission side for frequency multiplexing is generated on the reception side, and an upstream band signal frequency-multiplexed using this demodulation carrier is generated. The frequency is converted to the original frequency band. Therefore, since the frequency of the carrier included in the upstream band signal and the frequency of the demodulated carrier completely match, it is possible to completely prevent the interference of the carrier included in the upstream band signal during frequency conversion (during demodulation), A predetermined upstream band signal (for example, 10 to 55 MHz) of each electric transmission line can be obtained.

[0007] According to the frequency converter in the CATV optical transmission system according to the second aspect, the frequency converter further includes a filter for extracting each carrier included in the electric signal, and a voltage control for generating a demodulated carrier. An oscillator,
A first frequency divider for dividing the carrier extracted by the filter, a second frequency divider for dividing the demodulated carrier output from the voltage controlled oscillator, a carrier divided by the first frequency divider, A phase comparator for comparing the phase of the demodulated carrier divided by the 2 frequency divider and controlling the oscillation frequency of the voltage controlled oscillator so that the phase difference is constant (including 0). I do.

The carrier and the demodulated carrier output from the voltage controlled oscillator are frequency-divided, for example, by a factor of ten. Then, the phases of the divided carrier waves are compared. The feedback control of the voltage controlled oscillator is performed so that the phase difference is always constant, for example, 0. As a result, a demodulated carrier wave that is phase-synchronized with a predetermined carrier wave included in the upstream band signal is obtained. That is, even if the carrier has a change in frequency and phase, the demodulated carrier output from the voltage controlled oscillator is synchronized in real time with the phase of the carrier due to the phase synchronization. Therefore, if frequency conversion (demodulation) is performed with the demodulated carrier, an interference signal based on the frequency difference between the carrier and the demodulated carrier is not generated, so that an accurate signal of a predetermined upstream band can be obtained.

According to a third aspect of the present invention, in the CATV optical transmission system, the frequency converter further includes an intermediate frequency oscillator for generating the first intermediate frequency carrier, and an intermediate frequency oscillator. An intermediate demodulator for demodulating an electric signal by the first intermediate frequency carrier and band-converting the upstream band signal to a predetermined low band signal; a filter for extracting a carrier component from the low band signal by the intermediate demodulator; A voltage-controlled oscillator for generating a frequency carrier;
A first frequency divider for dividing the carrier component extracted by the filter, a second frequency divider for dividing the second intermediate frequency carrier output from the voltage controlled oscillator, and a frequency divider divided by the first frequency divider The phase of the carrier component is compared with the phase of the second intermediate frequency carrier divided by the second frequency divider, and the phase difference is constant.
And a phase comparator for controlling the oscillation frequency of the voltage controlled oscillator so that the first intermediate frequency carrier output from the intermediate demodulator is modulated by the second intermediate frequency carrier output from the voltage controlled oscillator. An output modulator.

The present invention has solved the problem that it is difficult to extract only the carrier wave from the frequency region where the frequency is multiplexed and the frequency is increased, from the viewpoint of the separation characteristics of the filter. Therefore, first, the upstream band signal frequency-multiplexed by the intermediate demodulator is once frequency-converted to a predetermined low band. For example, it is converted to a specified band of 10 to 55 MHz.
Since the band-converted signal is not used for demodulating the signal, the intermediate frequency oscillator that generates the first intermediate frequency carrier does not need to be phase-synchronized with the carrier included in the upstream band signal. A carrier component (named as a carrier component because the frequency of the carrier on the transmitting side has been frequency-converted) is extracted from the signal that has been band-converted to a predetermined low band, as in claim 2. A second intermediate frequency carrier phase-locked to this carrier component is generated in the same manner as in claim 2. By modulating and frequency-converting the first intermediate frequency carrier with the second intermediate frequency carrier, it is possible to obtain a demodulated carrier phase-synchronized with a predetermined carrier included in the frequency-multiplexed upstream band signal. Therefore, by demodulating and frequency-converting the upstream band signal frequency-multiplexed with the demodulated carrier, a predetermined upstream band signal (for example, 10 to 55 MHz) of each electric transmission line can be obtained. At this time, since the carrier and the demodulated carrier are in phase synchronization, interference by the carrier during frequency conversion is prevented.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following examples. In the following embodiments, since frequency conversion is obtained by amplitude modulation and amplitude demodulation using a carrier, unless otherwise specified, frequency conversion, modulation, and demodulation are used in the same sense. . Similarly, since modulation, demodulation, and frequency conversion are performed by the multiplier, the multiplier, the modulator, the demodulator, and the frequency converter are used with the same meaning unless otherwise specified. (First Embodiment) FIG. 1 shows a CATV optical transmission system to which a frequency converter according to the present invention is applied. The figure is a system block diagram. The upstream band data signal is, for example, a band of 10 to 55 MHz. The present embodiment relates to an upstream band frequency converter on the receiving side in a CATV optical transmission system in which the upstream band is multiplexed. The frequency converter in the downstream band is the same as the conventional one.

CA to which the frequency converter of the present invention is applied
The TV optical transmission system includes a station-side system 200 that is a CATV station, an optical fiber cable 125 laid from the station-side system 200, and a subscriber-side system 100 connected to the optical fiber cable 125. The subscriber-side system 100 transmits each carrier (frequency: f _C1 ,
f _C2 , f _C3 ), a frequency converter 105 for frequency-converting the modulated signals, a mixer 110 for mixing and transmitting the modulated upstream band signals, and an electric signal transmitted from the mixer 110 to an optical signal. , And a photoelectric converter 120 for sending the optical fiber cable 125 to the optical fiber cable 125.

The optical line system includes an optical fiber cable 12.
5, a photoelectric converter 210 for converting the optical signal transmitted from the photoelectric conversion device 5 into an electric signal, a distributor 220 for distributing the upstream band signal obtained by the photoelectric converter 210, and a frequency conversion of the upstream band signal transmitted from the distributor 220. Frequency converter 23
0, a central device 240 for demodulating the frequency-converted upstream band signal into a data signal and reading the data signal, and the Internet 10 and an Internet interface 250 for transmitting data according to the read data. If not connected to the Internet, in particular, the Internet interface 250 is not necessary. The CATV optical transmission system to which the frequency converter 230 of the present invention is applied is configured as described above.

Each function will be described along the signal flow.
In the subscriber system 100, the frequency converter 105 converts the upstream band signal from each terminal device into a signal of a predetermined band. The frequency of the carrier used for the band conversion is, for example, the frequency f _C1 = 165 MHz and f _C2 = 26.
5 MHz, f _C3 = 365 MHz. The respective upstream band signals obtained by modulating the respective carrier waves with the respective upstream band signals f _D and performing frequency conversion are mixed by the mixer 110. Thereby, frequency multiplexing is performed. That is, each upstream band signal transmitted from the mixer 110 is widened as shown in FIG. The figure is a band diagram schematically illustrating frequency on the horizontal axis and signal strength on the vertical axis. Then, the frequency-multiplexed upstream band signal is input to the semiconductor laser, the carrier light is modulated, and transmitted to the optical fiber cable 125 as an optical signal.

The optical signal transmitted through the optical fiber cable 125 is converted into an electric signal by the photoelectric converter 210 of the central office system 200 and sent to the distributor 220 having a distribution function. The distributor 220 distributes the upstream band signal using a plurality of distributors (not shown). Next, each of the distributed upstream band signals is transmitted to the frequency conversion device 230.

FIG. 3 shows the configuration of the frequency converter 230. The frequency conversion device 230 includes a branching device 231,
A band-pass filter (hereinafter, BPF) 232, a frequency divider 233a as a first frequency divider, and a frequency divider 23 as a second frequency divider
3b, a phase comparator 234, and a voltage controlled oscillator (hereinafter, VC
O) 235, multiplier 23 which is a demodulator (frequency converter)
7. It comprises an amplifier 238. The frequency-multiplexed upstream band signal input from the input terminal IN is supplied to the splitter 2
31 is partially branched. Here, for example, it is assumed that the frequency-multiplexed upstream band signal is a signal obtained by modulating the frequency of the carrier of the frequency f _C1 with the upstream band signal of the frequency f _D and performing frequency conversion. The other of the upstream band signal is supplied to an amplifier 238.
And is input to the multiplier 237. The split signal is input to the BPF 232, and the carrier f _C1 is extracted from the upstream band signal. The extracted carrier f _C1 is then divided by a frequency divider 233a,
The frequency-divided signal is sent to the phase comparator 234. The frequency division at this time is, for example, 1/10 frequency division.

As shown in FIG. 3, the frequency converter 230 which is a feature of the present invention has a VCO 235.
Then, one output OUT2 of the VCO 235 is input to the frequency divider 233b, and the oscillation signal, which is a demodulated carrier, is frequency-divided. The divided signal is output from the phase comparator 2
34, and is compared with the phase of the divided signal of the previous carrier. Then, the phase difference is always constant, for example, 0
By outputting a voltage from the output terminal OUT3 of the phase comparator 234 so as to satisfy the following condition, the phase of the demodulated carrier output from the VCO 235, that is, the frequency, is feedback-controlled.
Thereby, from the output terminal OUT1 of the VCO 235,
A demodulated carrier that is always in phase with the carrier of the frequency f _C1 is output.

In this state, the up-band signal (frequency: -f _D + f _C1 ) from the amplifier 238
235 is demodulated by the output signal f _C1 from the output terminal OUT1. Then, if a signal in the frequency band of the difference between them is extracted by a filter (not shown), an original upstream band signal (frequency f _D ) is obtained from the output terminal OUT (FIG. 3).

The original upstream band signal is demodulated into data and read by the central unit 240 of FIG. 1, and if an Internet connection is designated, the Internet interface 25
0 to the Internet 10. Therefore, if such a frequency conversion device is employed, at the time of frequency conversion, there is no interference due to the carrier wave included in the frequency multiplexed upstream band signal, so that the upstream band signal transmitted by frequency multiplexing using the optical fiber cable is used. Can be accurately demodulated. Therefore, a multi-channel CAT
It becomes a useful frequency converter for data communication of the V optical transmission system.

(Second Embodiment) In the first embodiment, a demodulated carrier for frequency conversion is divided by a carrier of an upstream band signal,
It is obtained by synchronizing the phase with the frequency-divided signal. However, it is difficult to extract a carrier in a high frequency region. In particular, when the upstream band is further increased in band and wide-multiplexed, there is a problem that it becomes difficult to design a band-pass filter for extracting only a carrier wave, for example. That is, when the frequency is further increased, it becomes difficult to accurately separate only the carrier with the above method. Therefore, a second embodiment is provided in which an intermediate frequency oscillator and an intermediate demodulator are provided to cope with this problem.

FIG. 4 shows a second embodiment of the present invention. In the figure, VCO 239C is an intermediate frequency oscillator, and multiplier 239B is an intermediate demodulator. The frequency divider 233a is a first frequency divider, and the frequency divider 233b is a second frequency divider and a multiplier 23.
9D is a modulator. The multiplier 237 is a demodulator (frequency converter). The VCO 235 corresponds to the voltage controlled oscillator of the first embodiment. In the figure, the frequency converter 230 of the first embodiment is further advanced to be a frequency converter 230A. Note that the frequency conversion device 23 of the first embodiment
Portions having the same function as 0 are assigned the same numbers.

The function will be described along the flow of signals.
The upstream band signal (frequency:-
f _D + f _C3 ) is partially branched by the branching unit 231. Here, as in the first embodiment, the frequency f _D is the frequency of the upstream band signal, and the frequency f _C3 is the frequency of the carrier used for frequency conversion on the transmission side. The carrier frequency f _C3 is, for example, 365 MHz or higher.

The other signal is input to the multiplier 237 via the amplifier 238. The branched signal is input to the multiplier 239B via the amplifier 239A. The first intermediate frequency carrier f _B (for example, 300 MHz) output from the VCO 239C is input to the multiplier 239B and demodulated by the input. As a result, if a signal having a frequency of the difference between the two is extracted, the frequency becomes (−f _D +
is converted into f _C3) -f _B. That is, the signal has a predetermined low band, a carrier frequency of 65 MHz, and a bandwidth of 10 to 55 MHz.
The frequency is converted to an upstream band signal of z.

Then, a carrier component (frequency: f _C3 −f _B ) is extracted from the frequency-converted upstream band signal by a surface acoustic wave (SAW) filter 236 at the next stage. The SAW filter 238 is a kind of band-pass filter having a narrow band, and is a filter that accurately extracts a carrier (frequency: f _C3 −f _B ). As in the first embodiment, the VCO 235
From the output terminal OUT1, a second intermediate frequency carrier phase-locked to the carrier component of the frequency (f _C3 −f _B ) (for example, 65 MHz) is obtained.

In this state, the second intermediate frequency carrier (frequency: f _C3 −f _B ) from the VCO 235 and the first intermediate frequency carrier (frequency: f _B ) output from the VCO 239C are multiplied by the multiplier 239D. That is, the second intermediate frequency carrier (frequency: f _C3 −f _B ) from the VCO 235
The first intermediate frequency carrier (frequency: f _B ) of CO 239C is modulated. If a signal having the sum frequency of the two results is extracted, a multiplier 239D outputs a demodulated carrier wave that is phase-synchronized with the carrier wave (frequency f _C3 ) of the frequency-multiplexed upstream band signal. The demodulation carrier is passed through an amplifier 239e, a multiplier 237, an uplink band signal output from the amplifier 238 (Frequency: -f _D + f _C3) to be multiplied. In other words, the upstream band signal output from the amplifier 238 is demodulated by the demodulated carrier wave to obtain the original upstream band (10 to 10).
55 MHz).

Thus, the demodulated carrier (frequency f _C3 )
Is phase-synchronized with the carrier (frequency f _C3 ) used for frequency conversion on the transmission side of the frequency-multiplexed upstream band signal, and prevents interference of the carrier during frequency conversion by demodulation on the reception side. .

In the second embodiment, when demodulating with the first intermediate frequency carrier, the original upstream band (10 to 55 MHz) is used.
However, this may be achieved by lowering the frequency band, and need not be in this band. In the first embodiment and the second embodiment, in FIG. 2, the upstream band (10 to 55 MHz)
Carrier is not specified. This indicates that band conversion has not been performed on the side to be transmitted to the optical transmission line, and there is no carrier for the band conversion, but there is a carrier when output at the terminal device of each customer. I do.

(Modifications) While one embodiment of the present invention has been described, various other modifications are possible. In the above embodiment, the band is mainly multiplexed into four bands including the band not subjected to band conversion, but the number of bands to be multiplexed is not limited. In the first embodiment, if an output signal that matches the frequency of the carrier is obtained from the VCO 235 by improving the frequency division ratio of the frequency divider, the frequency of the carrier is changed to 465 MHz, 565 MHz, or 665 MHz.
Or the like. Further, it is possible to increase the number of channels.

In the second embodiment, the VC0239C controlled by the PLL is employed as the intermediate frequency oscillator. However, the oscillator may be not a VCO as long as it is a highly stabilized oscillator. For example, a crystal oscillator of several hundred MHz may be used. The type does not matter. In the second embodiment, the carrier in the band whose frequency has been converted to the low band is extracted by the SAW filter 236. However, an ordinary filter including a coil and a capacitor or an active filter having a coil, a capacitor, and an amplifier may be used.

[Brief description of the drawings]

FIG. 1 is a configuration block diagram of a CATV optical transmission system to which a frequency converter according to a first embodiment of the present invention is applied.

FIG. 2 is an explanatory diagram of a band of an upstream band signal having a wide band according to the first embodiment of the present invention.

FIG. 3 is a configuration block diagram of a frequency conversion device according to the first embodiment of the present invention.

FIG. 4 is a block diagram illustrating a configuration of a frequency conversion device according to a second embodiment of the present invention.

FIG. 5 is a configuration block diagram of a CATV optical transmission system using a conventional frequency conversion device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Internet 100 Subscriber side system 105 Frequency converter 110 Mixer 120 Photoelectric converter 125 Optical fiber cable 200 CATV station 210 Photoelectric converter 220 Distributor 230 Frequency converter 231 Splitter 232 Bandpass filter 233a, b 1 frequency divider, 2nd frequency divider) 234 Phase comparator 235 Voltage controlled oscillator 236 Surface acoustic wave filter 237 Multiplier (demodulator) 238 Amplifier 239B Multiplier (intermediate demodulator) 239C Voltage controlled oscillator (intermediate frequency oscillator) 239D Multiplier (modulator) 240 Central unit 250 Internet interface

Claims (3)

[Claims] 1. A transmission side for performing electric / optical conversion in an optical transmission line to which a plurality of electric transmission lines are connected, modulating each carrier with each upstream band signal of the electric transmission line to perform band conversion. In a CATV optical transmission system for modulating carrier light with the frequency-multiplexed electric signal and transmitting the modulated optical signal to a central unit.
In a frequency conversion device used on the receiving side that performs electrical conversion, a demodulated carrier wave that is phase-synchronized with each of the carriers included in the optical / electrically converted electrical signal is generated, and the electrical signal is demodulated by the demodulated carrier wave. And a frequency conversion device in a CATV optical transmission system, wherein each of the upstream band signals is obtained. 2. The frequency conversion device further includes: a filter for extracting each carrier included in the electric signal; a voltage controlled oscillator for generating the demodulated carrier; and a frequency divider for dividing the carrier extracted by the filter. A 1 divider; a second divider for dividing the demodulated carrier output from the voltage controlled oscillator; a carrier divided by the first divider; and a divider divided by the second divider. The phase comparator according to claim 1, further comprising: a phase comparator that compares a phase with the demodulated carrier wave and controls a transmission frequency of the voltage-controlled oscillator so that a phase difference is constant (including 0). A frequency converter in a CATV optical transmission system. 3. The frequency conversion device further includes: an intermediate frequency oscillator for generating a first intermediate frequency carrier; and demodulating the electric signal by the first intermediate frequency carrier generated by the intermediate frequency oscillator. An intermediate demodulator that band-converts an upstream band signal to a predetermined low-band signal, a filter that extracts a carrier component from the low-band signal by the intermediate demodulator, a voltage-controlled oscillator that generates a second intermediate-frequency carrier, A first frequency divider for dividing a carrier component extracted by the filter, a second frequency divider for dividing a second intermediate frequency carrier output from the voltage controlled oscillator, and a frequency divider by the first frequency divider The phase of the obtained carrier component is compared with the phase of the second intermediate frequency carrier divided by the second frequency divider, and the oscillation frequency of the voltage controlled oscillator is adjusted so that the phase difference is constant (including 0). A phase comparator to be controlled, and a modulator that outputs the demodulated carrier by modulating the first intermediate frequency carrier output from the intermediate demodulator with a second intermediate frequency carrier output from the voltage controlled oscillator. 3. The frequency converter in the CATV optical transmission system according to claim 2, comprising: