JPH1188266A - Optical space transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate amplitude distortion within each multiplex signal frequency band by storing an amplitude ratio among multiplexed signals. SOLUTION: Pluralities of signals received by a multiplex modulator 41 are given to frequency modulation sections 46a-46c, where the signals are frequency-modulated by different carrier frequencies f1-f3 as frequency band signals. Then the signals are frequency-multiplexed at a multiplexer 48 via output amplitude selection circuits 47a-47c in response to a characteristic of a coaxial cable 42. The multiplexed signal is sent to an optical transmitter 40 via a coaxial cable 42, led to an AGC amplifier 49, where the signal is amplified with an amplitude suitable for optical modulation. In this case, the signal received by the AGC amplifier section 49 is subject to signal amplitude operation corresponding to an attenuation of the coaxial cable 42, then all signal amplitudes are arranged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical space transmission apparatus for transmitting free space by frequency-modulating and frequency-multiplexing a plurality of signals.

[0002]

2. Description of the Related Art In a conventional optical space transmission apparatus, light such as infrared light is used as a transmission medium in the same manner as generally used wireless communication, and signals are transmitted according to the intensity of the light. In order to use light as a transmission medium, it is necessary to set the operation state to line-of-sight communication, and to install the optical transmitter and receiver in a place where they can see each other. Further, since light can have a wider transmission band than radio waves and is not subject to regulations such as the Radio Law, the degree of freedom regarding its use increases. Therefore, since it is easy to frequency-multiplex a plurality of signals and transmit them, they are generally used when simultaneously transmitting a plurality of channels such as video and audio.

FIG. 8 is a block diagram of a conventional optical space transmission apparatus. On the transmission side, an electric signal frequency-multiplexed by a multiplex modulator 1 is guided to an optical transmitter 3 via a coaxial cable 2 and the optical signal is transmitted. And the light is emitted toward the transmission space. On the receiving side, the optical signal propagating in the space is received by the optical receiver 4, converted into an electric signal, and then guided to the multiplex demodulator 6 via the coaxial cable 5, and the multiplexed signal is converted into each electric signal. To be restored.

In the multiplex modulator 1, input terminals 7a, 7
The outputs of b and 7c are frequency modulation sections 8a, 8b and 8 respectively.
c, and the outputs of the frequency modulators 8a to 8c are connected via a multiplexer 9 to an equalizer circuit 10 corresponding to the frequency-amplitude characteristics of the coaxial cable 2. The output of the equalizing circuit unit 10 is connected from the output terminal 11 to the input terminal 12 of the optical transmitter 3 via the coaxial cable 2.

In the optical transmitter 3, the input terminal 12 is A
The output of the AGC amplifier 13 is connected to an electro-optical modulator 14 and a light source 15 such as a laser diode or an LED.
6 are arranged.

In the optical receiver 4, a receiving optical system 17 for receiving an optical signal and a light receiving element 18 such as an avalanche photodiode or a PIN diode are disposed. The output of the light receiving element 18 is an optical-electrical conversion unit. 19, AG
The C amplifying unit 20 and the output terminal 21 are sequentially connected, and the output terminal 21 is connected to the input terminal 22 of the multiplex demodulator 6 via the coaxial cable 5.

In the multiplex modulator 6, the input terminal 22 is connected to an equalizing circuit section 23 for compensating the frequency-amplitude characteristics of the coaxial cable 5, and the output of the equalizing circuit section 23 separates a multiplex signal into respective bands. The frequency demodulation units 25a, 25b, 25c, the output terminals 26a, 26b,
26c.

In the spatial optical transmission apparatus having the above-described configuration, the three-channel video signals input to the input terminals 7a to 7c of the multiplex modulator 1 are frequency-modulated to different carrier frequencies by the frequency modulators 8a to 8c. . The modulated signals are located in frequency bands that do not overlap each other, and are frequency-multiplexed in the multiplexing unit 9. Further, the signal is guided to the equalization circuit unit 10, and after preliminarily compensating for the characteristics of the coaxial cable 2, the signal is guided to the optical transmitter 3 via the coaxial cable 2.

The frequency multiplexed signal guided to the optical transmitter 3 is
After being amplified to a constant amplitude by the AGC amplifier 13 and converted into an optical signal by the electro-optical converter 14, the light is transmitted from the light source 15 to the space via the transmission optical system 16.

On the other hand, the optical signal propagating in the space reaches the optical receiver 4 and is condensed by the receiving optical system 17 so that the light receiving element 1
The light-to-electricity conversion unit 20 converts the light into an electric signal. Since the amplitude of the signal propagating in the space is affected by attenuation in the space and fluctuation of the atmosphere, the signal is converted to a constant amplitude by the AGC amplifier 20 and then transmitted to the multiplex demodulator 6 by the coaxial cable 5. Be guided.

The multiplexed signal input to the multiplex demodulator 6 is compensated for its amplitude characteristic in the band system in the equalizer circuit 23. Further, the multiplexed signal is divided into three by the demultiplexer 24, demodulated by the respective frequency demodulators 25a to 25c, and the respective video signals are output to the output terminals 26a to 26c.

The optical transceivers 3 and 4 and the multiplex modulator 1 and the multiplex demodulator 6 are connected to each other by coaxial cables 2 and 5, respectively, assuming that they are installed at remote locations. However, there is no problem even if the optical transmitter 3 and the multiplex modulator 1 are integrated in the same housing.
Since it is necessary to install the device in a place where it can be seen, it is more convenient to actually separate the device as in the conventional example.

FIG. 9 shows the attenuation characteristics of the coaxial cables 2 and 5, and the relationship between the amount of attenuation per unit length and frequency is generally expressed by the following equation. L = A · f ^1/2 + B · f [dB / m]

Here, L is the attenuation [dB] per unit length,
f is a frequency [Hz], and A and B are constants representing attenuation determined by the material and shape of the coaxial cables 2 and 5. That is,
A is a loss factor relating to the conductors of the coaxial cables 2 and 5 and indicates a conductor loss due to a skin effect, and this term increases in proportion to the square root of the frequency f. B is the coaxial cable 2, 5
Is a loss coefficient related to a dielectric used in the above equation, and represents an attenuation amount due to a dielectric loss, and increases in proportion to the frequency f. And
Generally, the value of B is much smaller than that of A and can be almost ignored at a frequency of 1 GHz or less.

The coaxial cable 2 having such characteristics,
5 is used for connection of a frequency multiplexed signal, each has a different carrier frequency. Therefore, when passing through the coaxial cables 2 and 5, which are the same transmission path, the output amplitude decreases as the carrier frequency increases. Become. Further, when each multiplexed modulated wave is frequency-modulated in a wide band like a video signal, distortion occurs in a band signal even within the band.

For example, even when the frequency-modulated signals subjected to FM modulation from the multiplex modulator 1 are output with equal and flat amplitude characteristics as shown in FIG. The signal input to the optical transmitter 3 is a signal having a different amplitude according to the attenuation characteristic A of the cable as shown in FIG. At this time, A
Since the GC amplifying unit 13 has a function of keeping the amplitude obtained by combining the multiplexed signals constant, the difference between the respective signal amplitudes is preserved, and the relationship between the input converted noise power in the optical transmitter 3 and the input signal amplitude is obtained. The higher the frequency, the lower the C / N (Carrier to Noise Ratio) becomes.

Therefore, before outputting a signal from the multiplex modulator 1, an equalizing circuit section 10 exhibiting an attenuation characteristic opposite to that of the coaxial cable 2 to be used is inserted, and multiplexed at the input end of the optical transmitter 3. Set so that the signal amplitudes are the same. In addition,
This equalization circuit unit 10 may be inserted at the input end of the optical transmitter 3. Similarly, on the receiving side, the optical signal received by the optical receiver 4 is converted into an electric signal, amplified, and then subjected to the same distortion in the process of being guided to the multiplex demodulator 6 by the coaxial cable 5. An equalizing circuit 2 is connected to the input terminal of the demodulator 6
3 is inserted to compensate for the amplitude characteristic.

FIG. 11 shows a configuration diagram of the equalization circuit. Three types of cables having different lengths or cables having different attenuation characteristics are used. High frequency switching circuits 27, 2 with one contact and three contacts
8 are arranged one by one on the input / output side, and between the high-frequency switching circuits 27 and 28, basic blocks formed by circuits 29 to 37 for providing the reverse characteristics of the coaxial cables 2 and 5 are arranged in three rows. ing. In this case, a primary type or secondary type bridging circuit as shown in FIG. 12 (a) or (b) is used since individual characteristics can be easily connected in succession.

In the case of these circuits, it is possible to match the characteristic impedance of the signal source and the load over the entire band in which each bridge circuit is used, so that the individual frequency characteristics can be easily synthesized. It is possible to In some cases, a combination of a resistor and a capacitor or an inductor and a capacitor is provided with an active element such as a transistor or an FET. With such a method, it is possible to equalize the amplitude of each multiplexed signal and remove distortion in the band system.

[0020]

However, in the above-mentioned conventional example, the types of the coaxial cables 2 and 5 that can be used are limited to the number of blocks of the prepared equalizing circuit sections 10 and 23, and therefore, three types of coaxial cables 10 and 23 are provided. Only coaxial cables 2 and 5 can be supported. Further, in order to increase the types of the coaxial cables 2 and 5 to be used, it is necessary to prepare an equalizing circuit block corresponding thereto, but at the same time, the number of contacts of the high frequency switching circuits 27 and 28 for switching the same increases. There is a need.

The usable high-frequency switching circuits 27 and 28 include:
Since the passing signal band is very wide, a semiconductor switch such as a diode or an FET generally used cannot be used, and a mechanical high-frequency relay represented by a coaxial relay or the like must be used. In general, there are many types of coaxial relays having one contact and two contacts, which are relatively inexpensive. However, if the configuration shown in FIG. 11 is used, at least four coaxial relays are required, which increases the circuit size and increases the cost. Become.

Further, when configuring the equalizing circuit sections 10 and 23, a large amount of simulation and confirmation experiments are required to realize the reverse characteristics of the coaxial cables 2 and 5, and furthermore, the equalizing characteristics are continuously reduced. There is a problem that it is very difficult to make the change dynamically.

An object of the present invention is to solve the above-mentioned problems,
It is an object of the present invention to provide an optical space transmission apparatus that preserves an amplitude ratio between multiplexed signals and compensates for amplitude distortion in each multiplexed signal band.

[0024]

According to the present invention, there is provided a space optical transmission apparatus according to the present invention, wherein one or a plurality of signals are frequency-modulated and frequency-multiplexed, and an optical signal is used as a transmission medium. An optical space transmission apparatus using a transmission path as a free space, characterized by having an equalizing circuit function for compensating for the characteristics of a coaxial cable used for transmitting and receiving electric signals during transmission.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the embodiments shown in FIGS. FIG. 1 shows a configuration diagram of an optical space transmission apparatus according to an embodiment, in which an optical transmitter 40 and a multiplex modulator 41 are connected by a coaxial cable 42 on the transmission side, and an optical receiver 43 and a multiplex demodulator 44 are coaxial on the reception side. Cable 45
Connected by

In the multiplex modulator 41, the outputs of the frequency modulators 46a, 46b and 46c having different carrier frequencies f1, f2 and f3 are output amplitude selecting circuits for giving attenuations according to the characteristics of the coaxial cable 42, respectively. It is connected to the multiplexer 48 via 47a, 47b, 47c. In the optical transmitter 40, the output of the AGC amplifying unit 49 is sequentially connected to an electro-optical conversion unit 50 and a light source 51 such as a laser diode or an LED. A system 52 is arranged.

In the optical receiver 43, a receiving optical system 53 and a light receiving element 54 such as an avalanche photodiode or a PIN diode are arranged in the light receiving direction. 56 are sequentially connected. In the multiplex demodulator 44, the output of the demultiplexer 57 is output to the FM demodulators 58a, 58b,
8c.

With the above-described configuration, the plurality of signals input to the multiplex modulation device 41 are frequency-modulated by the frequency modulation units 46a to 46c to different carrier frequencies f1 to f3, respectively, to become band-based signals. These signals are frequency-multiplexed by the multiplexer 48 via the output amplitude selection circuits 47a to 47c according to the characteristics of the coaxial cable 42, respectively.

The multiplexed signal is transmitted to the optical transmitter 40 via the coaxial cable 42, guided to the AGC amplifier 49, and amplified to a signal amplitude suitable for subsequent optical modulation. At this time, all the signal amplitudes of the signals input to the AGC amplifying section 49 are uniform because the signal amplitude operation corresponding to the attenuation of the coaxial cable 42 has been performed in advance.

FIG. 2 shows the attenuation characteristics of the coaxial cables 42 and 45 at a predetermined length. Since the attenuation characteristics are almost proportional to the square root of the frequency, the attenuation increases as the frequency increases. Accordingly, the three types of signals are f1 to f3, respectively.
When the frequency is modulated by (f1 <f2 <f3), if the attenuation amounts are L1, L2, and L3, respectively, L
The relationship 1 <L2 <L3 holds, and when the signal amplitude before transmission is constant, the signal amplitude after transmission is more attenuated as the frequency increases as shown in FIG. 3 as the frequency increases. Therefore, by giving predetermined attenuation to each signal by the output amplitude selection circuits 47a to 47c, the amplitude after transmission by the coaxial cable 42 can be made uniform.

That is, the signal of the carrier frequency f1 is attenuated by the difference between the attenuations L3 and L1, and the signal of the carrier frequency f2 is attenuated by the difference between the attenuations L3 and L2. By not attenuating the signals of the carrier frequencies f1 to f3, the signals of the carrier frequencies f1 to f3 become uniform after transmission by the coaxial cable 42. At this time, attenuation may be given to the carrier frequency f3,
Assuming that the amount of attenuation at this time is La, the amounts of attenuation added to the carrier frequency f1 and the carrier frequency f2 are (L3
-L1 + La) and (L3-L2 + La).

Further, if the attenuation of the carrier frequencies f1 to f3 for other cables of different lengths or types is known,
Similarly, the amplitude after transmission can be made constant. At this time, the sum of the cable attenuation and the previously given attenuation is always equal to the cable attenuation L with respect to the carrier frequency f3.
3 so that the signal amplitude at the input end of the optical transmitter 40 can always be constant regardless of the type of cable.
The GC range can be narrowed.

As for the output amplitude selection circuits 47a to 47c, the amount of attenuation given in advance depends on the length and type of the coaxial cable 42 to be used, and the value has a different value depending on the signal frequency to be multiplexed. Fixed attenuators of the number obtained from the type and multiplex number of the cable 42,
A switching circuit for switching between them is required. For example, when three types of coaxial cables are used and the multiplex number is 3, 3 × 3 = 9 types of fixed attenuators and their switching circuits are required.

FIG. 4 shows the configuration of a fixed attenuator, which is composed of a simple basic block circuit such as a general resistor attenuator. In addition, since a switching circuit for switching these basic block circuits does not require a wide band, a conventional semiconductor high-frequency switch circuit using a diode or the like can be used.

FIG. 5 shows a configuration diagram of the output amplitude selection circuit.
Three fixed attenuators 61a to 61c, and high-frequency switching circuits 62a to 62c and 63a to 63c such as three semiconductor switches are connected before and after these fixed attenuators 61a to 61c. Thus, when multiplexing three signals, it is necessary to configure three types of this block circuit.

In this embodiment, by using a fixed attenuator which can be configured very simply for a circuit having an attenuation characteristic, it becomes unnecessary to use an expensive coaxial relay as a switching circuit for switching between them.

In the conventional example, an equalizing circuit section is provided on both the communication side and the receiving side. In this embodiment, fixed attenuators 61a to 61c and high frequency switching circuits 62a to 62c, 6 are provided on the communication side.
Output amplitude selection circuits 47a to 47c each including 3a to 63c
Is arranged. Considering that the influence of the distortion of the odd object in the band system is small, the limiting function or the AGC function of each of the FM demodulators 48a to 58c for the respective signals in the multiplex demodulator 44 is multiplexed with the optical receiver 43. Regarding the difference in attenuation depending on the type of the coaxial cable 45 connecting the demodulators 44, if the function is sufficiently satisfied, the output amplitude selection circuits 47a to 47c are provided on the receiving side.
The configuration may be simplified without using.

FIG. 6 shows a configuration diagram of an equalizer circuit using an attenuator circuit of a variable attenuator that continuously varies the amount of attenuation.
Three PIN diodes Da, Db, Dc are arranged,
The direct currents Ia and Ib flowing through the terminals a and b are controlled according to the coaxial cable 45 used. As a result, the input / output impedance of the attenuator circuit can be kept constant, a desired amount of attenuation can be obtained, and equalization can be realized for an arbitrary cable. As described above, since the amount of attenuation can be continuously varied without having a switching circuit, it is possible to cope with more types of cables.

FIG. 7 shows a configuration diagram of an equalization circuit based on an AGC circuit using active elements. The equalization circuit includes a variable gain amplifier 65, a detection circuit 66, and a voltage comparison circuit 67. This AGC circuit is a closed loop and has a function of making the output signal amplitude constant according to the voltage applied to the output level setting terminal c.

Also in this case, equalization can be realized for an arbitrary cable. By using an attenuator circuit or an AGC circuit that can be continuously varied, types of cables that can be equalized are not limited.

[0041]

As described above, the optical space transmission apparatus according to the present invention can significantly reduce the time required for design and experiment, and does not use expensive coaxial relays. With the configuration described above, a sufficient equalization circuit function can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical free space transmission apparatus according to an embodiment.

FIG. 2 is a graph of attenuation of a coaxial cable.

FIG. 3 is an equalization circuit

FIG. 5 is a configuration diagram of an output amplitude selection circuit.

FIG. 6 is a configuration diagram of a variable attenuator.

FIG. 7 is a configuration diagram of another AGC amplifier.

FIG. 8 is a configuration diagram of a conventional optical space transmission apparatus.

FIG. 9 is a graph of attenuation of a coaxial cable.

FIG. 10 is a graph showing amplitude distortion caused by a coaxial cable.

FIG. 11 is a configuration diagram of an equalization circuit.

FIG. 12 is a configuration diagram of a bridging circuit.

[Explanation of symbols]

 Reference Signs List 40 optical transmitter 41 multiplex modulator 43 optical receiver 44 multiplex demodulator 46a to 46c frequency modulator 47 to 47c output amplitude selection circuit 48 multiplexer 49, 56 AGC amplifier 51 light source 54 light receiving element 57 duplexer 58a to 58c FM demodulation unit

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[Procedure amendment]

[Submission date] December 11, 1997

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Change

[Correction contents]

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical free space transmission apparatus according to an embodiment.

FIG. 2 is a graph of attenuation of a coaxial cable.

FIG. 3 is a graph showing characteristics of an equalization circuit.

FIG. 4 is a configuration diagram of a fixed attenuator.

FIG. 5 is a configuration diagram of an output amplitude selection circuit.

FIG. 6 is a configuration diagram of a variable attenuator.

FIG. 7 is a configuration diagram of another AGC amplifier.

FIG. 8 is a configuration diagram of a conventional optical space transmission apparatus.

FIG. 9 is a graph of attenuation of a coaxial cable.

FIG. 10 is a graph showing amplitude distortion caused by a coaxial cable.

FIG. 11 is a configuration diagram of an equalization circuit.

FIG. 12 is a configuration diagram of a bridging circuit.

[Description of Signs] 40 Optical transmitter 41 Multiplexer 43 Optical receiver 44 Multiplexer / demodulator 46a-46c Frequency modulator 47-47c Output amplitude selection circuit 48 Multiplexer 49, 56 AGC amplifier 51 Light source 54 Light receiving element 57 Demultiplexer 58a-58c FM demodulation unit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI H04B 10/152 H04B 9/00 S 10/142 10/04 10/06 H04J 14/00 14/02 H04B 10/14

Claims (4)

[Claims] 1. An optical space transmission apparatus in which one or a plurality of signals are frequency-modulated and frequency-multiplexed, an optical signal is used as a transmission medium, and a transmission path is a free space. An optical space transmission device having an equalization circuit function for compensating the characteristics of a coaxial cable used for transmission and reception. 2. The optical space transmission apparatus according to claim 1, wherein a fixed attenuator is used for the equalization circuit function. 3. The optical space transmission apparatus according to claim 1, wherein a variable attenuator is used for the equalization circuit function. 4. The optical space transmission apparatus according to claim 1, wherein an AGC amplifier is used for the equalization circuit function.