JPH06164548A - Optical spacial transmission device - Google Patents

Abstract

PURPOSE:To avoid the deterioration of transmission quality by setting the frequency of a modulation signal or the frequency of an intermediate frequency signal lest the second higher harmonics of a modulation signal and spurious output are mixed. CONSTITUTION:A system control circuit 34 transmits control data DA-DF to respective slots 22A-22F and sets the frequency of a transmission/reception channel. In such a case, a private band is set to 26MHz and the frequency intervals between adjacent channels to 45MHz. When the center frequency f1 of the modulation signal is set higher than 315MHz and the frequency of the intermediate frequency signal to the range of 620-661MHz, a specified expression is satisfied and the deterioration of transmission quality can be avoided. The band of an income circuit 26 is set so that six channels can be secured, and therefore the frequency band of the modulation signal inputted/outputted to and from the circuit 26 is set in the upper frequency 605-625MHz of a sixth modulation signal. Thus, the information signals of the plural channels can be transmitted by multiplexing the frequencies.

Description

Detailed Description of the Invention

[0001]

[Table of Contents] The present invention will be described in the following order. Field of Industrial Application Conventional Technology Problem to be Solved by the Invention (FIG. 10) Means for Solving the Problem (FIGS. 1 and 8) Action (FIGS. 1 and 8) Example (1) Overall Configuration (FIG. 1) (2) Slot connection (FIGS. 2 and 3) (3) Transmission board (FIG. 4) (4) Reception board (FIG. 5) (5) Income circuit (FIG. 6) (6) Frequency allocation (FIG. 7) (FIG. 9) (7) Effect of the embodiment (8) Other embodiment Effect of the invention

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical space transmission device,
In particular, it can be applied to the case of transmitting and receiving a desired information signal to and from a transmission target through a light beam propagating in space.

[0003]

2. Description of the Related Art Conventionally, in this type of optical space transmission device, a portion of a transmission light beam sent to a transmission object is folded back and received, whereby the irradiation position of the transmission light beam can be easily detected on the transmission side. The one that is designed to be obtained is proposed (Japanese Patent Application No. 63-138120).

That is, in this type of optical space transmission device, a laser diode is driven by a predetermined information signal, and a transmission light beam having a predetermined plane of polarization is emitted from the laser diode. As a result, in the optical space transmission device, the transmission light beam is sent out to the transmission target via the predetermined optical system, and the information signal is transmitted via the transmission light beam L1.

Further, in the optical free space transmission apparatus, the received light beam coming from the transmission object is received by a predetermined light receiving element, and the output signal of this light receiving element is processed by a predetermined signal processing circuit. As a result, in the optical space transmission device, the information signal to be transmitted is received via this reception light beam.

Further, in the optical space transmission device, a part of the transmission light beam is separated and folded back in parallel with the optical axis of the transmission light beam, and the folded back transmission light beam is imaged together with the scenery of the transmission target. As a result, in the optical space transmission device, the irradiation position of the transmission light beam can be detected as a bright luminescent spot in the captured image of the transmission target, which can simplify the installation work and can be carried outdoors, for example. It is designed so that a video signal picked up by a television camera can be easily relayed.

[0007]

By the way, the optical communication system of this type is characterized in that it can secure a large frequency band for transmission as compared with a normal micro line or the like. Therefore, in this type of optical space transmission device, for example, by frequency-multiplexing and transmitting a video signal or the like,
It is considered that information signals of multiple channels can be efficiently transmitted.

However, the laser diode used for this kind of optical communication has a characteristic that non-linear distortion occurs. Especially, in the laser diode used for the optical space transmission device, by using a high-power radar diode, The characteristic of this non-linear distortion is large. Further, in this type of optical space transmission device, it may be carried and used outdoors. In this case, the temperature greatly changes due to direct sunlight, etc., and the characteristics of the laser diode greatly change. Characteristics deteriorate.

Therefore, in this type of space optical transmission apparatus, the generation of harmonics due to this type of non-linear distortion is unavoidable, and if an attempt is made to use a large band efficiently, this harmonic will interfere with other channels. There was a fear of giving. Further, when the information signal is once converted into the intermediate frequency signal when the information signal is frequency-multiplexed in this way, spurious signals at the time of frequency conversion may also interfere with other channels. Under the influence of
In the information signal, there is a problem that the transmission quality deteriorates.

One method for solving this problem is shown in FIG.
As shown in 0, the frequency fIF of the intermediate frequency signal SIF is selected to be higher than the frequency band used for transmission, and the second-order harmonics of each channel CH1, CH2, CH3, ... Are the channels CH1, CH2, CH3, ... A method of arranging the channels CH1, CH2, CH3, ... so that they are located between them can be considered. However, with this method,
In the second harmonic of each channel CH1, CH2, CH3, ..., Channels CH1, CH2, CH3 ,.
In order not to interfere with each channel CH1, CH2, CH3, ... by expanding the band twice for ..., the distance D between each channel CH1, CH2, CH3, ... is sufficient. Need to be wide.

Therefore, when the frequency allocation is selected in this way, the frequency band cannot be used efficiently,
Regarding the local oscillation circuit at the time of frequency conversion, it is necessary to greatly switch the oscillation frequency for each channel,
The entire structure becomes complicated accordingly. Further, in this case, the intermodulation component SB of the intermediate frequency signal SIF and one channel (in this case, the fifth channel CH5) is mixed with the other channel (in this case, the first and second channels CH1 and CH2).
However, the transmission quality is deteriorated in this case as well.

On the other hand, a method of reducing the generation of harmonics by using the linear region of the laser diode by making the modulation depth of the laser diode shallow can be considered.
In the case of this method, the transmitted light beam cannot be used efficiently, and the transmission efficiency is deteriorated accordingly.

Furthermore, in the case of transmitting a video signal with this type of optical space transmission device, if the band used for transmission is selected to be a low frequency band, it can be carried around and used in various places. It may be affected by the television broadcasting signal.

The present invention has been made in consideration of the above points, and proposes an optical space transmission apparatus capable of frequency-multiplexing and transmitting information signals of a plurality of channels while effectively avoiding deterioration of transmission quality. It is what

[0015]

In order to solve such a problem, according to the present invention, an information signal SVA of a plurality of channels is used.
Multiplexing circuits 24 and 28 for frequency-multiplexing I to SVFI and SAAI to SAFI to generate a frequency multiplexed signal S1.
And the transmission light beam L1 modulated by the frequency-multiplexed signal S1
And a transmission optical system 15B, 15C for transmitting the transmission light beam L1 emitted from the laser light source 13 to a transmission target, and frequency multiplexing circuits 24, 2
Reference numeral 8 denotes a plurality of channels of information signals SVAI to SVFI,
A plurality of modulation circuits 40 for respectively modulating the SAAI to SAFI to generate intermediate frequency signals SIF of a plurality of channels held at a predetermined frequency, and a plurality of modulation circuits 40 for converting the intermediate frequency signals SIF of the channels to a predetermined frequency. It has a plurality of frequency conversion circuits 42, 44 and 46 for generating a plurality of channels of modulation signals SMA to SMF, and an adder circuit 24 for adding a plurality of channels of modulation signals SMA to SMF to generate a frequency multiplexed signal S1. , The lowest frequency of the intermediate frequency signal SIF is set higher than the highest frequency of the frequency multiplexed signal S1, and the modulation signal SMA of multiple channels
The highest frequency of the intermediate frequency signal SIF is set to be lower than the frequency twice the lowest frequency of the modulation signal SMA for the modulation signal SMA having the lowest frequency among the SMFs.

Further, in the present invention, the information signals SVAI to SVFI and SAAI to SAFI of a plurality of channels are frequency-multiplexed, and the multiplexing circuits 24 and 28 for generating the frequency multiplexed signal S1 and the frequency multiplexed signal S1 are modulated. The laser light source 13 for generating the transmission light beam L1 and the transmission optical systems 15B, 15C for transmitting the transmission light beam L1 emitted from the laser light source 13 to the transmission target are provided, and the frequency multiplexing circuits 24, 28 are provided with a plurality of channels. Of the plurality of channels and each of the plurality of channels of modulation signals SMA to SMF for modulating each of the information signals of FIG.
Adder circuit 2 for adding MF to generate a frequency multiplexed signal
4 and the modulation signal S having the lowest frequency and the modulation signal S having the highest frequency among the modulation signals SMA of a plurality of channels.
For the MF, the highest frequency of the modulation signal SMF having the highest frequency is set to be lower than twice the lowest frequency of the lowest frequency modulation signal SMA.

Further, in the present invention, the frequency multiplexing circuits 24 and 28 are provided with the modulation signals SMA to SM of a plurality of channels.
Of F, the first, second, and third modulated signals SMA, SMB, and SMD in which the frequencies are adjacently and sequentially set,
After the first and second modulation signals SMA and SMB, the frequency difference of 2 between the first and second modulation signals SMA and SMB is 2
The frequency of the third modulation signal SMD is set at a frequency equal to or more than twice the frequency.

Further, in the present invention, there is provided a channel number detecting means 34 for detecting the channel number of a plurality of channel information signals, and the frequency multiplexing circuits 24, 28.
Is based on the detection result of the channel number detecting means 34,
When the number of channels is large, the second and third modulation signals SM
The frequency of the fourth modulation signal SMC is set to a frequency band approximately in the middle of B and SMD.

[0019]

The lowest frequency of the intermediate frequency signal SIF is set higher than the highest frequency of the frequency-multiplexed signal S1, and the lowest one of the modulation signals SMA having the lowest frequency among the modulation signals SMA to SMF of a plurality of channels. By setting the maximum frequency of the intermediate frequency signal SIF to be lower than twice the frequency, it is possible to reduce the interference due to the second harmonic and spurious of the modulation signals SMA to SMF.

Further, regarding the modulation signal having the lowest frequency and the modulation signal SMF having the highest frequency among the modulation signals SMA having a plurality of channels, the modulation signal SMA having the lowest frequency.
If the highest frequency of the modulation signal SMF having the highest frequency is set to be lower than twice the lowest frequency, the direct information signals SVAI to SVFI and SAAI to SAFI are modulated to generate the modulation signals SMA to SMF. The interference due to the second harmonic of the modulation signals SMA to SMF and spurious can be reduced.

[0021]

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

(1) Overall Configuration In FIG. 1, reference numeral 11 denotes an optical space transmission device as a whole, and a driver 12 drives a laser diode 13 so that the laser diode 13 emits a transmission light beam L1 having a predetermined plane of polarization. .

Further, in the optical space transmission device 11, after the transmission light beam L1 is converted into a parallel light beam by the lens 15A, it is transmitted through the deflection beam splitter 16 and the lens 15B.
And 15C, thereby transmitting the transmission light beam L1 in a predetermined beam shape. Thereby, the optical space transmission device 1
In No. 1, the transmission light beam L1 is transmitted to the transmission target, and the desired information signal S1 is transmitted via the transmission light beam L1.

Further, in the optical space transmission device 11, the reception light beam L2 coming from the transmission object is received by the lens 15C and then guided to the deflection beam splitter 16 via the lens 15B. Here, the reception light beam L2 is transmitted from the transmission object so that the polarization plane is orthogonal to the transmission light beam L1, and thus, in the optical space transmission device 11, the reception light beam L2 is transmitted by the deflection beam splitter 16. It is reflected and guided to the lens 15D.

Here, the lens 15D focuses the received light beam L2 on the light receiving element 16, and the light receiving element 16 outputs the output signal, which is the light receiving result, to the demultiplexing circuit 20 via the AGC circuit 18.
Output to.

Further, in this embodiment, the optical space transmission device 11 has first to sixth slots 22A to 22F adapted to house card-like substrates, respectively.
The output signals of the transmission boards housed in the slots 22A to 22F are added by a multiplexing circuit 24 having an addition circuit configuration, whereby the modulated signals generated by the respective transmission boards are frequency-multiplexed to generate an information signal S1. Further, in the free-space optical transmission device 11, in addition to this transmission substrate, the modulation signal generated by the intercom circuit 26 is frequency-multiplexed by the multiplexing circuit 24, whereby various signals for communication can be transmitted toward the transmission target. It is done like this.

Further, in the optical free space transmission apparatus 11, the demultiplexing circuit 20 band-separates the information signal S2 and outputs it to the first to sixth slots 22A to 22F and the intercom circuit 26. Slots 22A-22
By inserting the receiving board into F instead of the transmitting board,
The information signal coming from the transmission object can be processed, and the communication signal sent from the transmission object via the intercom circuit 26 can be demodulated.

As a result, in the optical free space transmission apparatus 11, the first to sixth slots 22A to 22F are provided as necessary.
A transmission board or a reception board can be inserted into the to secure a desired transmission / reception channel.

(2) Connection of Slots As shown in FIG. 2, in each of the slots 22A to 22F, the card-shaped substrate is connected to the mother substrate via a predetermined connector, and in the optical space transmission device 11, The multiplexing circuit 24, the demultiplexing circuit 20, and the income circuit 26 are formed on this mother substrate.

That is, in the mother board, when the transmission boards 28A and 28B are housed in the respective slots by connecting the connectors to the multiplexing circuit 24 and the demultiplexing circuit 20, the transmission boards 28A and 28B output the signals. While the modulated signals SMA to SMF to be output are output to the multiplexing circuit 24, when the receiving boards 30C and 30D are inserted in the slot instead of the transmitting board, the output signal SDA of the demultiplexing circuit 20 is output.
~ SDF can be supplied to the receiving boards 30C and 30D.

At this time, the system control circuit 34 outputs the control data DA to DF assigned to the respective slots 22A to 22F to the connectors of the respective slots 22A to 22F, so that the transmission data inserted in the respective slots. Set the operating frequency of the board and the receiving board to each slot.
The frequencies are assigned to 2A to 22F. Thus, as shown in FIG. 3, in the optical free space transmission apparatus 11, when the transmission boards 28A and 28B are housed in the first and second slots 22A and 22B, the modulation output from the transmission boards 28A and 28B is performed. The center frequencies of the signals are set to the first and second channels C, respectively.
Set to the frequencies of H1 and CH2.

Further, when the receiving boards 30C and 30D are housed in the third and fourth slots 22C and 22D,
The center frequencies for demodulation by the receiving boards 30C and 30D are set to the third and fourth channels CH3 and CH4, respectively.
Set to the frequency of. Therefore, in the optical free space transmission apparatus 11, for the optical free space transmission apparatus 11B to be transmitted, the receiving substrate 30A is provided on the first and second slots 22A and 22B so as to correspond to the first and second slots 22A and 22B. , 30B can be accommodated and the modulated signal can be transmitted from the optical space transmission device 11 to the transmission target 11B for the first and second channels CH1 and CH2.

Furthermore, the third and fourth slots 22C
In order to correspond to the optical space transmission device 11B to be transmitted, the transmission substrates 28C and 28D are housed in the third and fourth slots 22C and 22D so as to correspond to the third and fourth channels CH3 and CH4. Target 1
The modulated signal sent from 1B can be received.

Thus, in the optical space transmission device 11,
By forming a plurality of board storage positions so that the transmission boards and reception boards can be selectively stored, and setting the operating frequency of this board to the frequency set for each board storage position, The transmission / reception channels can be set only by selectively storing desired transmission boards or reception boards in this board storage position so that the frequency of each channel can be easily set.
At this time, in the optical free space transmission apparatus 11, since the six slots are shared by the transmission board and the reception board, the entire shape can be downsized and the transmission / reception channel can be set easily.

Further, in the system control circuit 34,
Set each bit of a predetermined input port to each slot 22A to 22A.
The connector is assigned to each of the F connectors, and correspondingly, the connection line of this connector is connected to the power supply in the transmission board 28, while the connection line of this connector is grounded in the reception board 30. There is.
Thus, in the system control circuit 34, it is determined which of the transmission substrate 28 and the reception substrate 30 is housed in each of the slots 22A to 22F, and the transmission and reception frequencies of the intercom circuit 26 are determined based on the determination result. To set.

That is, in the system control circuit 34, when the transmission board 28 is housed in the first slot 22A, the first and the second assigned to the intercom circuit.
Of the income channels CHA and CHB of the intercom channel of the first and second income channels CHA and CH of the income circuit 26, respectively.
Set to B. On the contrary, in the system control circuit 34, when the receiving board 30 is housed in the first slot 22A, the transmitting channel and the receiving channel of the intercom circuit 26 are set to the second and first income channels CHB and CHA, respectively. Set to.

As a result, in the free-space optical transmission device 11, the frequency of the intercom circuit 26 is set according to the type of the substrate accommodated in the first slot 22A, and thereby the communication is performed without impairing the compatibility. It is designed to secure a line for.

Further, in the optical free space transmission apparatus 11, when the transmitting substrate 28 and the receiving substrate 30 are not accommodated in the first slot 22A, the substrate accommodated in the subsequent second slot 22B is detected, and the first substrate is detected here. When the transmission board 28 is stored in the slot 22B of No. 2, the intercom circuit 26
The transmission channel and the reception channel of are set to the first and second income channels CHA and CHB, respectively. On the other hand, the transmission board 28 is attached to the first slot 22A.
And the receiving board 30 is not accommodated
When the receiving circuit board 30 is housed in the slot 22B, the transmitting channel and the receiving channel of the intercom circuit 26 are set to the second and first intercom channels CH, respectively.
Set to B and CHA.

Further, in the optical space transmission device 11, the transmission substrate 28 is provided on the first and second slots 22A and 22B.
When neither the receiving board 30 nor the receiving board 30 is housed, the board housed in the subsequent third slot 22C is detected, and when the transmitting board 28 or the receiving board 30 is housed in the third slot 22C, respectively. , The transmitting and receiving channels of the income circuit 26 are set to the first and second income channels CHA and CHB or the second and first income channels CHB and CHA, respectively.

Similarly, in the optical free space transmission apparatus 11, when none of the substrates is accommodated in the first to third slots 22A to 22C, the intercom circuit corresponding to the substrates accommodated in the subsequent fourth slot 22D. 26 transmission / reception channels are set, and first to fourth slots 22A to 22 are set.
D, when no substrate is accommodated in the first to fifth slots 22A to 22E, the following fourth slot 22D,
The transmission / reception channel of the intercom circuit 26 is set according to the substrate housed in the fifth slot 22E.

(3) Transmission board As shown in FIG. 4, when the transmission board 28 is housed in each of the slots 22A to 22F, the video signal SVAI input from a predetermined connector arranged in the housing.
(SVBI to SVFI) and two-channel audio signal SAAI (SABI to SAFI) are applied to each slot 22.
This video signal S is input through the A to 22F connector.
VAI (SVBI to SVFI) and audio signal SAA
I (SABI to SAFI) is input to the modulation circuit 40.
The modulation circuit 40 controls the video signal SVAI at a predetermined center frequency.
Also, the audio signal SAAI is frequency-modulated to form an intermediate frequency signal SIF composed of a modulation signal of a predetermined frequency, and the intermediate frequency signal SIF is output to the mixer 42.

The mixer 42 is composed of a double balanced mixer having a ring modulation circuit configuration. The mixer 42 multiplies the local oscillator signal SOSC output from the local oscillator circuit 44 by the center frequency SIF and outputs it via a low pass filter circuit (LPF) 46. Output. At this time, the optical free space transmission apparatus 11 switches the frequency of the local oscillation signal SOSC according to the control data DA (DB to DF) output from the system control circuit 34, and thereby the center assigned to each slot 22A to 22F. The frequency modulation signal SMA (SMB to SMF) is output from each modulation substrate 28.

That is, in the local oscillator circuit 44, the distributed output of the distributor 50 is supplied to the mixer 42 as the local oscillator signal SOSC, and the remaining distributed output is frequency-divided by the prescaler 52 and output to the PLL circuit 54. Here, the PLL circuit 54 is
The reference signal generated from the crystal unit 56 is cyclically counted, and the result of phase comparison between this count result and the frequency division output is output. This count value is the control data output from the system control circuit 34. DA (DB ~ D
It is adapted to switch according to F).

The local oscillator circuit 44 outputs this phase comparison result to the voltage control type oscillation circuit 60 via the loop filter circuit 58, and outputs the output signal of this voltage control type oscillation circuit 60 to the distribution circuit 50. As a result, in the local oscillator circuit 44, the control data DA (DB to DF) can be switched to switch the center frequency of the modulation signal SMA (SMB to SMF), and in the optical space transmission device 11, the slots 22A to 22A. By outputting the control data DA to DF for each 22F, the frequency of the modulation signal SMA (SMB to SMF) is adjusted to each slot 22A using a common transmission board.
It can be set to a frequency corresponding to .about.22F.

(4) Receiving Substrate On the other hand, as shown in FIG. 5, in the receiving substrate 30, the output signal SDA (SDB to SDF) of the distribution circuit 20 is received.
Is received by the mixer 64 via the low-pass filter circuit (LPF) 62, where it is multiplied by the local oscillator signal SOSC of the local oscillator circuit 44. Here, the mixer 62 and the local oscillator circuit 4
4, the mixer 42 and the local oscillator circuit 44 of the transmitting board 28 have the same configuration, and as a result, the local oscillator signal SOSC having a frequency corresponding to each of the slots 22A to 22F is generated as in the case of the transmitting board 28.

The demodulation circuit 66 band-limits and inputs the intermediate frequency SIF which is the output signal of the mixer 64, demodulates this output signal, and outputs the 2-channel audio signal SAAO (S
ABO-SAFO) and video signal SVAO (SVBO-
SVFO) is output. As a result, the optical space transmission device 11
In this case, even when the receiving board 30 is housed in place of the transmitting board 28, the audio signal and the video signal can be demodulated with respect to the channels assigned to the slots 22A to 22F. You can set up.

(5) Income Circuit As shown in FIG. 6, in the optical space transmission device 11, an operator can communicate with the transmission target via the microphone jack M and the earphone jack I provided on the casing operation surface. The relay manager's voice signals SA and SB can be input / output, and this voice signal is transmitted / received to / from a transmission target as a communication signal. That is, in the intercom circuit 26, the audio signal SA input from the microphone jack M is amplified by the amplifier circuit 7.
After being amplified by 0, it is frequency-modulated by the modulation circuit 72, frequency-converted by the subsequent mixer 74, and output to the multiplexing circuit 24 via the low-pass filter circuit 76.

Further, in the intercom circuit 26, the output signal SINI of the demultiplexing circuit 20 is supplied to the low-pass filter circuit 7.
The signal is supplied to the mixer 80 via 8, and the frequency is converted here and demodulated by the demodulation circuit 82, and the output signal of this demodulation circuit 82 is output to the earphone jack I via the amplifier circuit 84.
In contrast to the transmission system and the reception system, in the intercom circuit 26, the local oscillation circuits 86 and 88 of the transmission system and the reception system are formed by the PLL circuit like the transmission substrate 28 and the reception substrate 30, respectively, and thereby the system control is performed. In accordance with the control data DIA, DIB output from the circuit 34, the local oscillator circuit 86
The oscillation frequency can be switched.

As a result, in the optical free space transmission apparatus 11, the control data DI corresponding to the transmission board 28 and the reception board 30 housed in the first to sixth slots 22A to 22F is provided.
A and DIB are output to switch the oscillation frequency of the local oscillator circuit 86, whereby the communication lines can be set to frequencies that do not interfere with each other as necessary.

(6) Frequency allocation In this way, the frequencies of the transmitting substrate 28 and the receiving substrate 30 are set to the frequencies assigned to the slots 22A to 22F. In order to effectively avoid the deterioration of the transmission quality, the frequency of the intermediate frequency signal SIF is selected to a predetermined frequency, and the frequency of each channel is set to the predetermined frequency with respect to this frequency.

That is, as shown in FIG. 7, the frequency-converted n-channel modulation signals SM1 to SMn are frequency-multiplexed, with the exclusive band per channel being B [MHz] and the frequency interval between adjacent channels being D [MHz]. If you do
If the frequency fIF of the intermediate frequency signal SIF is selected to be lower than the frequencies of the modulation signals SM1 to SMn, spurious signals such as the intermediate frequency signal SIF and the local oscillation signal SOSC may enter the frequency band of the modulation signals SM1 to SMn. There is. In this case, by setting the frequency band of the intermediate frequency signal SIF to be higher than that of the modulation signals SM1 to SMn, it is possible to prevent spurs of this kind from entering the frequency band of the modulation signals SM1 to SMn. obtain.

That is, assuming that the center frequencies of the modulation signals SM1 to SMn are f1 to fn,

[Equation 1] If the frequencies f1 to fn of the modulation signals SM1 to SMn and the frequency fIF of the intermediate frequency signal SIF are selected so that the relationship of 1 is satisfied, it is possible to prevent spurs of this kind from entering the frequency band of the modulation signals SM1 to SMn. .

On the other hand, two modulation signals SM1 to SMn
In the second harmonic, the second harmonic of the information signal SM1 having the lowest frequency among the n channels has the lowest frequency, and in the higher frequency band, the second harmonic of the sequentially modulated signals SM2 to SMn. Occurs. Therefore, if the intermediate frequency signal SIF is set to a frequency band lower than the second harmonic of the information signal SM1, the interference of the second harmonic can be avoided.

That is, assuming that the center frequencies of the modulation signals SM1 to SMn are f1 to fn,

[Equation 2] If the frequencies f1 to fn of the modulation signals SM1 to SMn and the frequency fIF of the intermediate frequency signal SIF are selected so that the relationship is established, the interference of the second harmonic can be avoided.

Further, in television broadcasting, since the frequency band is selected to be 300 [MHz] or less, the frequencies f1 to fn of the modulated signals SM1 to SMn are set to 300.
If it is set to [MHz] or higher, it is possible to effectively avoid the deterioration of transmission quality due to television broadcasting.

That is, the following equation is obtained corresponding to the equations (1) and (2).

[Equation 3] If the frequencies f1 to fn of the modulation signals SM1 to SMn and the frequency fIF of the intermediate frequency signal SIF are selected so that the relationship is established, the deterioration of the transmission quality due to television broadcasting can be effectively avoided.

In the system control circuit 34, each slot 2 is made to satisfy the relations of the expressions (1) to (3).
Control data DA to DF are sent to 2A to 22F, and the frequency of the transmission / reception channel is set by this. That is, in the case of this embodiment, the exclusive band B is 26 [MHz], and the frequency interval D between the adjacent channels is 45 [M] from the frequency characteristics on the demodulation side.
Hz].

As a result, the center frequency f of the modulated signal SM1
If 1 is set to a frequency higher than 315 [MHz] and the frequency fIF of the intermediate frequency signal SIF is set to a range of 620 [MHz] to 661 [MHz], the relations of equations (1) to (3) are satisfied. ,
It can be seen that deterioration of transmission quality can be avoided. As a result, the system control circuit 34 outputs the control data DA to DF to the slots 22A to 22F as shown in FIG. 8 to set the frequency of each channel.

In this embodiment, the band of the intercom circuit 26 is secured for 6 channels, so that the upper frequencies 605 to 625 of the sixth modulation signal SMF are secured.
The frequency band of the modulation signals SINO and SINI input to and output from the intercom circuit 26 is set to [MHz]. Thus, by setting each frequency so as to satisfy the relationships of the equations (1) to (3), the modulation signal SMA
~ It is possible to effectively avoid the interference of the second harmonic of SMF, and further, the modulation signals SMA to S generated from the mixer.
Spurious having a difference frequency between the MF and the intermediate frequency signal SIF can also be arranged in a frequency band lower than that of the modulation signal SMA, whereby the deterioration of the transmission quality can be effectively avoided.

Further, by ensuring the transmission quality in this way, the modulation depth of the laser diode can be deepened by effectively avoiding the deterioration of C / N by that amount, whereby the transmission distance can be further expanded. .

By the way, as shown in FIG. 9, the first and second
The third-order intermodulation distortion S3B of the modulated signals SMA and SMB occurs in the upper and lower frequency bands of the first and second modulated signals SMA and SMB with the frequencies 2f1-f2 and 2f2-f1 as the center frequencies. Therefore, when the frequency allocation is set as shown in FIG. 8, the upper third-order intermodulation distortion S3B of the first and second modulated signals SMA and SMB is set.
Are mixed in the third modulated signal SMC.

Further, the third-order intermodulation distortion S3B mixed in the third modulation signal SMC is also generated by the fourth and fifth modulation signals SMD and SME, and the fourth and fifth modulation signals are generated. Third-order intermodulation distortion S of SMD and SME
In 3B, it is also mixed in the sixth modulated signal SMF.
Such a third-order intermodulation distortion S3B has a characteristic that the level is small and the deterioration of the transmission quality is practically small. However, if the third-order intermodulation distortion S3B is mixed in the modulation signal,
The transmission quality deteriorates accordingly.

Therefore, in the system control circuit 34, the frequency assigned to each slot is switched based on the detection result of the substrates housed in the slots 22A to 22F. That is, when either the transmission board 28 or the reception board 30 is accommodated in all of the six slots 22A to 22F, the system control circuit 34 selects the frequency allocation described above with reference to FIG.

On the other hand, six slots 22A to 22
When one slot of F is unused, the system control circuit 34 changes the frequency allocation so that the third modulation signal SMC becomes an empty channel, while the six slots 22A to 22F are selected. If the two slots are unused, the third and sixth modulated signals SM
Change the frequency allocation so that C and SMF are empty channels. As a result, the optical space transmission device 11
When the number of channels is small, the frequency band used for transmission can be effectively used by that amount to transmit and receive a high-quality modulated signal.

The process of changing the frequency allocation is executed after confirming the usage status of the slot to be transmitted when the scan mode is switched to the communication mode. Even if an unused board is left unattended, the channel is set to correspond to the transmission target.

(7) Effects of the Embodiments With the above configuration, the modulation signal SMA having the lowest frequency is obtained.
The second order of the modulation signal by arranging the intermediate frequency signal in the frequency band lower than twice the frequency of It is possible to prevent harmonics and spurious waves from being mixed in advance, thereby effectively avoiding deterioration of transmission quality and transmitting a plurality of information signals by frequency multiplexing.

(8) Other Embodiments In the above embodiments, the case where the intermediate frequency signal is once generated to generate the modulation signal has been described, but the present invention is not limited to this, and the direct modulation signal is generated. It can be widely applied to the case. In this case, in the frequency allocation, it is sufficient to arrange the entire modulation signal in a frequency band that is twice the frequency band of the lowest frequency modulation signal.

Further, in the above embodiment, the case where the channels are set for the six slots and the income circuit has been described, but the present invention is not limited to this, and various channel numbers can be selected as necessary. The intercom circuit may be omitted.

Further, in the above embodiment, the case where the video signal and the audio signal are transmitted has been described.
The present invention is not limited to this, and can be widely applied to an optical space transmission device that transmits various information signals.

[0070]

As described above, according to the present invention, the frequency of the modulation signal and / or the frequency of the intermediate frequency signal are set so that the second harmonic and spurious of the modulation signal are not mixed, so that the transmission quality can be improved. By effectively avoiding the deterioration, it is possible to obtain an optical space transmission apparatus capable of frequency-multiplexing and transmitting information signals of a plurality of channels.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a bidirectional transmission device according to an embodiment of the present invention.

FIG. 2 is a block diagram showing the connection of the slots.

FIG. 3 is a schematic diagram for explaining channel assignment.

FIG. 4 is a block diagram showing a transmission board.

FIG. 5 is a block diagram showing a receiving board.

FIG. 6 is a block diagram showing an income circuit.

FIG. 7 is a characteristic curve diagram showing frequency allocation required for improving transmission quality.

FIG. 8 is a characteristic curve diagram showing an actual frequency allocation.

FIG. 9 is a schematic diagram for explaining third-order intermodulation distortion.

FIG. 10 is a characteristic curve diagram showing frequency allocation when a second harmonic is arranged between adjacent modulation signals.

[Explanation of symbols]

1, 2, 4A, 4B, 11 ... Optical space transmission device, 6A-
6F, 22A-22F ... Slot, 10A, 10B,
24 ... Multiplexing circuit, 12A, 12B, 20 ... Splitting circuit,
13 ... Laser diode, 26 ... Income circuit, 3
4 ... System control circuit.

Claims (4)

[Claims] 1. A multiplexing circuit for frequency-multiplexing a plurality of channels of information signals to generate a frequency-multiplexed signal, a laser light source for generating a transmission light beam modulated by the frequency-multiplexed signal, and an emission from the laser light source. And a transmission optical system for outputting the transmitted transmission light beam to a transmission target, wherein the frequency multiplexing circuit modulates each of the information signals of the plurality of channels to generate an intermediate frequency signal of the plurality of channels held at a predetermined frequency. A plurality of modulating circuits for generating, a plurality of frequency converting circuits for respectively converting the intermediate frequency signals of the plurality of channels to generate a plurality of channels of modulation signals held at a predetermined frequency, and adding the plurality of channels of modulation signals And an adder circuit for generating the frequency-multiplexed signal, and the lowest frequency of the intermediate frequency signal is set to the frequency-multiplexed signal. The highest frequency of the intermediate frequency signal is set lower than the frequency twice the lowest frequency of the modulation signal for the lowest modulation frequency of the modulation signals of the plurality of channels. An optical space transmission device characterized in that 2. A multiplexing circuit for frequency-multiplexing a plurality of channels of information signals to generate a frequency-multiplexed signal, a laser light source for generating a transmission light beam modulated by the frequency-multiplexed signal, and an emission from the laser light source. And a transmission optical system for transmitting the transmitted transmission light beam to the transmission target, wherein the frequency multiplexing circuit modulates the information signals of the plurality of channels, respectively, and modulates the plurality of channels held at respective predetermined frequencies. A plurality of modulation circuits for generating signals, and an adder circuit for adding the modulation signals of the plurality of channels to generate the frequency multiplexed signal, wherein the modulation signal of the lowest frequency among the modulation signals of the plurality of channels And for the modulated signal with the highest frequency, 2 of the lowest frequency of the modulated signal with the lowest frequency.
An optical space transmission device characterized in that the highest frequency of the modulated signal having the highest frequency is set to be lower than the doubled frequency. 3. The frequency multiplexing circuit includes first and second modulation signals for the first, second and third modulation signals whose frequencies are adjacently and sequentially set among the modulation signals of the plurality of channels. The frequency of the third modulated signal is set so as to be separated from the first and second modulated signals by a frequency equal to or more than twice the frequency difference between the first and second modulated signals. Optical space transmission equipment. 4. The information signal of the plurality of channels,
The frequency multiplexing circuit has a channel number detecting means for detecting the number of channels, and the frequency multiplexing circuit, based on the detection result of the channel number detecting means, when the number of channels is large, the frequency of the second and third modulated signals is almost the same. The optical space transmission device according to claim 3, wherein the frequency of the fourth modulated signal is set to an intermediate frequency band.