JP2004040242A - Signal transmission method and signal transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a signal transmission method and a signal transmission system capable of easily (e.g. with a simple configuration) transmitting a transmission signal such as a radio frequency signal for a long distance with high quality. <P>SOLUTION: The signal transmission system uses a local oscillation signal to convert a transmission signal into an intermediate frequency band signal, modulates the intermediate frequency band signal with a pulse frequency, converts the modulated signal into a first optical signal, converts the local oscillation signal into a second optical signal, and transmits the first and second optical signals through an optical transmission line. The signal transmission system mixes a signal obtained by applying photoelectric conversion to the first optical signal and demodulating the result with the local oscillation signal obtained by applying photoelectric conversion to the second optical signal when receiving the first and second optical signal via the optical transmission line to reproduce the transmission signal. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a signal transmission method and system for converting a transmission signal such as a radio frequency signal into an optical signal for transmission.
[0002]
[Prior art]
Utilizing the broadband and low-loss properties of optical fibers, wireless signal optical fiber transmission, in which a transmission signal such as a radio frequency signal is directly converted into an optical signal and transmitted, has been performed. Conventionally, a coaxial cable has been used for transmission from a wireless device to an antenna. An optical fiber is thinner than a coaxial cable, is easier to use, and has a smaller loss, so that long distance transmission is possible. By replacing the coaxial cable with an optical fiber, a transmission signal in a higher frequency band can be transmitted over a long distance.
[0003]
As a method of converting a transmission signal into an optical signal, a semiconductor laser is mainly used. As the current flowing through the semiconductor laser is increased, the optical output starts to increase from the threshold current value at which laser oscillation starts. In a semiconductor laser, the ratio between the amount of increase in current and the amount of increase in light output, that is, the differential quantum efficiency, becomes almost constant after the threshold current value is exceeded. By utilizing this characteristic, the semiconductor laser is directly modulated by adding an AC current as a transmission signal and a DC bias current set to a threshold current value or more. By appropriately selecting the amplitude of the transmission signal, the transmission signal can be reproduced from the optical signal received via the optical fiber.
[0004]
As described above, conventionally, a semiconductor laser is driven while a transmission signal is an analog signal, and the transmission signal is converted into an optical signal and transmitted.
[0005]
According to this method, when the amplitude of the transmission signal is increased on the transmission side, the effect of the nonlinear distortion of the semiconductor laser increases, and the SN ratio of the transmission signal reproduced on the reception side is good, but the distortion characteristics are poor. Become. On the other hand, if the amplitude of the transmission signal is reduced on the transmission side, the transmission signal reproduced on the reception side has a good distortion characteristic but a low SN ratio. Further, the SN ratio is also affected by the transmission-side optical power and the optical fiber loss.
[0006]
That is, when a transmission signal is directly converted into an optical signal, the non-linearity of the E / O converter and the O / E converter becomes a problem. Further, if the linearity is improved by reducing the amplitude in the E / O converter, there is a problem that the SN ratio is deteriorated on the receiving side.
[0007]
Therefore, in the case of a system that directly modulates a transmission signal by driving the semiconductor laser with the transmission signal, the transmittable distance greatly fluctuates according to the performance variation of the semiconductor laser, and the system design has an effect on the performance of the semiconductor laser. There was a problem that it was greatly affected.
[0008]
On the other hand, Japanese Patent No. 3264415 discloses a system in which a frequency signal is subjected to frequency conversion, FM (Frequency Modulation) modulation, and then converted to an optical signal and transmitted. FM modulation is a modulation method in which an oscillation frequency is changed by a voltage controlled oscillator (VCO) according to an input signal.
[0009]
When converted into an optical signal after performing FM modulation, (1) the influence of intermodulation distortion generated in the optical transmission line can be ignored, (2) the optical modulation degree is increased, and (3) the CNR is improved by the FM broadband gain. There is an advantage that it is.
[0010]
However, since the spectrum of an FM-modulated signal does not include a baseband component, in FM demodulation, for example, a high-cost FM demodulator including a frequency discriminating filter (passive) and a diode detector is used. I need. Furthermore, in this FM demodulator, the intermodulation distortion eventually increases due to the nonlinear distortion caused by the diode detector. As a result, it is difficult for the receiving side to accurately restore the transmission signal, and the transmission quality is poor. was there. Further, the FM demodulation method includes a delay detection method for calculating a product of a delayed self signal and a non-delayed signal. This system has a feature that intermodulation distortion is reduced, but the configuration is complicated.
[0011]
[Problems to be solved by the invention]
As described above, conventionally, when a transmission signal is converted into an optical signal and transmitted, there has been a problem that transmission over a long distance with high quality cannot be easily performed (for example, with a simple configuration).
[0012]
In view of the above problems, the present invention provides a signal transmission method and a signal transmission system capable of easily (for example, with a simple configuration) and high-quality long-distance transmission of a transmission signal such as a radio frequency signal. The purpose is to provide.
[0013]
[Means for Solving the Problems]
The signal transmission method according to the present invention converts a transmission signal into an intermediate frequency band signal using a local oscillation signal, converts the intermediate frequency band signal into a first optical signal after pulse frequency modulation, and converts the local oscillation signal into a first optical signal. Converting the signal into a second optical signal, transmitting the first and second optical signals via an optical transmission path, and receiving the first and second optical signals via the optical transmission path; After the first optical signal is photoelectrically converted, the signal obtained by demodulation is mixed with the local oscillation signal obtained by photoelectrically converting the second optical signal, thereby reproducing the transmission signal. It is characterized by doing.
[0014]
A signal transmission system according to the present invention is a signal transmission system comprising: a transmission device that converts a transmission signal into an optical signal and transmits the optical signal; and a reception device that receives the optical signal and reproduces the transmission signal, connected by an optical transmission line. A transmitter for converting the transmission signal into an intermediate frequency band signal using a local oscillation signal, and then modulating a pulse frequency, and a first pulse obtained as a result of the modulation by the modulator. A first conversion means for converting a signal into a first optical signal; and a second conversion means for converting the local oscillation signal into a second optical signal, wherein the first and second optical signals are converted. The transmission is performed via the optical transmission line, and the receiving device receives the first and second optical signals via the optical transmission line, and converts the first and second optical signals. Third converting means for performing photoelectric conversion, and the first optical signal; Reproducing means for reproducing the transmission signal by mixing a signal obtained by demodulating a signal obtained as a result of photoelectrically converting the signal and the local oscillation signal obtained as a result of photoelectrically converting the second optical signal. And characterized in that:
[0015]
According to the present invention, a transmission signal such as a radio frequency signal is pulse-frequency-modulated and then converted to an optical signal and transmitted, so that high-quality long-distance transmission can be performed. Demodulation may be performed using only a low-pass filter (LPF). Therefore, according to the present invention, a transmission signal can be easily (for example, with a simple configuration) and transmitted over a long distance with high quality.
[0016]
The signal transmission method according to the present invention converts a transmission signal into an intermediate frequency band signal using a local oscillation signal, and a first pulse signal obtained as a result of pulse frequency modulation of the intermediate frequency band signal; Converting the second pulse signal obtained by taking the logical product of the AND into an optical signal, transmitting the optical signal via an optical transmission path, and receiving the optical signal via the optical transmission path, The transmission signal is reproduced by mixing a signal obtained as a result of demodulating the second pulse signal obtained by photoelectrically converting the optical signal and the local oscillation signal extracted from the second pulse signal. It is characterized by doing.
[0017]
A signal transmission system according to the present invention is a signal transmission system comprising: a transmission device that converts a transmission signal into an optical signal and transmits the optical signal; and a reception device that receives the optical signal and reproduces the transmission signal, connected by an optical transmission line. A transmitter for converting the transmission signal into an intermediate frequency band signal using a local oscillation signal, and then modulating a pulse frequency, and a first pulse obtained as a result of the modulation by the modulator. A first conversion unit for converting a second pulse signal obtained by ANDing the signal and the local oscillation signal into an optical signal, and transmitting the optical signal via the optical transmission line Wherein the receiving device receives the optical signal via the optical transmission line, and obtains the optical signal by photoelectrically converting the optical signal. Said second pulse Extraction means for extracting the local oscillation signal from the signal, a signal obtained as a result of demodulating the second pulse signal obtained by photoelectrically converting the optical signal, and a local oscillation signal extracted by the extraction means. And reproducing means for reproducing the transmission signal.
[0018]
According to the present invention, a transmission signal such as a radio frequency signal is pulse-frequency-modulated and then converted to an optical signal and transmitted, so that high-quality long-distance transmission can be performed. Demodulation may be performed using only a low-pass filter (LPF). Therefore, according to the present invention, a transmission signal can be easily (for example, with a simple configuration) and transmitted over a long distance with high quality. Also, since the local oscillation signal is transmitted on the first pulse signal obtained as a result of modulating the transmission signal, only one optical fiber cable is required, and signal transmission with reduced cost is realized. be able to.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, a case where the transmission signal is a radio frequency (RF) signal will be described.
[0020]
(1st Embodiment)
FIG. 1 is a block diagram illustrating a configuration example of a signal transmission system according to the first embodiment, which is roughly divided into a transmission device 1 and a reception device 2.
[0021]
In FIG. 1, a transmission device 1 includes a first mixer (mixer) 101, a local oscillator 102, a pulse frequency modulator (hereinafter, referred to as a “PFM modulator”) 103, and a first electro-optical converter ( Hereinafter, a first E / O converter or a first E / O 104 and a second electric / optical converter (hereinafter referred to as a second E / O converter or a second E / O converter) will be described. E / O) 105.
[0022]
The receiving device 2 includes a first optical / electrical converter (hereinafter, referred to as a first O / E converter or a first O / E) 106 and a second optical / electrical converter ( Hereinafter, a second O / E converter or a second O / E 107, a pulse frequency demodulator (hereinafter referred to as “PFM demodulator”) 108, and a second mixer (mixer) 109. And a band-pass filter 110.
[0023]
The transmission device 1 and the reception device 2 are connected by two optical fiber cables 121 and 122.
[0024]
First, the transmitting device 1 will be described.
[0025]
The local oscillation signal output from the local oscillator 102 is input to the first mixer 101 and the second E / O converter 105.
[0026]
The first mixer (mixer) 101 receives a local oscillation signal output from a local oscillator 102 and a radio frequency signal (RF signal). The first mixer 101 mixes (multiplies) a local oscillation signal and a radio frequency signal and outputs an intermediate frequency signal.
[0027]
The intermediate frequency signal (IF signal) output from the first mixer 101 is input to the PFM modulator 103. The PFM modulator 103 performs pulse frequency modulation (PFM modulation) on the intermediate frequency signal using a publicly known / public technology, and as a result, outputs a PFM modulated signal corresponding to the input intermediate frequency signal.
[0028]
PFM modulation is a modulation method in which the repetition frequency of an electric pulse having a constant pulse width changes according to the amplitude of an input signal (for example, the IF signal) input to a PFM modulator, as shown in FIG. FIG. 2 shows an example in which the pulse repetition frequency is high when the input signal amplitude is large, and the pulse repetition frequency is low when the input signal amplitude is small.
[0029]
Note that, in the present embodiment, the relationship between the IF signal amplitude of the PFM modulator 103 and the pulse repetition frequency is not limited. The pulse repetition frequency may be increased when the reverse relationship shown in FIG. 2 is satisfied, that is, when the IF signal amplitude is small.
[0030]
The output signal of the PFM modulator 103 is converted into an optical signal by the first E / O converter 104.
[0031]
Although various realization methods can be considered as the first E / O converter 104, in the present embodiment, direct modulation of a semiconductor laser is used. As a method of driving the semiconductor laser with an electric pulse which is an output signal of the PFM modulator 103, the following method can be considered. That is, a method in which a semiconductor laser biased near a threshold current value is driven by a signal obtained by adding a pulse current to a bias current to obtain an optical pulse similar to an electric pulse. By biasing the semiconductor laser in advance, the delay time from when the electric pulse is input to when the semiconductor laser emits light can be reduced.
[0032]
On the other hand, the local oscillation signal output from the local oscillator 102 is converted into an optical signal by the second E / O converter 105. Similarly to the first E / O converter 104, the second E / O converter 105 can be realized by direct modulation of a semiconductor laser. That is, the semiconductor laser biased near the threshold current value is driven by a signal obtained by adding the output signal of the local oscillator 102 to the bias current.
[0033]
The first and second E / O converters 104 and 105 have no problem even if the nonlinear distortion is large.
[0034]
The signal for driving the first E / O converter 104 is the output of the PFM modulator 103. The PFM modulator 103 changes the repetition frequency of an output pulse having a constant width according to the amplitude of a signal input to the PFM modulator 103. Therefore, the first E / O converter 104 that converts the output pulse of the PFM modulator 103 into an optical pulse may have a large distortion and the optical pulse may have a constant width.
[0035]
The second E / O converter 105 converts a constant repetition electrical signal of the local oscillator 102 into an optical signal. The optical signal output from the second E / O converter 105 only needs to be able to convert a constant repeating electrical signal into an optical signal. Therefore, the second E / O converter 105 may have a large distortion.
[0036]
A semiconductor laser is used as an E / O converter which may have a large distortion as described above. Semiconductor lasers are roughly classified into two types: a DFB laser capable of dynamically performing a single longitudinal mode oscillation, and an FP laser that oscillates by reflection from an end face. The DFB laser has a higher cost than the FP laser because the grating is formed inside the semiconductor laser. In the first embodiment, the first and second E / O converters 104 and 105 can be configured by either a DFB laser or an FP laser. Further, there is no upper limit of the magnitude of the signal amplitude for driving the semiconductor laser. That is, the signal amplitude can be increased as long as the semiconductor laser is not broken.
[0037]
The optical signal output from the first E / O converter 104 is transmitted to the receiving device 2 via the optical fiber cable 121, and the optical signal output from the second E / O converter 105 is received via the optical fiber cable 122. It is transmitted to the device 2.
[0038]
Next, the receiving device 2 will be described.
[0039]
The first O / E converter 106 converts an optical signal received via the optical fiber cable 121 into an electric signal. In addition, the second O / E converter 107 reproduces a local oscillation signal by converting an optical signal received via the optical fiber cable 122 into an electric signal.
[0040]
The signal output from the first O / E converter 106 is input to the PFM demodulator 108. The PFM demodulator 108 demodulates the PFM modulated signal as the input signal by using a known / public technology.
[0041]
The signal output from the PFM demodulator 108 and the signal output from the second O / E converter 107, that is, the local oscillation signal output from the local oscillator 102 sent from the transmitting device 1 is the second signal. To the mixer 109. The second mixer 109 mixes (multiplies) the signal output from the PFM modulator 108 with the local oscillation signal, and passes the resulting signal through a band-pass filter 110 to thereby generate a radio frequency signal (RF Signal) can be reproduced.
[0042]
As described above, the PFM modulation is a method in which the repetition frequency of a pulse having a constant pulse width is changed according to the amplitude of an input signal. FIG. 3 shows the PFM modulation spectral density. The PFM modulation spectrum exists in a portion denoted by “PFM” and two portions denoted by “IF” in FIG. If the spectrum of the portion "PFM" is removed from the two spectra "PFM" and "IF" and only "IF" is left, PFM modulation is demodulated. That is, demodulating PFM modulation is equivalent to inserting a low-pass filter and equalizing.
[0043]
Therefore, demodulating the signal output from the first O / E converter 106 with the PFM demodulator 108 means extracting the spectrum of the portion indicated by IF shown in FIG. 3 with a low-pass filter.
[0044]
A second mixer 109 mixes a spectrum part called “IF” extracted by the PFM demodulator 108 and a local oscillation signal obtained by converting the spectrum part into an electric signal by the second O / E converter 107. By doing so, signals having spectra indicated by “LO” and “RF” shown in FIG. 3 are obtained. FIG. 3 does not show the spectrum of the lower sideband of “LO”.
[0045]
The band-pass filter 110 extracts only the spectral components indicated by “RF” shown in FIG. 3 to output a radio frequency signal.
[0046]
Here, the first and second O / E converters 106 and 107 may have a large nonlinear distortion like the first and second E / O converters 104 and 105.
[0047]
In the first embodiment, transmission between the first and second E / O converters 104 and 105 and the first and second O / E converters 106 and 107 is performed using two optical fiber cables. I did it. By making the wavelengths of the first E / O converter 104 and the second E / O converter 105 different, an optical signal as an output of the first E / O converter 104 and a second E / O converter An optical signal as an output of the converter 105 is multiplexed by a multiplexer, transmitted by one optical fiber, and then demultiplexed by a demultiplexer to form a first O / E converter 106 and a second O / E converter 106. It is also conceivable that the signal is received by the / E converter 107.
[0048]
The pulse frequency modulation is a modulation method in which the repetition frequency of a pulse having a constant width changes according to the amplitude of an input intermediate frequency band signal. Therefore, since the semiconductor laser for converting into an optical signal only needs to output a pulse having a constant width, the modulation distortion does not matter. On the other hand, when converting a local oscillation signal into an optical signal, the amount of modulation distortion is not limited. The semiconductor laser does not need to faithfully output an optical signal in accordance with the waveform of the local oscillation signal, but only needs to faithfully transmit frequency information. Therefore, a semiconductor laser for obtaining an optical signal may be a semiconductor laser for ordinary digital communication, and a low-cost device can be realized. When the above-mentioned two semiconductor lasers are used, transmission to the receiving end through two optical fibers can be considered. Further, it is conceivable that the oscillation frequencies of the two semiconductor lasers are made different, wavelength-division multiplexed and transmitted to the receiving end by one optical fiber, and the wavelengths are separated at the receiving end.
[0049]
As described above, according to the first embodiment, a transmission signal such as a radio frequency signal is PFM-modulated, converted to an optical signal, and transmitted. As described above, the PFM is a modulation method in which the repetition frequency of a pulse having a constant pulse width is changed according to an input signal (for example, an IF signal in this case), and the spectrum of the PFM-modulated signal is shown in FIG. As is clear, a baseband component is included. Therefore, the demodulation of the PFM only requires a low-pass filter (LPF). Since this is a passive device, the configuration is simple and the power consumption is low, so that the cost of the entire system can be reduced.
[0050]
Transmission signals such as radio frequency signals are converted to optical signals after PFM modulation and transmitted, so that high-quality long-distance transmission can be performed. On the receiving side, PFM demodulation is performed by a low-pass filter (LPF). Only good. Therefore, according to the present invention, a transmission signal can be easily (for example, with a simple configuration) and transmitted over a long distance with high quality.
[0051]
Here, the means for converting an electric signal to an optical signal is a semiconductor laser. However, it is needless to say that a combination of a semiconductor laser that oscillates constantly and an external modulator of lithium niobate or a semiconductor can be realized.
[0052]
(Second embodiment)
FIG. 4 is a block diagram illustrating a configuration example of a signal transmission system according to the second embodiment, which is roughly divided into a transmission device 3 and a reception device 4.
[0053]
In FIG. 4, a transmission device 3 includes a first mixer (mixer) 201, a local oscillator 202, a pulse frequency modulator (hereinafter, referred to as a "PFM modulator") 203, and an electro-optical converter (hereinafter, E). / O converter or E / O) 204 and a logical product circuit 205.
[0054]
The receiving apparatus 2 includes an optical-to-electrical converter (hereinafter, referred to as an O / E converter or O / E) 206, a clock extraction circuit 207, and a pulse frequency demodulator (hereinafter, referred to as a “PFM demodulator”). ) 208, a second mixer (mixer) 209, and a band-pass filter 210.
[0055]
The transmission device 3 and the reception device 4 are connected by one optical fiber cable 221.
[0056]
First, the transmitting device 3 will be described.
[0057]
The local oscillation signal output from local oscillator 202 is input to first mixer 201 and AND circuit 205.
[0058]
The first mixer (mixer) 201 receives a local oscillation signal output from the local oscillator 202 and a radio frequency signal (RF signal). The first mixer 201 mixes (multiplies) a local oscillation signal and a radio frequency signal, and outputs an intermediate frequency signal.
[0059]
The intermediate frequency signal (IF signal) output from the first mixer 201 is input to the PFM modulator 203. The PFM modulator 203 performs pulse frequency modulation (PFM modulation) on the intermediate frequency signal using a publicly known / public technology, and as a result, outputs a PFM modulated signal corresponding to the input intermediate frequency signal.
[0060]
The AND circuit 205 performs an AND operation on the PFM modulated signal output from the PFM modulator and the local oscillation signal output from the local oscillator 202. As a result, for example, an AND operation pulse train as shown in FIG. Is output.
[0061]
Since the logical history circuit 205 calculates the logical product of the PFM modulation signal output from the PFM modulator and the local oscillation signal output from the local oscillator 202, the local oscillation signal can be added to the PFM modulation signal. is there.
[0062]
PFM modulation is a modulation method in which the repetition frequency of an electric pulse having a constant pulse width changes according to the amplitude of an input signal. Therefore, as shown in FIG. It can be seen that the repetition frequency of the product operation pulse train group has changed. In FIG. 5, when the amplitude of the IF signal as the input signal is large, the repetition frequency of the AND operation pulse train increases, and when the amplitude of the IF signal as the input signal is small, the repetition frequency of the AND operation pulse train decreases. Examples were shown. However, similarly to the first embodiment, the relationship between the IF signal amplitude of the PFM modulator 203 and the pulse (logical product operation pulse train) repetition frequency is not limited. For example, when the IF signal amplitude is large, the pulse (logical product operation pulse train) repetition frequency may be lowered.
[0063]
An AND operation pulse train as a PFM modulation signal on which a local oscillation signal is output, which is an output signal of the AND circuit 205, is converted into an optical signal by the E / O converter 204.
[0064]
Although various realization methods can be considered as the E / O converter 204, in this embodiment, direct modulation of a semiconductor laser is used. The method of driving the semiconductor laser with the output signal of the AND circuit 205 is the same as in the first embodiment described above, and a description thereof will be omitted.
[0065]
The E / O converter 204 has no problem even if the nonlinear distortion is large. A signal for driving the E / O converter 204 is an output of the AND circuit 205. The input signal of the AND circuit 205 is obtained by adding the output signal of the local oscillator 202 to the output signal of the PFM modulator 203.
[0066]
The PFM modulator 203 changes the repetition frequency of an output pulse having a constant width according to the amplitude of a signal input to the PFM modulator 203. Therefore, the E / O converter 204 that converts a signal obtained by calculating the logical product of the output pulse of the PFM modulator 203 and the output signal of the local oscillator 202 into an optical pulse may have a large distortion.
[0067]
As in the first embodiment, the E / O converter 204 uses a semiconductor laser, and there is no restriction on the type of the semiconductor laser. Therefore, the E / O converter can be configured by either a DFB laser or an FP laser. Further, there is no upper limit of the magnitude of the signal amplitude for driving the semiconductor laser. That is, the signal amplitude can be increased as long as the semiconductor laser is not broken.
[0068]
The optical signal output from the E / O converter 204 is transmitted to the receiving device 4 via the optical fiber 221.
[0069]
Next, the receiving device 4 will be described.
[0070]
The O / E converter 206 converts an optical signal received via the optical fiber cable 221 into an electric signal, and outputs the electric signal to the PFM modulator 208 and the clock extraction circuit 207.
[0071]
An electric signal output from the O / E converter 206 is input to the PFM demodulator 208, and the electric signal is demodulated using a publicly known technique (PFM modulation).
[0072]
The PFM demodulation can be realized by removing the PFM modulation spectrum component using the low-pass filter, as described in the first embodiment. The PFM demodulator 208 according to the second embodiment determines the cutoff frequency of the low-pass filter so as to remove the PFM modulation spectrum.
[0073]
The electric signal output from the O / E converter 206 is input to the clock extraction circuit 207, where the local oscillation signal component included in the PFM modulated signal is extracted. This extraction method may use a publicly known / public technology, and is not the gist of the present invention, so that the description is omitted.
[0074]
The signal obtained as a result of demodulation by the PFM demodulator 208 and the local oscillation signal extracted by the clock extraction circuit 207 are mixed by the second mixer 209.
[0075]
A radio frequency signal is extracted from the signal obtained as a result of mixing the two signals by the second mixer 209 using the band-pass filter 210.
[0076]
Here, similarly to the E / O converter 204, the O / E converter 206 may have a large nonlinear distortion.
[0077]
In the second embodiment, a local oscillation signal is placed on a modulated transmission signal, converted to an optical signal and transmitted, and the receiving apparatus 4 outputs the output of the O / E converter 206 to the PFM demodulator 208 and the clock extraction. The signal is input to the circuit 207. Then, the output signal of the PFM demodulator 208 and the output signal of the clock extraction circuit 207 are mixed by the second mixer 209 to reproduce the radio frequency signal.
[0078]
In the transmitting device 3, not only one semiconductor laser for converting to an optical signal is required, but also pulsed light may be output, so that modulation distortion is not required. A semiconductor laser for obtaining an optical signal may be a semiconductor laser for ordinary digital communication, and a low-cost device can be realized.
[0079]
To simplify the configuration, the output of the O / E converter 206 may be passed through a bandpass filter 210 to output a radio frequency signal. In this case, since the receiving device 4 can be configured only by the O / E converter 206 and the band-pass filter 210, the cost can be reduced.
[0080]
As described above, according to the second embodiment, a transmission signal such as a radio frequency signal is subjected to PFM modulation, converted to an optical signal, and transmitted. As described above, the PFM is a modulation method in which the repetition frequency of a pulse having a constant pulse width is changed according to an input signal (for example, an IF signal in this case), and the spectrum of the PFM-modulated signal is shown in FIG. As is clear, a baseband component is included. Therefore, the demodulation of the PFM only requires a low-pass filter (LPF). Since this is a passive device, the configuration is simple and the power consumption is low, so that the cost of the entire system can be reduced.
[0081]
Transmission signals such as radio frequency signals are converted to optical signals after PFM modulation and transmitted, so that high-quality long-distance transmission can be performed. Only may be. Therefore, according to the present invention, a transmission signal can be easily (for example, with a simple configuration) and transmitted over a long distance with high quality.
[0082]
Here, the means for converting an electric signal to an optical signal is a semiconductor laser. However, it is needless to say that a combination of a semiconductor laser that oscillates constantly and an external modulator of lithium niobate or a semiconductor can be realized.
[0083]
【The invention's effect】
As described above, according to the present invention, a transmission signal such as a radio frequency signal can be easily (for example, with a simple configuration) regardless of the nonlinear distortion of the E / O converter and the O / E converter. Moreover, high-quality long-distance transmission can be performed.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration example of a signal transmission system according to a first embodiment of the present invention.
FIG. 2 is a diagram for explaining a relationship between an input signal (IF signal) and an output pulse of a PFM modulator.
FIG. 3 is a diagram for explaining the spectral density of each signal component (IF component, PFM component, RF component).
FIG. 4 is a block diagram showing a configuration example of a signal transmission system according to a second embodiment of the present invention.
FIG. 5 is a diagram for explaining a relationship between an input signal (IF signal) of a PFM modulator and a pulse output from an AND circuit.
[Explanation of symbols]
101, 201 ... first mixer (mixer)
102, 202 ... local oscillator
103, 203 ... PFM modulator
104: first electric / optical converter (first E / O converter)
105: second electric / converter (second E / O converter)
106: first optical / electrical converter (first O / E converter)
107: second optical / electrical converter (second O / E converter)
204 E / O converter
205: AND circuit
206 ... O / E converter
207 ... Clock extraction circuit
108, 208 ... PFM demodulator
109, 209: second mixer (mixer)
110, 210 ... band pass filter

Claims (4)

The transmission signal is converted into an intermediate frequency band signal using a local oscillation signal, and after the intermediate frequency band signal is pulse frequency modulated, the signal is converted into a first optical signal, and the local oscillation signal is converted into a second optical signal. Converting and transmitting these first and second optical signals via an optical transmission line,
When the first and second optical signals are received via the optical transmission line, the first optical signal is photoelectrically converted, and then the signal obtained by demodulation and the second optical signal are photoelectrically converted. Reproducing the transmission signal by mixing the local oscillation signal obtained as a result of the transmission. The transmission signal is converted into an intermediate frequency band signal using the local oscillation signal, and the first pulse signal obtained as a result of pulse frequency modulation of the intermediate frequency band signal is ANDed with the local oscillation signal. Converting the second pulse signal into an optical signal, transmitting the optical signal via an optical transmission line,
When the optical signal is received through the optical transmission path, a signal obtained as a result of demodulating the second pulse signal obtained by photoelectrically converting the optical signal, and a signal extracted from the second pulse signal A signal transmission method, wherein the transmission signal is reproduced by mixing the local oscillation signal. A signal transmission system in which a transmission device that converts a transmission signal into an optical signal and transmits the signal, and a reception device that receives the optical signal and reproduces the transmission signal are connected via an optical transmission path,
The transmitting device,
After converting the transmission signal to an intermediate frequency band signal using a local oscillation signal, a modulation means for pulse frequency modulation,
First conversion means for converting a first pulse signal obtained as a result of modulation by the modulation means into a first optical signal;
Second conversion means for converting the local oscillation signal into a second optical signal;
And transmitting the first and second optical signals via the optical transmission line,
The receiving device,
A third conversion unit that receives the first and second optical signals via the optical transmission line and photoelectrically converts the first and second optical signals;
By mixing a signal obtained by demodulating a signal obtained as a result of photoelectrically converting the first optical signal and the local oscillation signal obtained as a result of photoelectrically converting the second optical signal, the transmission is performed. Reproducing means for reproducing a signal;
A signal transmission system comprising: A signal transmission system in which a transmission device that converts a transmission signal into an optical signal and transmits the signal, and a reception device that receives the optical signal and reproduces the transmission signal are connected via an optical transmission path,
The transmitting device,
After converting the transmission signal to an intermediate frequency band signal using a local oscillation signal, a modulation means for pulse frequency modulation,
First conversion means for converting a second pulse signal obtained by taking the logical product of a first pulse signal obtained as a result of modulation by the modulation means and the local oscillation signal into an optical signal;
Comprising, transmitting the optical signal via the optical transmission line,
The receiving device,
A second conversion unit that receives the optical signal via the optical transmission path and photoelectrically converts the optical signal;
Extracting means for extracting the local oscillation signal from the second pulse signal obtained by photoelectrically converting the optical signal;
Reproduction means for reproducing the transmission signal by mixing a signal obtained as a result of demodulating the second pulse signal obtained by photoelectrically converting the optical signal with a local oscillation signal extracted by the extraction means; When,
A signal transmission system comprising:

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