JP3863463B2 - Signal transmission method and signal transmission system - Google Patents

Description

[0001]
BACKGROUND 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 and transmitting the optical signal, for example.
[0002]
[Prior art]
Radio signal optical fiber transmission has been performed in which a transmission signal such as a radio frequency signal is directly converted into an optical signal and transmitted by taking advantage of the broadband property and low loss property of the optical fiber. Conventionally, coaxial cables have been used for transmission from a wireless device to an antenna. An optical fiber is thinner than a coaxial cable, is easy to use, and has low loss, so it can be transmitted over a long distance. 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 for 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 the semiconductor laser, the ratio of the current increase amount to the light output increase amount, that is, the differential quantum efficiency becomes almost constant from the point where the threshold current value is exceeded. Using this characteristic, the semiconductor laser is directly modulated by adding an alternating current as a transmission signal and a direct current 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]
Thus, conventionally, the semiconductor laser is driven while the transmission signal remains 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 influence of nonlinear distortion of the semiconductor laser becomes large, and the SN ratio of the transmission signal reproduced on the reception side is good but the distortion characteristic is bad. 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 good distortion characteristics but a poor SN ratio. Furthermore, the S / N ratio is also affected by the transmission side optical power and optical fiber loss.
[0006]
That is, when the transmission signal is directly converted into an optical signal, the nonlinearity of the E / O converter and the O / E converter becomes a problem. In addition, if the E / O converter reduces the amplitude and improves the linearity, 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 the transmission signal by driving the semiconductor laser with the transmission signal, the transmittable distance greatly varies depending on the performance variation of the semiconductor laser, and the system design is affected by the performance of the semiconductor laser. There was a problem of being greatly influenced.
[0008]
On the other hand, Japanese Patent No. 3264415 discloses a system in which a frequency signal is subjected to frequency conversion and FM (Frequency Modulation) modulation, and then converted to an optical signal for transmission. FM modulation is a modulation method in which an oscillation frequency is changed according to an input signal by a voltage controlled oscillator (VCO).
[0009]
When converted to an optical signal after FM modulation, (1) the influence of intermodulation distortion generated in the optical transmission line can be ignored, (2) the degree of optical modulation is increased, and (3) CNR is improved by FM broadband gain. There is an advantage that.
[0010]
However, since the spectrum of the FM-modulated signal does not include a baseband component, in FM demodulation, for example, a high-cost FM demodulator composed of a frequency discrimination filter (passive) and a diode detector is used. I need. Further, in this FM demodulator, the intermodulation distortion eventually increases due to the nonlinear distortion caused by the diode detector, and as a result, it is difficult for the receiving side to accurately restore the transmission signal, and the transmission quality is poor. was there. As an FM demodulation method, there is a delay detection method in which a product of a delayed self signal and a non-delayed signal is calculated. This system has a feature that the 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 it is not possible to transmit a long distance with high quality easily (for example, with a simple configuration).
[0012]
Therefore, in view of the above problems, the present invention provides a signal transmission method and a signal transmission system capable of easily transmitting a transmission signal such as a radio frequency signal (for example, with a simple configuration) and transmitting it over a long distance with high quality. The purpose is to provide.
[0013]
[Means for Solving the Problems]
In the signal transmission method of the present invention, the transmission signal is converted into an intermediate frequency band signal using a local oscillation signal, the intermediate frequency band signal is subjected to pulse frequency modulation, and then converted into a first optical signal. When the signal is converted into a second optical signal, the first and second optical signals are transmitted via the optical transmission line, and the first and second optical signals are received via the optical transmission line, The transmission signal is regenerated by mixing the signal obtained by demodulating the first optical signal and then demodulating it with the local oscillation signal obtained as a result of the photoelectric conversion of the second optical signal. It is characterized by doing.
[0014]
The signal transmission system of the present invention is a signal transmission in which a transmission apparatus that converts a transmission signal into an optical signal and transmits the transmission apparatus and a reception apparatus that receives the optical signal and regenerates the transmission signal is connected by an optical transmission line. In the system, the transmission device converts the transmission signal into an intermediate frequency band signal using a local oscillation signal and then modulates the pulse frequency, and a first pulse obtained as a result of modulation by the modulation unit 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 into Wherein the receiver receives the first and second optical signals via the optical transmission path, and receives the first and second optical signals. Third conversion means for photoelectric conversion and the first optical signal Reproduction means for reproducing the transmission signal by mixing the signal obtained by demodulating the signal obtained as a result of photoelectric conversion of the signal and the local oscillation signal obtained as a result of photoelectric conversion of the second optical signal It was characterized by comprising.
[0015]
According to the present invention, a transmission signal such as a radio frequency signal is subjected to pulse frequency modulation and then converted into an optical signal for transmission, so that long-distance transmission can be performed with high quality. Demodulation only requires a low-pass filter (LPF). Therefore, according to the present invention, a transmission signal can be transmitted over a long distance with high quality easily (for example, with a simple configuration).
[0016]
In the signal transmission method of the present invention, a transmission signal is converted into an intermediate frequency band signal using a local oscillation signal, and the first pulse signal obtained as a result of pulse frequency modulation of the intermediate frequency band signal, the local oscillation signal, When the second pulse signal obtained by taking the logical product of the above is converted into an optical signal, the optical signal is transmitted through the optical transmission path, and the optical signal is received through the optical transmission path, The 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 are mixed to reproduce the transmission signal It is characterized by doing.
[0017]
The signal transmission system of the present invention is a signal transmission in which a transmission apparatus that converts a transmission signal into an optical signal and transmits the transmission apparatus and a reception apparatus that receives the optical signal and regenerates the transmission signal is connected by an optical transmission line. In the system, the transmission device converts the transmission signal into an intermediate frequency band signal using a local oscillation signal and then modulates the pulse frequency, and a first pulse obtained as a result of modulation by the modulation unit A first conversion means for converting a second pulse signal obtained by taking a logical product of the signal and the local oscillation signal into an optical signal, and transmitting the optical signal through the optical transmission line The receiving device receives the optical signal via the optical transmission line, and photoelectrically converts the optical signal, 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, a local oscillation signal extracted by the extraction means, And a reproducing means for reproducing the transmission signal.
[0018]
According to the present invention, a transmission signal such as a radio frequency signal is subjected to pulse frequency modulation and then converted into an optical signal for transmission, so that long-distance transmission can be performed with high quality. Demodulation only requires a low-pass filter (LPF). Therefore, according to the present invention, a transmission signal can be transmitted over a long distance with high quality easily (for example, with a simple configuration). Further, 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, thereby realizing signal transmission with further cost reduction. be able to.
[0019]
DETAILED DESCRIPTION OF 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]
(First embodiment)
FIG. 1 is a block diagram showing a configuration example of the signal transmission system according to the first embodiment, and is roughly composed of a transmission device 1 and a reception device 2.
[0021]
In FIG. 1, a transmission apparatus 1 includes a first mixer (mixer) 101, a local oscillator 102, a pulse frequency modulator (hereinafter referred to as “PFM modulator”) 103, and a first electric / optical converter ( Hereinafter, the first E / O converter or the first E / O 104 and the second electric / optical converter (hereinafter referred to as the second E / O converter or the second E / O converter) (Referred to as E / O) 105.
[0022]
The receiving apparatus 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 a “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 transmission device 1 will be described.
[0025]
A local oscillation signal output from the local oscillator 102 is input to the first mixer 101 and the second E / O converter 105.
[0026]
A local oscillation signal output from the local oscillator 102 and a radio frequency signal (RF signal) are input to the first mixer (mixer) 101. The first mixer 101 mixes (multiplies) the local oscillation signal and the 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 technique, and as a result, a PFM modulated signal corresponding to the input intermediate frequency signal is output.
[0028]
As shown in FIG. 2, 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 the PFM modulator. 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]
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 reverse relationship shown in FIG. 2, that is, when the IF signal amplitude is small, the pulse repetition frequency may be increased.
[0030]
The output signal of the PFM modulator 103 is converted into an optical signal by the first E / O converter 104.
[0031]
Various realization methods are conceivable as the first E / O converter 104, but in this embodiment, direct modulation of the semiconductor laser is used. As a method of driving a semiconductor laser with an electric pulse that is an output signal of the PFM modulator 103, the following method can be considered. That is, this is a method of obtaining an optical pulse similar to an electric pulse by driving a semiconductor laser biased in the vicinity of a threshold current value with a signal obtained by adding a pulse current to a bias current. By biasing the semiconductor laser in advance, the delay time from when the electric pulse is input until 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 also 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]
A signal for driving the first E / O converter 104 is an output of the PFM modulator 103. The PFM modulator 103 changes the repetition frequency of the output pulse having a certain width according to the amplitude of the 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 only needs to have a constant width.
[0035]
The second E / O converter 105 converts the electric signal of the local oscillator 102 that is constant repetition into an optical signal. The optical signal output from the second E / O converter 105 only needs to be able to convert a constant and repeated electrical signal into an optical signal. Accordingly, the second E / O converter 105 may have a large distortion.
[0036]
A semiconductor laser is used as an E / O converter that may have a large strain as described above. Semiconductor lasers can be broadly classified into two types of lasers: DFB lasers capable of dynamic single longitudinal mode oscillation and FP lasers oscillated by reflection at the end face. The DFB laser is more expensive than the FP laser because it produces a grating 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. There is no upper limit on the amplitude 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 that is the output of the first E / O converter 104 is transmitted to the receiving device 2 via the optical fiber cable 121, and the optical signal that is the output of the second E / O converter 105 is received via the optical fiber cable 122. 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 electrical signal. Further, the second O / E converter 107 regenerates the local oscillation signal by converting the 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 using a publicly known / public technique.
[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 transmitted from the transmission device 1 is the second To the mixer 109. In the second mixer 109, the signal output from the PFM modulator 108 and the local oscillation signal are mixed (multiplied), and the resulting signal is passed through the band-pass filter 110, whereby a radio frequency signal (RF Signal) can be reproduced.
[0042]
As described above, PFM modulation is a method of changing the repetition frequency of a pulse having a constant pulse width in accordance with the input signal amplitude. This PFM modulation spectral density is shown in FIG. The PFM modulation spectrum exists in the part indicated by “PFM” and the two parts indicated by “IF” in FIG. If the spectrum of the portion indicated by “PFM” is removed from the two spectra indicated by “PFM” and “IF” and only “IF” is left, the PFM modulation is demodulated. That is, demodulating the PFM modulation is equivalent to putting a low-pass filter.
[0043]
Therefore, to demodulate the signal output from the first O / E converter 106 by the PFM demodulator 108, the spectrum of the portion indicated by IF shown in FIG. 3 is taken out by the low-pass filter.
[0044]
The second mixer 109 mixes the spectrum portion called “IF” taken out by the PFM demodulator 108 and the local oscillation signal obtained by converting it into an electric signal by the second O / E converter 107. By doing so, the signal of the spectrum described as "LO" and "RF" shown in FIG. 3 will be obtained. In FIG. 3, the spectrum of the low sideband of “LO” is not shown.
[0045]
By extracting only the spectral component indicated by “RF” shown in FIG. 3 by the band-pass filter 110, a radio frequency signal can be output.
[0046]
Here, like the first and second E / O converters 104 and 105, the first and second O / E converters 106 and 107 may have large nonlinear distortion.
[0047]
In the first embodiment, transmission is performed by two optical fiber cables between the first and second E / O converters 104 and 105 and the first and second O / E converters 106 and 107. I tried to do it. By making the wavelengths of the first E / O converter 104 and the second E / O converter 105 different from each other, the optical signal as the output of the first E / O converter 104 and the second E / O converter 104 are changed. The optical signal as the output of the converter 105 is multiplexed by a multiplexer, transmitted through a single optical fiber, demultiplexed by a demultiplexer, and the first O / E converter 106 and the second O / E converter 106. Reception by the / E converter 107 is also conceivable.
[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 that converts to an optical signal only needs to output a pulse having a certain width, the modulation distortion does not matter. On the other hand, when the local oscillation signal is converted into an optical signal, the amount of modulation distortion is not limited. The semiconductor laser does not need to output an optical signal faithfully to the waveform of the local oscillation signal, and only needs to faithfully transmit frequency information. Therefore, the semiconductor laser for obtaining the optical signal may be a normal semiconductor laser for digital communication, and a low-cost apparatus can be realized. When the above-described two semiconductor lasers are used, transmission to the receiving end with two optical fibers can be considered. Furthermore, it is also conceivable that the oscillation frequencies of the two semiconductor lasers are made different from each other, wavelength-division-multiplexed and transmitted to the receiving end through 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 converted into an optical signal after being PFM modulated and transmitted. As described above, PFM is a modulation method in which the repetition frequency of a pulse having a constant pulse width is changed in accordance with an input signal (for example, an IF signal here), and the spectrum of a PFM-modulated signal is shown in FIG. As is clear, a baseband component is included. Therefore, the demodulation of the PFM may be performed only by the 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]
Since a transmission signal such as a radio frequency signal is PFM modulated and then converted into an optical signal and transmitted, high-quality long-distance transmission is possible. 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 transmitted over a long distance with high quality easily (for example, with a simple configuration).
[0051]
Here, the means for converting an electrical signal into an optical signal is a semiconductor laser, but it goes without saying that it can also be realized by a combination of a semiconductor laser oscillating constantly and lithium niobate or a semiconductor external modulator.
[0052]
(Second Embodiment)
FIG. 4 is a block diagram showing a configuration example of a signal transmission system according to the second embodiment, and is roughly composed of a transmission device 3 and a reception device 4.
[0053]
In FIG. 4, a transmission apparatus 3 includes a first mixer (mixer) 201, a local oscillator 202, a pulse frequency modulator (hereinafter referred to as “PFM modulator”) 203, and an electric / optical converter (hereinafter referred to as E). / O converter or E / O) 204 and a logical product circuit 205.
[0054]
The receiving apparatus 2 includes an optical / 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 a single optical fiber cable 221.
[0056]
First, the transmission device 3 will be described.
[0057]
The local oscillation signal output from the local oscillator 202 is input to the first mixer 201 and the AND circuit 205.
[0058]
A local oscillation signal output from the local oscillator 202 and a radio frequency signal (RF signal) are input to the first mixer (mixer) 201. The first mixer 201 mixes (multiplies) the local oscillation signal and the 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 technique, and as a result, a PFM modulated signal corresponding to the input intermediate frequency signal is output.
[0060]
The AND circuit 205 performs an AND operation on the PFM modulation 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 put on 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 in accordance with the input signal amplitude. Therefore, as shown in FIG. It can be seen that the repetition frequency of the product operation pulse train group changes. 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 is high, and when the amplitude of the IF signal as the input signal is small, the repetition frequency of the AND operation pulse train is low. An example was given. However, as in the first embodiment, the relationship between the IF signal amplitude of the PFM modulator 203 and the pulse (logical product pulse train) repetition frequency is not limited. For example, when the IF signal amplitude is large, the pulse (logical product pulse train) repetition frequency may be lowered.
[0063]
An AND operation pulse train as a PFM modulation signal carrying a local oscillation signal, 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 for driving the semiconductor laser with the output signal of the AND circuit 205 is the same as in the first embodiment described above, and is therefore omitted here.
[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 putting the output signal of the local oscillator 202 on the output signal of the PFM modulator 203.
[0066]
The PFM modulator 203 changes the repetition frequency of the output pulse having a certain width according to the amplitude of the signal input to the PFM modulator 203. Accordingly, the E / O converter 204 that converts a signal obtained by ANDing 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 is not restricted by the type of the semiconductor laser. Therefore, the E / O converter can be configured by either a DFB laser or an FP laser. There is no upper limit on the amplitude 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 through the optical fiber 221.
[0069]
Next, the receiving device 4 will be described.
[0070]
The O / E converter 206 converts the optical signal received via the optical fiber cable 221 into an electrical signal, and outputs the electrical signal to the PFM modulator 208 and the clock extraction circuit 207.
[0071]
The 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]
As described in the first embodiment, the PFM demodulation can be realized by removing the PFM modulation spectrum component with a low-pass filter. The PFM demodulator 208 in the second embodiment determines the cutoff frequency of the low-pass filter so as to remove the PFM modulation spectrum.
[0073]
The clock extraction circuit 207 receives the electrical signal output from the O / E converter 206, and extracts a local oscillation signal component included in the PFM modulation signal. As this extraction method, a publicly known / public technique may be used, and since it is not the gist of the present invention, description thereof 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 both signals by the second mixer 209 using the band-pass filter 210.
[0076]
Here, like the E / O converter 204, the O / E converter 206 may have large nonlinear distortion.
[0077]
In the second embodiment, a local oscillation signal is put on the modulated transmission signal and then converted into an optical signal and transmitted. In the receiving device 4, the output of the O / E converter 206 is extracted with the PFM demodulator 208 and the clock. 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 transmission device 3, not only one semiconductor laser is required to convert into an optical signal, but it is only necessary to output pulsed light, so that the modulation distortion does not matter. A semiconductor laser for obtaining an optical signal may be a normal semiconductor laser for digital communication, and a low-cost apparatus can be realized.
[0079]
In order to simplify the configuration, the output of the O / E converter 206 can be passed through the band pass filter 210 to output a radio frequency signal. In this case, since the receiving apparatus 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 converted into an optical signal after being PFM modulated and transmitted. As described above, PFM is a modulation method in which the repetition frequency of a pulse having a constant pulse width is changed in accordance with an input signal (for example, an IF signal here), and the spectrum of a PFM-modulated signal is shown in FIG. As is clear, a baseband component is included. Therefore, the demodulation of the PFM may be performed only by the 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]
Since a transmission signal such as a radio frequency signal is PFM modulated and then converted into an optical signal and transmitted, it can be transmitted over a long distance with high quality. It ’s OK. Therefore, according to the present invention, a transmission signal can be transmitted over a long distance with high quality easily (for example, with a simple configuration).
[0082]
Here, the means for converting an electrical signal into an optical signal is a semiconductor laser, but it goes without saying that it can also be realized by a combination of a semiconductor laser oscillating constantly and lithium niobate or a semiconductor external modulator.
[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 (eg, with a simple configuration) regardless of the non-linear distortion of the E / O converter and the O / E converter. In addition, high-quality transmission over a long distance is possible.
[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 that is an 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 electricity / 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 (2)

The transmission signal is converted into an intermediate frequency band signal using a local oscillation signal, and this intermediate frequency band signal is subjected to pulse frequency modulation, and the repetition frequency of the output pulse having a constant width is changed according to the amplitude of the intermediate frequency band signal. In order to generate the first pulse signal, and to put the local oscillation signal on the first pulse signal, a logical product of the first pulse signal and the local oscillation signal is taken to obtain a second Generating a pulse signal, converting the second pulse signal into an optical signal, and transmitting the optical signal via an optical transmission line;
When the optical signal is received via the optical transmission line, the second pulse signal obtained by photoelectrically converting the optical signal is demodulated, and a signal obtained as a result of the demodulation is obtained from the second pulse signal. A signal transmission method comprising: reproducing the transmission signal by mixing the extracted local oscillation signal. A signal transmission system in which a transmission device that converts a transmission signal into an optical signal and transmits the transmission device and a reception device that receives the optical signal and reproduces the transmission signal are connected by an optical transmission line,
The transmitter is
Means for converting the transmission signal into an intermediate frequency band signal using a local oscillation signal;
Modulation means for modulating the intermediate frequency band signal by pulse frequency and generating a first pulse signal in which the repetition frequency of the output pulse having a constant width is changed according to the amplitude of the intermediate frequency band signal;
Means for generating a second pulse signal by taking a logical product of the first pulse signal and the local oscillation signal in order to put the local oscillation signal on the first pulse signal;
Means for converting the second pulse signal into an optical signal;
Comprising transmitting the optical signal through the optical transmission line,
The receiving device is:
Means for photoelectrically converting the optical signal received via the optical transmission path and outputting the second pulse signal;
Extraction means for extracting the local oscillation signal from the second pulse signal;
Demodulation means for demodulating the second pulse signal;
A reproduction means for reproducing the transmission signal by mixing the signal obtained as a result of demodulation by the demodulation means and the local oscillation signal extracted by the extraction means;
A signal transmission system comprising:

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