JPH08316908A - Optical fiber transmitter - Google Patents

Abstract

PURPOSE: To secure an economical constitution of an optical fiber transmitter in order to transmit the high frequency signals with high quality and without suffering any limitation caused by the linearity or the S/N of every laser diode. CONSTITUTION: An RF signal is converted into an IF signal by a frequency converter means 21, and the IF signal is converted into a digital signal by an A/D converter 25. Then the digital signal and the local signal of the means 21 are converted into the optical signals by the laser diodes 12 and 34 and then made incident on the optical fibers 13 and 35 respectively. The optical fibers 13 and 35 convert the optical signals into electric signals through the photodiodes 14 and 36. The reproduced digital signals of the electric signals are converted into the analog signals by a D/A converter 32. The IF signal is reproduced. Then the reproduced IF signal is converted and reproduced into an RF signal by a frequency converter means 33 and based on a reproduced local signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber transmission device for transmitting a radio frequency signal using an optical fiber.

[0002]

2. Description of the Related Art Since an optical fiber has a smaller diameter and lighter weight than a coaxial cable, a technique for utilizing an optical fiber transmission line as a transmission line for a radio frequency signal has been developed. In the conventional optical fiber transmission of a high frequency signal, as shown in FIG.
As shown in (1), the transmission side modulates the emission intensity of the laser diode 12 as the electrical / optical conversion means with a high frequency signal (RF signal) from the input terminal 11, and transmits this optical signal through the optical fiber 13. On the receiving side, the optical signal from the optical fiber 13 is converted into a high frequency signal by the photodiode 14 as an optical / electrical converting means and output to the output terminal 15.

Generally, in order to transmit a signal with high quality,
The transmission line must have low distortion and low noise. In particular, since strong signals and weak signals having different frequencies coexist in radio signals such as mobile radio signals, it is necessary to suppress intermodulation distortion in the transmission line as small as possible. In mobile communication, for example, in order to provide services to underground malls, in underground malls and ground relay equipment, uplink and downlink communications with base stations on the ground are performed, and downlink radio waves from base stations on the ground are converted into optical signals underground. Then, it enters the optical fiber, transmits it to the repeaters installed in various places in the underground mall with the optical fiber, radiates it as a radio wave in each of the repeaters, and the upstream radio wave from the mobile station in the underground mall is transmitted to the repeater in the underground mall. The signal is received, converted into an optical signal, transmitted to the relay device through the optical fiber, and then transmitted as a radio wave to the ground base station.

When high-frequency signals having different frequencies are transmitted through an optical fiber as described above, the intermodulation distortion in the optical fiber transmission shown in FIG. 7 is caused by the laser diode (LD) 12.
Depends on the linearity of the laser diode 12 when it is directly modulated with an RF signal. Therefore, in order to transmit the wireless signal through the optical fiber, it is necessary to select the LD having good linearity.

[0005]

The object of the present invention is not to depend on the linearity of the conversion characteristics of the electrical / optical conversion means, the intermodulation distortion can be suppressed to a small level, and the signal power to noise power ratio (S / An object of the present invention is to provide an optical fiber transmission device having a high N) and which can be constructed economically.

[0006]

According to the present invention, a high frequency signal is converted into an intermediate frequency signal on the transmitting side, the intermediate frequency signal is converted into a digital signal by an A / D converter, and the digital signal is converted into an optical signal. It is converted into a signal and transmitted through an optical fiber. On the other hand, on the receiving side, the optical signal transmitted through the optical fiber is converted into an electric signal to reproduce a digital signal, and the digital signal is reproduced as an intermediate frequency signal by a D / A converter, and the intermediate frequency signal is high frequency. It is converted back to a signal. At this time, the local signal on the transmitting side is transmitted to the receiving side, and the local signal is reproduced on the receiving side.

[0007]

[Operation] Since the optical fiber transmission device of the present invention transmits a digital signal through an optical fiber, the intermodulation distortion does not depend on the linearity of the electrical / optical conversion means, and a high frequency signal can be transmitted with low distortion. it can. Further, since the local signal on the transmitting side is used on the receiving side, it is not necessary to provide a highly stable local oscillator on the receiving side, and the structure can be economically constructed.

[0008]

First Embodiment FIG. 1 shows a first embodiment of the optical fiber transmission apparatus of the present invention. On the transmitting side, the high frequency signal to be transmitted is down-converted by the first frequency conversion means 21 to be converted into an intermediate frequency signal. That is, the input RF signal is frequency-mixed by the local signal from the local oscillator 22 in the mixer 23, and the intermediate-frequency signal is output from the frequency-mixed output to the low-pass filter 2.
Elected in 4. This intermediate frequency signal is sent to the A / D converter 25.
Is converted into a digital signal. Therefore, the frequency of the intermediate frequency signal is set to be equal to or lower than the upper limit frequency that can be converted by the A / D converter 25. For example, in mobile communication, 800MHz
Band, 1.5 GHz band, 2 GHz band, etc. are used, but A / D
As the converter 25, one having an operating frequency of 20 MHz and about 100 MHz in the future is conceivable, and is set to an intermediate frequency of such a frequency. A local signal for frequency conversion is output using a local oscillator 22 as shown in FIG. 2A, for example. That is, the oscillation output from the voltage controlled oscillator (VCO) 26 is frequency-divided by the variable frequency divider 27,
The frequency-divided output is compared in phase with a reference signal from a highly stable reference oscillator 28 such as a crystal oscillator in a phase comparator 29, and the phase comparison output is supplied to a control input terminal of the VCO 26 through a loop filter 31. A phase locked loop (PLL) is configured, and a local signal having a frequency that maintains the stability of the reference signal is output from the VCO 26. With this configuration, even an RF signal having a very high frequency can be converted into a digital signal.

The A / D converter 25, as shown in FIG.
Convert to a binary digital code proportional to the input signal amplitude. The linearity in this conversion is determined by the accuracy of the analog part of the A / D converter 25. The non-linearity of the A / D conversion can be easily compensated for by referring to, for example, a table after conversion into a digital signal. This digital signal is converted into an optical signal by the laser diode 12 as an electric / optical converting means and transmitted to the optical fiber 13.

On the receiving side, the optical signal transmitted by the optical fiber 13 is converted into an electric signal by a photodiode 14 as an optical / electrical converting means to reproduce a digital signal,
The digital signal is reproduced by the D / A converter 32 into an IF signal. Here, the linearity of the D / A converter 32 is determined similarly to the A / D converter 25. In addition, the A / D converter 25
Similarly, in the digital signal, the nonlinearity can be easily compensated. This analog signal, that is, the reproduced IF signal is up-converted by the second frequency conversion means 33 to reproduce the RF signal. In this case, the local signal is used on the transmitting side. For this reason, in this embodiment, the local signal from the local oscillator 22 on the transmission side is branched to generate electrical / electrical signals.
It is converted into an optical signal by a laser diode 34 as an optical converting means, and the optical signal is transmitted by an optical fiber 35. On the receiving side, the optical signal from the optical fiber 35 is converted into an electric signal by the photodiode 36 as the optical / electrical converting means to reproduce the local signal, and the local signal is amplified by the amplifying circuit 37 to generate the second frequency converting means. The reproduction I from the D / A converter 32 is input to the mixer 38 in 33.
The F signal is frequency-mixed, and the RF signal is selected by the band-pass filter 39 from the frequency-mixed output and output to the output terminal 15. It should be noted that the A / D converter 25 outputs the converted digital signal serially, and those which are converted in parallel perform parallel / serial conversion and output.

As described above, according to the present invention, the optical fiber 1
On the other hand, since the binary digital signal is transmitted on the upper part of FIG. 3, low distortion transmission is possible without depending on the nonlinearity of the laser diode 12. Further, since it is not necessary to provide the receiving side with a highly stable local oscillator similar to that on the transmitting side, the receiving circuit can be easily realized. Second Embodiment FIG. 3 shows a second embodiment of the optical fiber transmission device of the present invention, and the portions corresponding to those in FIG. 1 are designated by the same reference numerals. A
The digital signal from the / D converter 25 is converted into an optical signal with a wavelength λ1, whereas the local signal from the local oscillator 22 is converted into an optical signal with a wavelength λ2 different from the wavelength λ1 and the optical combiner 41 converts the optical signal of the laser diode 12 It is combined with the optical signal and is incident on the optical fiber 13. The optical signal from the optical fiber 13 is divided into an optical signal of wavelength λ1 and an optical signal of wavelength λ2 by the optical distributor 42, and converted into electric signals by the photodiodes 14 and 36, respectively. Others are the same as those in the first embodiment of FIG.

With this configuration, it is not necessary to provide the receiving side with the same highly stable local oscillator as the transmitting side, and a single optical fiber can transmit a digital signal and a local signal, resulting in a simpler configuration. And, the same effect as the first embodiment can be obtained. Third Embodiment FIG. 4 shows a third embodiment of the optical fiber transmission device of the present invention. In this third embodiment, before converting the IF signal into a digital signal, the local signal of the local oscillator 22 and the combiner 44 are used.
To synthesize. This local signal does not have to be a signal having the same frequency as the local signal input to the mixer 23, but it is output from the same local oscillator 22. The signal combined by the combiner 44 is converted into a digital signal by the A / D converter 25, converted into an optical signal, and then transmitted through the optical fiber 13.

On the receiving side, the transmitted optical signal is converted into an electric signal by the photodiode 14, and the D / A converter 3
2 reproduces the IF signal and the local signal, and distributes them to the distributor 45.
Then, the reproduced local signal is input to the local oscillator 46, a local signal based on the input local signal is generated by the local oscillator 46, and the local signal is supplied to the mixer 38.

With this structure, it is not necessary to provide the receiving side with a highly stable local oscillator similar to the transmitting side, and
A single optical fiber can transmit digital signals and local signals. As a result, the structure becomes simpler and the same effect as that of the first embodiment can be obtained. Fourth Embodiment FIG. 5 shows a fourth embodiment of the optical fiber transmission device of the present invention. On the transmitting side, the IF signal is converted into a digital signal by the same procedure as in the first embodiment and transmitted as an optical signal.
A / D converter 25 for converting an IF signal into a digital signal
Is operated with the local signal used for frequency conversion. This A /
The signal for operating the D converter 25 does not have to have the same frequency as the local signal input to the mixer 23.

On the receiving side, the photodiode 14
The electric signal is also branched and supplied to the clock reproduction circuit 47 to reproduce the clock signal. The recovered clock is input to the local oscillator 46 as a reference signal, a local signal is generated and supplied to the mixer 38. As the A / D converter 25 on the transmission side, for example, as shown in FIG. 6A, an analog signal (IF signal) is converted into an N-bit digital code by a digitizer 51, stored in a latch 52, and stored in a latch 5.
The output of 2 is converted into a serial digital signal by the serial conversion circuit 53 and is output. The conversion clock for converting the parallel code into the serial signal is performed by the local signal of the local oscillator 22. That is, the bit rate of this serial digital signal is the rate of the local signal. Clock recovery circuit 47 on the receiving side
Is a digital VCO 54, for example, as shown in FIG. 6B.
Clock signal from the photodiode 14 is compared with the received digital signal from the photodiode 14 in the digital phase comparator 55, and the phase comparison output is compared with the digital low-pass filter 5
A control signal is supplied to the digital VCO 54 through 6 and the output of the VCO 54 is converted into an analog signal by the D / A converter 57 to obtain a reproduction clock.

The sampling of the digitizer 51 in FIG. 6A may be performed with the local signal of the local oscillator 22, and the serial conversion of the serial conversion circuit 53 may be performed with a clock having a speed N times that of the local signal. In this case, the reception side divides the recovered clock by 1 / N to obtain a local signal. With the configuration shown in FIG. 5, it is not necessary to provide the receiving side with a highly stable local oscillator similar to the transmitting side, and a digital signal and a local signal can be transmitted by one optical fiber.

As a result, the structure becomes simpler and the same effects as those of the first embodiment can be obtained. In any of the above-described embodiments, the local signal transmitted from the transmitting side is the first frequency conversion means 2 as described in some of the embodiments.
It is not limited to the one used for frequency conversion in 1 and may be the one converted in frequency. When the frequency is converted, the frequency is converted on the receiving side to obtain a local signal having a desired frequency. Therefore, the local signal in the claims means both cases.

[0018]

As described above, the present invention is not limited to the linearity or S / N of electric / optical conversion means such as a laser diode in optical fiber transmission, and can transmit a higher quality RF signal. Therefore, it becomes possible to perform optical fiber transmission with excellent economical efficiency.

[Brief description of drawings]

FIG. 1 is a block diagram showing the first embodiment of the present invention.

2A is a block configuration diagram showing a specific example of a local oscillator 22 on the transmitting side in FIG. 1, and B is an A / D converter 25 in FIG.
It is a figure which shows the conversion characteristic of.

FIG. 3 is a block diagram showing a second embodiment of the present invention.

FIG. 4 is a block diagram showing a third embodiment of the present invention.

FIG. 5 is a block diagram showing a fourth embodiment of the present invention.

6A is a block diagram showing a specific example of the A / D converter 25 in FIG. 5, and B is a block configuration diagram showing a specific example of the clock recovery circuit 47 in FIG.

FIG. 7 is a block diagram showing a conventional optical fiber transmission device.

Claims (5)

[Claims] 1. An optical fiber transmission device for transmitting a high frequency signal through an optical fiber, wherein at a transmitting side, first frequency conversion means for converting the high frequency signal into an intermediate frequency signal, and converting the intermediate frequency signal into a digital signal. An A / D converter, an electric / optical conversion means for converting the digital signal into an optical signal and entering the optical fiber, and means for transmitting the local signal of the first frequency conversion means to the receiving side are provided. An optical / electrical conversion means for converting the optical signal transmitted through the optical fiber into an electric signal and reproducing the digital signal on the receiving side; and converting the reproduced digital signal into an analog signal for the intermediate processing. A D / A converter for reproducing a high frequency signal and a frequency conversion of the reproduced intermediate frequency signal by a local signal transmitted from the transmitting side to reproduce the high frequency signal. Optical fiber transmission system and the second frequency converting means, characterized in that it is provided that. 2. The means for transmitting the local signal comprises a second electric / optical converting means for converting the local signal into an optical signal, and an optical signal from the second electric / optical converting means for transmitting to the receiving side. Second optical / electrical converting means for converting the optical signal transmitted to the receiving side by the second optical fiber into an electric signal and supplying the electric signal to the second frequency converting means as a local signal. The optical fiber transmission device according to claim 1, further comprising: 3. The means for transmitting the local signal comprises a second electrical / optical converting means for converting the local signal into an optical signal having a second wavelength different from the first wavelength of the converted optical signal of the electrical / optical converting means. And an optical combiner for combining both the optical signals of the first and second wavelengths to enter the optical fiber, and the optical signal transmitted through the optical fiber is transmitted to the receiving side by the optical signal of the first wavelength. A signal and an optical signal of the second wavelength, and an optical distributor for supplying the optical signal of the first wavelength to the optical / electrical conversion means,
The optical fiber transmission according to claim 1, further comprising: second optical / electrical conversion means for converting the optical signal of the second wavelength into an electric signal and supplying it as a local signal to the second frequency conversion means. apparatus. 4. The means for transmitting the local signal is a synthesizer for synthesizing the local signal with the converted output of the first frequency converting means and supplying the synthesized signal to the A / D converter. The optical fiber according to claim 1, further comprising: a distributor that distributes an output of the D / A converter into a reproduced intermediate frequency signal and a reproduced local signal and supplies the reproduced local signal to the second frequency conversion means. Transmission equipment. 5. The means for transmitting the local signal is a means for operating the A / D converter by the local signal to include a component of the local signal in the digital signal, and the receiving side is provided with the above-mentioned. 2. The optical fiber transmission device according to claim 1, further comprising a clock regeneration circuit for regenerating a clock from an electric signal from the optical / electrical converting means and supplying it as a local signal to the second frequency converting means.