JPH0846568A - Optical reception device - Google Patents

Abstract

PURPOSE:To ensure the satisfactory identification and reproduction of a digital signal by suppressing the increase of an error rate despite the S/N deterioration of a received signal. CONSTITUTION:An optical signal is converted into an electric signal by a photodiode 1 and amplified by a variable gain amplifier 3. Then the amplified signal undergoes the voltage discrimination through a comparator 11 against the discrimination reference voltage Vth. Thus a data signal is discriminated and reproduced. Meanwhile the signal inputted to the comparator 11 undergoes the voltage discrimination through a comparator 12 against the voltage obtained by adding the control target voltage DELTAV to the voltage Vth. Thus a monitor signal is obtained and also the code error rate of the monitor signal is detected to the data signal by an error detection circuit 16 and an integration circuit 17. Then the gain of the amplifier 3 is controlled by an error amplifier 9 so that the amplitude of the signal inputted to the comparator 11 is changed. Thus the detected code error rate is set at a prescribed level Pe.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiver for receiving an optical signal, which is a digital signal represented by ON / OFF of light, and reproducing the digital signal.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing a configuration example of a conventional optical receiver. This optical receiver comprises a photodiode 1, a preamplifier 2, a variable gain amplifier 3, a clamp circuit 4, a timing extraction circuit 5, an identification reproduction circuit 6, an identification reference voltage source 7, a peak detection circuit 8 and an error amplifier 9. .

An optical signal arriving via an optical fiber (not shown) is incident on the photodiode 1. As a result, the photodiode 1 generates a current according to the incident optical signal. This current is converted into a voltage by the preamplifier 2 and then amplified by the variable gain amplifier 3. The output signal of the variable gain amplifier 3 is DC-regenerated by the clamp circuit 4 and then input to the identification / regeneration circuit 6. The output signal of the variable gain amplifier 3 is also input to the timing extraction circuit 5 and the timing is extracted. The signal extracted by the timing extraction circuit 5 is output as a clock signal and input to the identification reproduction circuit 6.

In the discriminating / reproducing circuit 6, the signal given from the clamp circuit 4 is subjected to voltage discrimination by the discriminating reference voltage Vth generated by the discriminating reference voltage source 7 and timing reproduction is carried out by the clock signal outputted from the timing extracting circuit 5. Then, the data signal is reproduced from the signal given from the clamp circuit 4.

A part of the output signal of the variable gain amplifier 3 is also input to the peak detection circuit 8. The peak detection circuit 8 performs peak detection on the output signal of the variable gain amplifier 3 and outputs a signal having a level indicating the peak value. The error signal from the output signal of the peak detection circuit 8 is amplified by the error amplifier 9 with respect to a predetermined reference value, and the variable gain amplifier 3
Is given to. This ensures that the output amplitude of the variable gain amplifier 3 is always at a constant level.
The gain is controlled and the AGC (automatic gain control) operation is realized.

By the way, the output signal of the clamp circuit 4 has a waveform as shown in FIG. 6 when the S / N of the received signal is good. The discrimination reference voltage Vth is normally set to increase the voltage discrimination margin and reduce the code error rate.
It is set near the center between the light emitting side (High level) and the non-light emitting side (Low level).

However, when the optical signal deteriorates due to dispersion in an optical fiber or the like, the light emitting side is deteriorated more than the non-light emitting side. FIG. 7 is a diagram for explaining this property, and shows the discrimination margin (eye opening) at the input of the discrimination reproduction circuit 6 when optical transmission is carried out in an optical fiber having a large dispersion. In this figure, the transmission distance is almost zero.
In the case of the direct connection state and the case of the transmission distance of 30 km, the points having the same code error rate when the phases of the identification reference voltage Vth and the timing extraction clock are changed are plotted and graphed. is there.

As is clear from this figure, the eye opening is wide in the direct connection state where the influence of dispersion in the optical fiber is small, but after 30 km transmission, the eye opening is greatly affected by dispersion in the optical fiber. Is getting smaller. In particular, the deterioration on the light emitting side is larger than that on the non-light emitting side.
The eye opening is closer to the non-emission side.

The output signal of the clamp circuit 4 when the S / N deteriorates in this way has a waveform as shown in FIG. As is clear from this figure, when AGC is performed so that the output amplitude of the variable gain amplifier 3 is always at a constant level,
When the S / N ratio deteriorates, the eye opening approaches the non-light emitting side, and the identification reference voltage Vth shifts to the light emitting side of the eye opening. Therefore, the discrimination margin on the light emitting side becomes small and the error rate becomes large.

[0010]

As described above, in the conventional optical receiving apparatus, the AGC is applied to the received signal so that the output amplitude always becomes a constant level, and then the digital signal is applied with a constant identification reference voltage. Since the identification reproduction is performed, the S / N of the received signal is deteriorated and the eye opening is displaced, so that the identification reference voltage is not an optimum value and the error rate becomes large.

The present invention has been made in consideration of such circumstances, and an object of the present invention is to suppress an increase in error rate even if the S / N of a received optical signal is deteriorated and suppress the increase of a digital signal. An object of the present invention is to provide an optical receiving device capable of favorably performing identification reproduction.

[0012]

To achieve the above object, the present invention provides a photoelectric conversion means such as a photodiode for converting an optical signal into an electric signal, and an electric signal obtained by the photoelectric conversion means. A variable gain amplifying means, such as a variable gain amplifier, which amplifies with a gain indicated by the control signal; and an electrical signal after being amplified by the variable gain amplifying means, identified by a predetermined first threshold value such as an identification reference voltage. Identification and reproduction means for identifying and reproducing the digital signal, for example, a comparator and a D-flip-flop, and an electric signal after being amplified by the variable gain amplifying means by a predetermined amount (for example, a control target voltage) than the first threshold value. A monitoring signal by discriminating with a predetermined second threshold which is only large, eg comparator and D
A monitoring signal generating means composed of a flip-flop, an error rate detecting means comprising an error detecting circuit and an integrating circuit for detecting an error rate of the monitoring signal with respect to the digital signal reproduced by the identification reproducing means, The control signal is generated to control the gain of the variable gain amplifying means so that the error rate detected by the error rate detecting means approaches a predetermined reference value such as an error rate setting voltage. For example, an error amplifier and an error rate. And a gain control means composed of a set voltage source.

[0013]

By taking such means, the electric signal obtained by converting the optical signal by the photoelectric converting means is amplified by the gain indicated by the control signal in the variable gain amplifying means, and then the identifying and reproducing means. Then, the digital signal is identified and reproduced by being identified by a predetermined first threshold value. At this time, the electric signal after being amplified by the variable gain amplifying means is identified by a predetermined second threshold value which is larger than the first threshold value by a predetermined second threshold value by the monitoring signal generating means to generate a monitoring signal. At the same time, the error rate detecting means detects the error rate of the monitoring signal with respect to the digital signal reproduced by the identifying and reproducing means, and the gain of the variable gain amplifying means is adjusted so that the error rate approaches a predetermined reference value. The control signal is generated by the gain control means to control the.

Therefore, the amplitude of the digital signal input to the identification / reproduction means is increased / decreased so that the error rate of the monitoring signal identified and generated by the second threshold value becomes a predetermined reference value. Thereby, the position where the predetermined error rate is obtained is controlled to be constant with respect to the first threshold value, and the eye opening is always stably formed around the first threshold value.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the optical receiving apparatus according to the present embodiment. The same parts as those in FIG. 5 are designated by the same reference numerals.

This optical receiver comprises a photodiode 1,
Preamplifier 2, variable gain amplifier 3, clamp circuit 4, timing extraction circuit 5, discrimination reference voltage source 7, error amplifier 9, comparators 11, 12, control target voltage source 13, D
-Flip-flops (D-FF) 14, 15, an error detection circuit 16, an integration circuit 17, and an error rate setting voltage source 18.

The photodiode 1 is arranged so that an optical signal coming through an optical fiber (not shown) is incident on the photodiode 1.
An electric signal having a current value according to the intensity of the incident optical signal is generated. Further, the photodiode 1 is a preamplifier 2
And supplies the generated electric signal to the preamplifier 2.

The preamplifier 2, the variable gain amplifier 3, the clamp circuit 4, the comparator 11 and the D-flip-flop 14 are serially connected in the same order as this description. Besides the output signal of the clamp circuit 4, the discrimination reference voltage Vth generated by the discrimination reference voltage source 7 is also input to the comparator 11. The preamplifier 2, the variable gain amplifier 3, the clamp circuit 4, the identification reference voltage source 7, the comparator 11 and the D-flip-flop 14 form an identification reproduction system for identifying and reproducing a data signal from the output signal of the photodiode 1. There is. Then, the output signal of the D-flip-flop 14 is output as the reproduced data signal.

The timing extraction circuit 5 is connected to the variable gain amplifier 3 and forms a clock generation system for generating a clock signal based on the output signal of the variable gain amplifier 3. Then, the timing extraction circuit 5 outputs the generated clock signal as a clock signal indicating the timing of the data signal in parallel with the data signal, and also supplies it to each of the D-flip-flops 14 and 15.

The comparator 12, the D-flip-flop 15, the error detecting circuit 16, the integrating circuit 17 and the error amplifier 9 are serially connected in the same order as described above. The sum of the output signal of the clamp circuit 4, the control target voltage ΔV generated by the control target voltage source 13, and the discrimination reference voltage Vth generated by the discrimination reference voltage source 7 is input to the comparator 12. In addition to the output signal of the D-flip-flop 15, the error detection circuit 16 includes D
The output signal of the flip-flop 14 is also input.
In the error amplifier 9, in addition to the output signal of the integrating circuit 17,
The error rate setting voltage Vs generated by the error rate setting voltage source 18 is input. These error amplifier 9 and comparator 1
2, the D-flip-flop 15, the error detection circuit 16, the integration circuit 17, and the error rate setting voltage source 18 form a control system for controlling the gain of the variable gain amplifier 3.

Next, the operation of the optical receiver configured as described above will be described. An optical signal arriving via an optical fiber (not shown) is converted into a current in the photodiode 1. This current is converted into a voltage by the preamplifier 2 and then amplified by the variable gain amplifier 3. The output signal of the variable gain amplifier 3 is DC-regenerated by the clamp circuit 4 and then input to the comparator 11. The output signal of the variable gain amplifier 3 is also input to the timing extraction circuit 5 and the timing is extracted. Then, the signal extracted by the timing extraction circuit 5 is output as a clock signal.

The comparator 11 performs voltage discrimination on the signal given from the clamp circuit 4 by the discrimination reference voltage Vth generated by the discrimination reference voltage source 7, and outputs a signal indicating the discrimination result to the D-flip-flop 14. . D-
The flip-flop 14 performs timing reproduction on the output signal of the comparator 11 by the clock signal output from the timing extraction circuit 5. As a result, the D-flip-flop 14 outputs a data signal synchronized with the clock signal.

By the way, when the data signal is reproduced as described above, the voltage discrimination and the timing reproduction are performed also in the comparator 12 and the D-flip-flop 15 similarly to the comparator 11 and the D-flip-flop 14. ing. However, in the comparator 12, the control target voltage ΔV generated by the control target voltage source 13 is added to the discrimination reference voltage Vth generated by the discrimination reference voltage source 7.
The voltage Vth + ΔV added with is input, and voltage discrimination is performed with a voltage different from that of the comparator 11. In the following, the signal generated by the comparator 12 will be referred to as a monitoring signal.

As described above, the comparator 11 and the comparator 12 have different reference voltages for voltage identification.
The data signal output from the flip-flop 14 and D-
A partial mismatch occurs with the monitoring signal output from the flip-flop 15. Therefore, the error detection circuit 16
Compares the data signal output from the D-flip-flop 14 with the monitoring signal output from the D-flip-flop 15, and outputs an error pulse when the two signals do not match as shown in FIG. . This error detection circuit 16
The output signal of is averaged in the integrating circuit 17, and is input to the differential amplifier 9. The output signal of the integrating circuit 17 becomes a signal having a voltage level Ve proportional to the error pulse generation rate, that is, the code error rate of the monitoring signal with respect to the data signal.

The error amplifier 9 outputs a signal having a voltage level corresponding to the difference between the voltage level of the output signal of the integrating circuit 17 and the error rate setting voltage Vs generated by the error rate setting voltage source 18. The output signal of the error amplifier 9 is given to the variable gain amplifier 3 as a control signal, and is varied so that the voltage level of the output signal of the integrating circuit 17 matches the error rate setting voltage Vs generated by the error rate setting voltage source 18. Gain amplifier 3
Gain is controlled. The error rate setting voltage Vs is set to a value corresponding to a predetermined code error rate Pe.

In this way, AGC is applied so that the code error rate of the supervisory signal with respect to the data signal becomes the predetermined code error rate Pe. Next, the operation of AGC will be described more specifically.

First, the discrimination reference voltage Vth is determined by the clamp circuit 4
When the amplitude of the output signal is the optimum amplitude for the comparator 11 to perform the voltage discrimination (eg, corresponding to the target amplitude of AGC in the conventional optical receiver shown in FIG. 5), the light emitting side level and the non-light emitting side level Set near the center of and. Thus, when the S / N of the optical signal received by the photodiode 1 is good, the discrimination margin in the comparator 11 is large and the data signal is error-free, as shown in FIG. 3, for example.

On the other hand, since the voltage Vth + ΔV deviates from the vicinity of the center between the light emitting side level and the non-light emitting side level, the monitor signal has more code errors than the data signal. However, when the S / N is good, even if the voltage Vth + ΔV is slightly deviated from the vicinity of the center between the light emitting side level and the non-light emitting side level, there is a sufficient margin for discrimination, so that a code error is less likely to occur. Therefore, in order to cause a code error of a predetermined code error rate Pe, the upper end of the voltage amplitude in the output signal of the clamp circuit 4, that is, the light emission side level and the voltage Vth + ΔV tend to approach each other. Thus,
The gain of the variable gain amplifier 3 is controlled so that the output signal of the clamp circuit 4 is in the state shown in FIG. 3, that is, the light emission side level is close to the voltage Vth + ΔV.

On the other hand, when the S / N of the optical signal received by the photodiode 1 is deteriorated, the voltage Vth +
Since ΔV is at a higher level than the identification reference voltage Vth,
The increase rate of the code error rate when the S / N is good is larger for the supervisory signal than for the data signal. Therefore, the code error rate of the monitoring signal with respect to the data signal tends to increase, and in order to cause a code error of a predetermined code error rate Pe, the light emission side level and the voltage Vth + ΔV in the output signal of the clamp circuit 4 are S. / N amount is high. Thus, the gain of the variable gain amplifier 3 is controlled so that the output signal of the clamp circuit 4 becomes the state shown in FIG. 4, that is, the light emission side level is separated from the voltage Vth + ΔV.

In this way, the gain of the variable gain amplifier 3 increases as the S / N deteriorates. Therefore S / N
The level on the light emitting side changes in accordance with the above, but the amplitude at which the code error rate becomes Pe is controlled to be constant, and the amplitude of the eye opening can be controlled to be constant. This allows
The identification reference voltage Vth can be fixed at a fixed position of the eye opening (especially near the center of the eye opening by optimally setting the control target voltage ΔV), and the code error rate of the data signal is increased. It can be prevented.

The present invention is not limited to the above embodiment. For example, in the above embodiment, the identification reference voltage Vth
The amplitude of the output signal of the clamp circuit 4 is
Although 1 is set near the center between the light emitting side level and the non-light emitting side level when the amplitude is optimum for performing voltage discrimination, it may be set so as to be biased to the light emitting side or the non-light emitting side. In addition, various modifications can be made without departing from the scope of the present invention.

[0032]

According to the present invention, photoelectric conversion means such as a photodiode for converting an optical signal into an electric signal,
A variable gain amplifying means, such as a variable gain amplifier, which amplifies the electric signal obtained by the photoelectric conversion means with a gain indicated by the control signal, and an electric signal after being amplified by the variable gain amplifying means, such as an identification reference voltage. The predetermined first of
Discrimination and reproduction of the digital signal by discriminating with a threshold value, for example, discrimination and reproduction means including a comparator and a D-flip flop, and an electric signal after being amplified by the variable gain amplifying means, by a predetermined amount from the first threshold value. A monitoring signal is generated by discriminating with a predetermined second threshold value that is larger (for example, a control target voltage), for example, a monitoring signal generating means including a comparator and a D-flip-flop, and a digital signal reproduced by the discrimination reproducing means. An error rate detecting means for detecting an error rate of the monitoring signal with respect to the signal, for example, an error detecting circuit and an integrating circuit, and an error rate detected by the error rate detecting means is a predetermined reference such as an error rate setting voltage. Generating the control signal to control the gain of the variable gain amplifying means to approach a value, eg By providing the gain control means including the error amplifier and the error rate setting voltage source, even if the S / N of the received optical signal is deteriorated, the increase of the error rate is suppressed and the discrimination and reproduction of the digital signal are performed well. It becomes an optical receiver capable of

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an optical receiving device according to an embodiment of the present invention.

2 is an error detection circuit 16 and an integration circuit 17 in FIG.
FIG.

3 is a diagram showing an example of an output signal waveform of a clamp circuit 4 when the S / N of an optical signal received by a photodiode 1 in FIG. 1 is good.

4 is a diagram showing an example of an output signal waveform of a clamp circuit 4 when S / N of an optical signal received by a photodiode 1 in FIG. 1 is deteriorated.

FIG. 5 is a block diagram showing a configuration example of a conventional optical receiver.

6 is a diagram showing an example of an output signal when the S / N of the received signal of the clamp circuit 4 in FIG. 5 is good.

FIG. 7 is a diagram showing a discrimination margin (eye opening) at the input of the discrimination / reproduction circuit 6 when optical transmission is carried out in an optical fiber having a large dispersion.

8 is a diagram showing an example of an output signal when the S / N of the received signal of the clamp circuit 4 in FIG. 5 is deteriorated.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Photodiode 2 ... Preamplifier 3 ... Variable gain amplifier 4 ... Clamp circuit 5 ... Timing extraction circuit 7 ... Identification reference voltage source 9 ... Error amplifier 11 ... Comparator 12 ... Comparator 13 ... Control target voltage source 14 ... D-flip floppy (D-FF) 15 ... D-flip-flop (D-FF) 16 ... Error detection circuit 17 ... Integration circuit 18 ... Error rate setting voltage source

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location H04B 10/28 10/26 10/14 10/04 10/06 H04L 25/03 D 9199-5K H04B 9/00 Y

Claims (1)

[Claims] 1. An optical receiving device for receiving an optical signal, which is a digital signal indicated by ON / OFF of light, and reproducing the digital signal, and a photoelectric conversion means for converting the optical signal into an electric signal, and the photoelectric conversion means. Variable gain amplifying means for amplifying the electric signal obtained by the converting means with a gain indicated by the control signal, and the digital signal by discriminating the electric signal after being amplified by the variable gain amplifying means with a predetermined first threshold value. Identifying and reproducing means for identifying and reproducing, and monitoring for generating a monitoring signal by identifying the electric signal after being amplified by the variable gain amplifying means with a predetermined second threshold value which is larger than the first threshold value by a predetermined amount. Signal generation means, error rate detection means for detecting the error rate of the monitoring signal with respect to the digital signal reproduced by the identification reproduction means, and the error rate detection means. An optical receiver comprising: a gain control means for generating the control signal to control the gain of the variable gain amplification means so that the error rate detected by the output means approaches a predetermined reference value.