JPH05191354A - Optical signal detection circuit - Google Patents

Abstract

PURPOSE:To obtain the optical input detection circuit with high stability in which malfunction of the optical signal detection circuit is eliminated while no optical signal is in existence. CONSTITUTION:The circuit is provided with a peak level detection circuit detecting an output amplitude of an amplifier circuit, a comparator 10 comparing an output signal level of the peak level detection circuit 4 with a reference level, a retriggerable monostable multivibrator circuit 12 driven by the output signal of the comparator and outputting a prescribed width of pulse, and a logic circuit 13 ANDing the output of the comparator with an output of the retriggerable monostable multivibrator circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal detecting circuit in an optical receiver for notifying the presence / absence of an optical signal input.

[0002]

2. Description of the Related Art FIG. 3 shows a circuit configuration of a conventional optical receiver. The optical receiver includes a light receiving element 1, a preamplifier 2, an AGC amplifier circuit 3, an amplitude detection circuit 4, an error amplifier circuit 5, a full-wave rectifier circuit 6, a SAW filter 7, a limit amplifier circuit 8, and an identification circuit 9. And an optical signal detection circuit using the comparator 10.

An optical signal transmitted through an optical fiber is
It is converted into an electric signal by the light receiving element 1 and the preamplifier 2, and is amplified to a constant amplitude by the AGC amplifier circuit 3. This signal is input to the full-wave rectification circuit 6, and the clock signal is reproduced through the SAW filter 7 and the limit amplification circuit 8. On the other hand, the identification and reproduction circuit identifies the amplified signal and reproduces the data signal.

The output of the preamplifier changes as the optical input level changes. In order to make the output of the AGC amplifier circuit 3, that is, the signal amplitude input to the identification circuit 9 and the full-wave rectifier circuit 6 constant, this amplitude is detected as a peak value, and a gain control signal generated by error amplification is used. , AGC amplifier circuit 3 gain is controlled.

In the optical receiver having such a structure, an optical signal detection circuit for comparing the output of the amplitude detection circuit as shown in FIG. 3 with a reference level (voltage) to determine the presence or absence of an optical signal is often used. ing.

However, in such a conventional optical receiver, if the S / N of the optical receiver is poor when no optical signal is input, the AG
Large noise is generated in the output of the C amplifier circuit. This noise is mainly the thermal noise of the preamplifier and the shot noise of the light receiving element amplified by the AGC amplifier circuit.

In such a state, the setting of the reference voltage of the comparator for discriminating the presence or absence of the optical signal has a delicate influence on the discrimination, so that it becomes difficult to set the reference voltage which enables discrimination without error. come.

When no signal is input, the normal AG
The gain of the C amplifier circuit is maximized. That is, the optical receiver is in the most sensitive state. Therefore, it is very easy to react to noise caused by sneak from the power source and other circuits.

In this case, the noise level at the output of the AGC amplifier circuit may fluctuate, the amplitude detection circuit may malfunction, and a correct optical signal detection output may not be obtained.

[0010]

SUMMARY OF THE INVENTION The object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a highly stable optical input detection circuit which eliminates the malfunction of the optical signal detection circuit in the absence of an optical signal. To do.

[0011]

DISCLOSURE OF THE INVENTION The gist of the present invention is to provide a peak value detection circuit for detecting the output amplitude of an amplifier circuit in an optical receiver, and a comparison for comparing an output signal level of the peak value detection circuit with a reference level. And a retriggerable monostable multivibrator circuit driven by an output signal of the comparator to output a pulse having a constant width, and a logical product of the output of the comparator and the retriggerable monostable multivibrator circuit output. And a logic circuit for taking an optical signal.

According to the above configuration, the light signal is converted into an electric signal by the light receiving element, and the AGC amplifier circuit oscillates it to a constant amplitude, the peak value of the amplified signal is detected, and the peak value is set as the reference level. By comparing and transmitting the comparison output, and by taking the logical product of the comparison output and the output of the retriggerable monostable multivibrator circuit in the logic circuit,
The presence or absence of an optical signal can be detected.

[0013]

1 is a block diagram of an optical receiver according to an embodiment of the present invention.

In FIG. 1, the optical receiver includes a light receiving element 1, a preamplifier 2, an AGC amplification circuit 3, an amplitude detection circuit 4, an error amplification circuit 5, a full wave rectification circuit 6, a SAW filter 7, a limit amplification circuit 8 and a discrimination circuit. The optical receiving circuit 9 includes a comparator for detecting an optical signal, a retriggerable monostable multivibrator circuit, and an AND gate circuit.

The retriggerable monostable multivibrator circuit outputs a pulse having a set width when the output of the comparator is input. The AND gate circuit takes a logical product (AND) of the output signal of the comparator and the output signal of the retriggerable monostable multivibrator circuit. AN
The output of the D gate circuit becomes an optical signal detection signal.

The operation of the circuit shown in FIG. 1 will be described with reference to the output waveforms of the respective parts of the circuit shown in FIG.

FIG. 2A is a graph showing the output waveform of the amplitude detection circuit which detects the peak value of the output amplitude of the AGC amplifier circuit 3.

(2) of FIG. 2 is a graph showing the output waveform of the comparator showing the result of inputting the output of (1) and comparing with the reference voltage V _ref .

(3) of FIG. 2 is a graph showing an output waveform when the comparator output of (2) is input to the retriggerable monostable multivibrator circuit.

FIG. 2 (4) is a graph showing the output waveform of the AND gate circuit showing the result after the logical product of the comparator of (2) and the output of the retriggerable monostable multivibrator circuit of (3). ..

In both graphs, the time t = t _f
3 is a graph showing the output waveform when the optical signal is input before and from t = t _f when the optical input is turned off.

The output of the amplitude detection circuit is V _o . When the optical signal is input, V _o is kept at a signal level equal to the level V _on . At this time, input to V _o (= V _on) a comparator for comparing the reference voltage V _ref. V _ref is set to a value smaller than the output V _on of the amplitude detection circuit when the optical signal is input and larger than the average level V _{off of the} output of the amplitude detection circuit when the optical signal is interrupted. Then comparator compares the V _o and V _ref, the time the optical signal input and outputs the H level. As a result, the re-triggerable monostable multivibrator circuit also outputs H level, and the logical product (AND) of them also becomes H level. Therefore, when an optical signal is input, the output of the optical signal detection circuit becomes H level.

[0023] When a light input break then, the amplitude detector output V _o is equal to V _off, lower than V _{re f.} However, if the S / N of the optical receiver is poor, a large amount of noise will be generated in the output of the AGC amplifier circuit 3. Moreover, the light receiving circuits such as the light receiving element 1, the preamplifier 2, and the AGC amplifier circuit 3 react to the noise caused by the sneak from the power source and other circuits.
Random impulse noise is generated at the output of the GC amplifier circuit 3.

In particular, the optical input level changes rapidly as shown in FIG.
Immediately after t = t _f of, the tendency becomes strong and the amplitude detection output may not be stable.

In that case, the noise level at the output of the AGC amplifier circuit 3 may fluctuate, and the amplitude detection circuit output V _o may respond as shown in (1) of FIG. The comparator output after t = t _f should normally maintain the L level. However, when this signal is input to the comparator, the H level is output while the amplitude detection circuit output V _o exceeds V _ref as shown in (2) of FIG.

The output of this comparator is sent to a retriggerable monostable multivibrator circuit. This circuit is triggered every time the input signal, that is, the output signal of the comparator, falls from the H level to the L level, and outputs an L level pulse. The retriggerable output pulse width (T) of the monostable multivibrator circuit is set to a value that is sufficiently larger than the time interval in which the noise appearing at the output of the amplitude detection circuit exceeds the reference voltage V _ref . Then, the output of this circuit can be maintained at the L level without being affected by noise as shown in (3) of FIG.

Therefore, by ANDing these two outputs using an AND gate circuit, the output of the monostable multivibrator circuit that can be re-triggered even if the output of the comparator changes to H level due to noise is L level. To be kept at AN
The output of the D gate circuit, that is, the output of the optical signal detection circuit is set to L
Can be held on a level.

On the other hand, when the output V _o of the amplitude detection circuit becomes smaller than the reference voltage V _ref and becomes longer than the output pulse width (T) set in the retriggerable monostable multivibrator circuit, this multivibrator circuit is generated. Output changes from L level to H level (t = in FIG. 2).
t _w after). At this time, the output of the comparator holds the L level because the output V _o of the amplitude detection circuit is kept below V _ref . Therefore, by ANDing these two outputs using an AND gate circuit, even if the output of the retriggerable monostable multivibrator circuit changes to H level without being retriggered, the output of the comparator is kept at L level. Since it is leaned, the output of the AND gate circuit, that is, the output of the optical signal detection circuit can be held at the L level.

As described above, the outputs of the comparator and the re-triggerable monostable multivibrator circuit are logically ANDed by using the AND gate circuit to output the comparator or the re-triggerable monostable multivibrator circuit. Even if the output changes depending on the presence or absence of noise, the output of the AND gate circuit, that is, the output of the optical signal detection circuit can always be held at the L level.

[0030]

According to the present invention, the output amplitude of the received signal is compared with the reference voltage to prevent the malfunction of the optical signal detection for determining the presence or absence of the optical signal. Is realized by taking the logical product with the signal passed through the retriggerable monostable multivibrator circuit.

According to this method, the presence or absence of an optical signal can be stably notified without being affected by noise when there is no optical signal input. In addition, even when the optical signal input changes abruptly, the presence or absence of the optical signal can be detected stably and the response speed can be fast.

All of these circuits can be integrated into a monolithic IC, and can be integrated on the same chip as an AGC amplifier circuit or the like. As a result, downsizing and high performance of the circuit can be easily achieved.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical receiver having an optical signal detection circuit of the present invention.

FIG. 2 is an input / output waveform of each part of the optical signal detection circuit of the present invention.

FIG. 3 is a block diagram of an optical receiver having a conventional optical signal detection circuit.

[Explanation of reference numerals] 1 light receiving element 2 preamplifier 3 AGC amplifier circuit 4 amplitude detection circuit 5 error amplifier circuit 6 full-wave rectifier circuit 7 SAW filter 8 limit amplifier circuit 9 identification circuit 10 comparator 12 retriggerable monostable multivibrator circuit 13 AND gate circuit

Claims (1)

[Claims] 1. In an optical receiver, a peak value detecting circuit for detecting an output amplitude of an amplifier circuit, a comparator for comparing an output signal level of the peak value detecting circuit with a reference level, and a drive by an output signal of the comparator. And a retriggerable monostable multivibrator circuit that outputs a pulse of a constant width, and a logic circuit that ANDs the output of the comparator and the retriggerable monostable multivibrator circuit output. Characteristic optical signal detection circuit.