JPS5925533B2 - Optical communication receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical communication receiver having an AGC circuit configured not to cause oscillation when the level of input light decreases.

2. Description of the Related Art Conventionally, an optical communication receiver using an avalanche photodiode (hereinafter abbreviated as APD in this specification) as an optical receiving element and provided with a feedback circuit has been known, and has the configuration shown in FIG.

The received input optical signal is irradiated to the APD and its output is sent to the preamplifier PRA, variable gain amplifier GCA, and equalizer EQL.
, when the received signal is passed through the amplifier AMP, the no-turn of the received signal becomes an eye-turn in the case of pulse communication. By slicing this eye pattern at an appropriate threshold level, the presence or absence of a signal is judged by the judgment circuit DEC, but this is because the amplitude between the peak values of the waveform changes over time depending on the received input. To prevent this, a loop circuit is provided and the loop gain is made variable to keep the output amplitude constant. That is, the amplitude value taken out by the peak value amplitude detection circuit P-PDET is compared and amplified with the comparison voltage Vr in the operational amplifier OP, and is applied to the DC-DC converter. The detection circuit P changes the gain of the APD according to the converter output.
- The amplitude input to PDET, that is, the amplitude of the eye pattern, is made constant. Gain adjustment using this feedback is called full AGC. Is the amplitude greater than the current comparison voltage Vr? Detection circuit P-PD
When the ET is manually operated, the output of the DC-DC converter is also controlled to be large and the gain of the APD is small. However, although this configuration can cope with a change of about +10 dB in the received light amplitude as an APD (■I) characteristic, it cannot follow a change exceeding +10 dB. Therefore, operational amplifier OP
The output of this stage is also applied to the variable gain amplifier GCA to make the gain of this stage variable. This gain adjustment is called electric AGC.The aforementioned full AGC and electric AGC are not actually operated at the same time, but are switched to operate electric AGC when the gain adjustment by full AGC is insufficient. ing. When the amount of light received decreases in a region where only full AGC is working, the loop gain of the full AGC circuit must be changed to a greater extent, and in this case, the feedback loop circuit tends to oscillate. Especially when the input light is interrupted, oscillation is likely to occur. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical communication receiver having a full AGC circuit which overcomes the above-mentioned drawbacks and is configured to prevent oscillation when the level of input light decreases.

Therefore, the present invention particularly employs a configuration in which the operational amplifier gain within the full AGC circuit is adjusted. Embodiments of the present invention shown in the drawings will be described below.

FIG. 2 shows only the portion corresponding to the operational amplifier OP in FIG. 1. The operational amplifier 0P in FIG. 2 operates the full AGC loop when the output EO is a positive value and operates the electric AGC loop when the output EO is a negative value, depending on the selection of the comparison voltage Vr and the configuration of other peripheral circuits. The amplification factor G, which is the ratio between the input voltage Er and the output voltage EO of the operational amplifier 0P, is given by:

Here, the capacitor C and the resistor R3 are connected in parallel, and are connected in series with the diode D to provide a feedback connection between the input terminals of the operational amplifier 0P. Therefore, during full AGC operation, the diode D is conductive, so the resistor R3 and capacitor C enter the feedback path and are connected in parallel with the conventional resistor R2, so when Er becomes small as in the conventional case, the amplifier gain G increases. Do not increase the value. Here, since the capacitor C is used for phase compensation, there is a margin for the phase shift transition of the oscillation operation. In the configuration shown in Figure 2, the DC outputs 0ut and A of the DC-DC converter
When examining the relationship with the gain M of the GC control system, it is known that it is not a linear relationship, but a nonlinear relationship as shown in FIG. Therefore, it is more effective to use a voltage Ibl leg type nonlinear element instead of a fixed resistance element as the resistor R3 in FIG. Figure 4 shows the voltage Er on the horizontal axis and the resistance value on the vertical axis.
Use as 3. When the value of EO suddenly increases, R3 suddenly decreases, so the gain of the operational amplifier, that is, AGClljl
The gain in the leg system is not large. The above describes the case where a common amplifier in the full AGC and electric AGC loops is used to control the amplification factor in the control system.
By providing separate amplifiers for the GC loop and the electric AGC loop and using a suitable AGC loop operation switching circuit, it is also possible to control the amplification factor of only the amplifier in the full AGC loop. In this way, according to the present invention, when the level of received input light changes greatly, such as in an optical communication receiver, the gain change of the AGC loop is prevented, so oscillation does not occur and stable operation is possible. Receiver is obtained.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the configuration of a conventional optical communication receiver, Fig. 2 is a diagram showing only the vicinity of the operational amplifier in Fig. 1 as an embodiment of the present invention, and Fig. 3 is an AGC control system. FIG. 4 is a diagram showing desirable characteristics as the resistance element R3. APD・・・Avalanche photodiode,
PRA, GCAAMP...Amplifier, EQL...
... Equalizer, DEC ... Judgment circuit,
P-PDET・・・Peak value detection circuit, DC/Dc
...DC-DC converter, D... Diode, C... Capacitor, Rl, R2, R3.
...Resistance, 0P...Operation amplifier.

Claims (1)

[Claims] 1. A full AGC circuit that changes the bias to the optical receiving element according to the output from the optical receiving element consisting of an avalanche photodiode, and an electric AGC circuit that changes the gain of an amplifier that amplifies the output from the optical receiving element. and,
In the optical communication receiver that performs a switching operation according to the output level from the light receiving element, the loop gain of the feedback loop constituting the full AGC circuit is nonlinearly deformed to suppress its steep fluctuation. Optical communication receiver.