JPH02237232A - Optical receiving circuit - Google Patents

Abstract

PURPOSE:To reduce a deviation from an optimum threshold voltage at a full AGC time by setting a voltage proportional to the bias voltage control voltage of an avalanche photo diode (APD) to be the threshold voltage of an identification circuit. CONSTITUTION:An output from APD 1 is given to the identification circuit 10 through an equalizing amplifier circuit 2 and is given to a peak value detection circuit 3. The peak value detection circuit 3 detects the peak value of a light-receiving level. The peak value signal is amplified in amplifier circuits 4 and 5 and is given to APD 1 through a DC/DC converter 6 as the bias voltage control voltage. The output of the amplifier circuit 4 is converted the gain level in a gain/level conversion circuit 9 and is given to the identification circuit 10 as the threshold voltage. The identification circuit 10 identifies the light- receiving level based on the threshold voltage. Thus, the deviation from the optimum threshold voltage at the time of AGC can be reduced, and identification in the identification circuit can be set optimum.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical receiving circuit, and more particularly to an optical receiving circuit using an avalanche photodiode (APD) as a light receiving element.

[Conventional technology]

Generally, in an optical receiving circuit using an APD, the output from the APD is given to an identification circuit via an equalization amplifier circuit, while the output of the equalization amplifier circuit is given to a peak detection circuit, which detects the output from the APD. The bias voltage of the APD is controlled based on the peak value.

The identification circuit identifies the received light level based on a predetermined threshold voltage, and this threshold voltage is provided by an equalizing amplifier or generated by the identification circuit itself.

In other words, in conventional optical receiving circuits, the threshold voltage is always a constant voltage with respect to the received light level.

[Problem that the invention seeks to solve]

As mentioned above, in the case of the conventional optical receiver circuit, the threshold voltage of the identification circuit is kept constant with respect to the light reception level.

However, in an APD, the noise distribution superimposed on the output of the APD changes as the current multiplication factor (M) increases or decreases. As a result, the optimal threshold voltage changes depending on the output of the AFD, that is, the level of received light.

Therefore, in the conventional optical receiver circuit, Fu11 (all) AGO
In this case, since the threshold voltage is always kept constant with respect to the light reception level, there is a problem that the threshold voltage deviates from the optimum threshold voltage.

[Means for solving problems]

According to the present invention, a light receiving element whose bias voltage is adjusted according to a received light level, and an identification circuit that receives an output from the light receiving element and identifies the level of the output based on a predetermined threshold voltage are provided. There is obtained an optical receiving circuit comprising: an adjusting means for adjusting the threshold voltage in accordance with a control voltage for controlling the bias voltage.

〔Example〕

The present invention will be explained below with reference to Examples.

Referring to FIG. 1, the optical receiving circuit according to the present invention is an APD.
1, equalization amplifier circuit 2, peak value detection circuit 5, amplifier circuit 4
and 5, a DC/DC converter 6, an identification circuit 9, and a gain/level conversion circuit 10.

The output from the APD 1 is given to the identification circuit 9 via the equalization amplifier circuit 2 and also given to the peak value detection circuit B. The peak value detection circuit 3 detects the peak value of the received light level and outputs it as a peak value signal. This peak value signal is amplified by amplifier circuits 4 and 5, and then Do/DC!
AP as bias voltage control voltage via converter 6
D 1 is given.

The output of the amplifier circuit 4 is applied to a gain/level conversion circuit 10, where the gain level is converted and the output is applied to the identification circuit 9 as a threshold voltage.

The identification circuit 9 identifies the received light level based on this threshold voltage.

By the way, during Full AGO, the bias voltage control voltage 7 of the APD 1 changes in accordance with the change in the light reception level of the APD 1, thereby changing the current multiplication factor (M) of the APD 1. At this time, the amount of excessive multiplication noise in the APD 1 increases or decreases in response to a change in the current multiplication factor (M) of the APD 1. This excessively multiplied noise amount is not superimposed equally on the high level and low level of the equalized waveform output of the equalizing amplifier circuit 2, but is biased toward one side. As a result, the threshold voltage from the gain/level conversion circuit 10 changes due to the change in the amount of excessive multiplication noise due to the current multiplication factor (M) of the APD. That is, by setting the threshold voltage of the identification circuit 9 to a voltage proportional to the bias voltage control voltage of the APD 1, deviation from the optimum threshold voltage can be reduced. As a result, the identification in the identification circuit can be optimized.

〔Effect of the invention〕

As explained above, in the present invention, since the voltage proportional to the bias voltage control voltage of the APD is used as the threshold voltage of the discrimination circuit, there is an effect that the deviation from the optimum threshold voltage during Full AGO can be extremely small. .

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of an optical receiving circuit according to the present invention. 1... Avalanche photodiode (APD), 2
... Equalization amplifier circuit, 3 ... Peak value detection circuit, 4,
5... Amplifier circuit, 6... Do/Do converter, 7
... APD bias voltage control voltage, 8... Threshold voltage, 9... Gain/level conversion circuit. Margin below

Claims (1)

[Claims] 1. An optical receiving circuit having a light receiving element whose bias voltage is adjusted according to the level of received light, and an identification circuit that receives the output from the light receiving element and identifies the level of the output based on a predetermined threshold voltage. An optical receiving circuit characterized in that it comprises an adjusting means for adjusting the threshold voltage according to a control voltage that controls the bias voltage.