JPS6378626A - Light modulation circuit - Google Patents

Abstract

PURPOSE:To keep the modulation amplitude of a low frequency analog signal at a constant level, and to stabilize a modulation factor, by keeping the amplitude of the low frequency analog signal at the constant level, in a light modulation circuit in which the low frequency analog signal is superimposed on a digital main signal. CONSTITUTION:In the light modulation circuit having a light emitting element driving circuit 1 that outputs a light signal on which the low frequency analog signal is superimposed for the amplitude of a light digital main signal to be inputted to a terminal 10, an LPF4 extracts the low frequency analog signal from the output of a light receiving element 3. The peak value of the signal is detected at a peak value detection circuit 6. After the detected voltage is compared with a reference voltage in a gain control signal generation circuit 7, the circuit 7 outputs a control signal to an amplifier 8 that is a gain variable amplifier, and applies a DC negative feedback on the circuit 1 so as to compress a difference between the above stated reference voltage and a peak value detecting voltage, and thereby, gain control is performed. Therefore, a new low frequency analog signal inputted from a terminal 11, after the amplitude being controlled, is superimposed on the main signal at the circuit 1. In this way, the modulation amplitude of the analog signal can be kept at the constant level.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an optical modulation circuit, and more particularly to an optical modulation circuit that superimposes a low frequency analog signal on the pulse amplitude of a variable digital main signal pulse train. .

[Conventional technology]

1 of the digital main signal pulse train? For the pulse amplitude,
In an optical modulation circuit that superimposes a low-frequency analog signal, conventionally, the degree of modulation of the low-frequency analog signal to be superimposed is arbitrarily initially set, and the degree of modulation is fixed.

[Problem that the invention seeks to solve]

Therefore, when the amplitude of the low frequency analog signal to be superimposed is large, the degree of modulation is large, and when the amplitude is small, the degree of modulation is small.

If the modulation degree of the analog signal changes, the transmission characteristics of the digital main signal will change (code error rate, etc.), resulting in instability.

Therefore, in view of the above drawbacks, the present invention provides an optical modulation circuit that can keep the modulation amplitude of the low frequency analog signal constant and stabilize the degree of modulation by keeping the amplitude of the low frequency analog signal constant. be.

[Means for solving problems]

According to the present invention, in an optical modulation circuit having a light emitting element driving circuit that outputs an optical signal in which a low frequency analog signal is superimposed on the pulse amplitude of an optical digital main signal pulse train,
From the output of a light receiving element that monitors the optical signal output from the light emitting element.

A variable gain amplifier that extracts a low frequency analog signal and controls the gain based on the low frequency analog signal, controls the amplitude of a new low frequency analog signal from the outside, and controls the amplitude of the new low frequency analog signal from the outside. By inputting a signal to the light emitting element drive circuit, an optical modulation circuit is obtained which is characterized in that the modulation amplitude with respect to the amplitude of the optical distal main signal is kept constant and the degree of modulation of the low frequency analog signal is stabilized. .

〔Example〕 Next, the present invention will be explained with reference to the drawings.

As shown in FIGS. 1 and 2, a light emitting element driving circuit 1 is a circuit that drives a light emitting element 2 with a digital signal obtained by superimposing a low frequency analog signal b'1 on the pulse amplitude of a digital main signal pulse train a. It is. The output of the photodiode 3, which is a light receiving element that monitors the light output of the light emitting element 2, is as follows.

The light is input to the light emitting element drive circuit 1 via the light output control circuit 9, and the light output is controlled. Further, the output of the photodiode 3 is input to a low-pass filter 4, and a low-frequency analog signal is extracted from the low-pass filter 4. The extracted low frequency analog signal is amplified by the first amplifier 5, and the peak value detection circuit 6 outputs a peak value detection voltage corresponding to the amplitude peak value of the low frequency analog signal.

The detected peak value detection voltage is input to the gain control signal generation circuit 7, and after being compared with the reference voltage of this gain control signal generation circuit 7, the gain control signal generation circuit 7
A control signal is output to the second amplifier 8, which is a variable gain amplifier having a variable gain function to which a low frequency analog signal is input.

By applying direct current negative feedback configured to compress the difference between the reference voltage and the peak value detection voltage.

Performs gain control. Therefore, a new low frequency analog signal from the low frequency analog signal input terminal 11 that is input to the second amplifier 8 is input to the second amplifier 8 .

After the amplitude is controlled, it is input to the 1 light emitting element drive circuit 1,
It is superimposed on the digital main signal input from the digital main signal input terminal 10.

〔Effect of the invention〕

As explained above, the present invention converts the output of a light receiving element that monitors the optical output of nine light emitting elements into a low frequency analog signal through a low pass filter, a first amplifier, a peak value detection circuit, and a gain control signal generation circuit. The gain of the input second amplifier is controlled to keep constant the amplitude of the new low frequency analog signal input to the light emitting element drive circuit via the second amplifier.

This has the effect of keeping the modulation amplitude of the low frequency analog signal constant with respect to the amplitude of the optical digital main signal and stabilizing the degree of modulation of the low frequency analog signal.

fair skinned

[Brief explanation of the drawing]

Figure 1 shows an optical modulation circuit according to the present invention! Rock diagram. FIGS. 2(a) and 2(b) are waveform diagrams showing input signals of the light emitting element driving circuit of the present invention, and FIG. 2(c) is a waveform diagram showing output signals of the light emitting element driving circuit of the present invention. 1... Light emitting element drive circuit, 2... Light emitting element, 3...
- Light receiving element, 4...Low pass filter, 5...Amplifier, 6...Peak value detection circuit, 7...Gain control signal generation circuit. 8... Amplifier, 9... Optical output control circuit, 10...
Digital main signal input terminal, 11... Low frequency analog signal input terminal, a... Digital main signal, b... Low frequency analog signal, C... Light emitting element drive circuit output signal.

Claims (1)

[Scope of Claims] 1. An optical modulation circuit having a light emitting element driving circuit that outputs an optical signal in which a low frequency analog signal is superimposed on the pulse amplitude of an optical digital main signal pulse train, comprising: A variable gain amplifier extracts a low frequency analog signal from the output of the light receiving element that monitors the optical signal, and controls the gain based on the low frequency analog signal to control the amplitude of a new low frequency analog signal from the outside. and by inputting the low frequency analog signal whose amplitude is controlled to the light emitting element drive circuit, the modulation amplitude with respect to the amplitude of the optical digital main signal is kept constant, and the degree of modulation of the low frequency analog signal is stabilized. A light modulation circuit featuring: