JPS6136604B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an AGC circuit for an optical receiver.

Conventionally, this type of AGC circuit has a photodiode 1 that converts an optical signal into an electrical signal, as shown in Figure 1.
It consisted of amplifiers 2, 4, and 6, a control circuit 3 for controlling the level of the output signal, a level detection circuit 5, and the like. The optical signal input to the photodiode 1 is converted into an electrical signal and passes through the amplifier 2 and the control circuit 3.
An output signal is obtained from an amplifier 4.
The output signal level is controlled by detecting the output level with a level detection circuit 5, amplifying it with an amplifier 6, and then adding it to the control circuit 3. The above level detection circuit has
There are the following types of AGC methods, but each has its own drawbacks.

(b) Detecting the pilot signal level This method requires a pilot signal generation circuit on the transmitting side and a pilot signal removal circuit on the receiving side, resulting in a large circuit size.

(b) A device that detects the signal peak level (c) A device that detects the signal average level (b) (c) may not be usable depending on the type of signal, especially for analog signals such as TV signals. It was unusable.

An object of the present invention is to provide an AGC circuit that solves the above-mentioned drawbacks by operating a control circuit so that the DC level of a received electrical signal is constant.

AGC according to the present invention to achieve the above object
The circuit is a light-to-voltage conversion circuit that converts the received optical signal into a voltage signal in the AGC circuit of the optical receiver, which is installed opposite the optical transmitter that outputs an optical signal whose average optical power is always kept constant. a control circuit that attenuates and outputs the output of the conversion circuit according to the level of a control signal; and a control circuit that detects an output DC voltage of the control circuit and adjusts the control signal so that the output DC voltage becomes equal to a reference DC voltage. The control voltage generating circuit is configured to include a control voltage generating circuit that generates .

According to the above configuration, the circuit configuration is simple, and it can also be used for analog signals, so that the object of the present invention is completely achieved.

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIG. 2 is a circuit diagram showing a first embodiment of the AGC circuit according to the present invention. In the figure, 7 is a wideband direct-coupled amplifier, 8 is a control circuit, 9 is a control voltage generation circuit, Es
is a reference voltage, and the other parts are the same as those explained in connection with FIG. The signal received by the photodiode 1 is amplified by a broadband direct-coupled amplifier 7 and input to a control circuit 8 . The control circuit attenuates this input signal by an amount corresponding to the control signal voltage and outputs it, and further amplifies this signal using the amplifier 4 to obtain an output signal. At this time, the output DC voltage of the control circuit is compared with the reference voltage Es by the control voltage generation circuit 9, and the error between the two voltages is amplified and supplied to the control circuit as a control signal. Because of this configuration, the DC level of the output of the control circuit 8 is always kept constant even if the optical input level changes and the output DC level of the broadband direct-coupled amplifier 7 changes.

By the way, in the case of an analog signal transmission system in which AC signal light is superimposed on DC light, the ratio of the DC light level to the AC signal light level is constant as long as the amplitude (or degree of modulation) of the AC signal is constant.

Therefore, controlling the DC signal level to a constant value also means controlling the AC signal level to a constant value.

3A, 3B, and 3C show examples of the configuration of the control circuit and control voltage generation circuit used in the embodiment of FIG. 2. Both are a combination of an attenuation circuit constructed using a resistor 10 and a voltage-controlled variable resistance element 11 (using an FET), and DC voltage amplification circuits 12 and 13. The DC voltage amplifier circuit 13 used in C is capable of obtaining bipolar DC voltages and controls two voltage-controlled variable resistance elements.

FIG. 4 is a circuit diagram showing a second embodiment of the present invention. This is the broadband direct-coupled amplifier 7 of the first embodiment.
is replaced with a circuit in which the input signal is amplified by a DC amplifier 14 and an AC amplifier 15, respectively, and then coupled through resistors 16 and 17. The other circuitry remains the same as described in connection with FIG.
The feature of this circuit is that when the signal component is at high frequency and it is not appropriate to use a wideband direct-coupled amplifier, the circuit configuration can be simplified by separately providing a narrowband high-frequency amplifier for the signal and a DC amplifier for the DC. It is. Furthermore, since the ratio of the DC component to the AC component can be freely selected, the bias point of the control circuit can be freely set as described later, and it is possible to operate the control circuit at the point with the best distortion, such as when handling analog signals. Become. This will be explained in detail using FIG. 5. FIG. 5A shows the input/output relationship of the control circuit in the first embodiment, and FIG. 5B shows the input/output relationship of the control circuit in the second embodiment. In B, if the amplitude of the AC signal and the DC component are equal,
In other words, this shows a case where the modulation degree is 100%, and the input/output relationship of the control circuit deviates from a linear relationship as the input voltage increases in the positive and negative directions, as shown in the figure. Therefore, when handling large signals, distortion may occur in the output signal as shown in the figure. On the other hand, in the second embodiment, the levels of alternating current and direct current can be changed. For example, if only the DC level is reduced to 1/4, it will be possible to use it near the center of the operating curve as shown in (B), and the distortion will be reduced accordingly.

As explained above, since the AGC circuit according to the present invention detects the DC light level and applies AGC, a pilot signal or the like is not required and the circuit becomes simple. Furthermore, AGC operation is performed normally even when there is no signal. Furthermore, by using a voltage-controlled variable resistance element in the control circuit for AGC, it becomes possible to use an AGC method that maintains a constant DC level with a simple circuit. Furthermore, by separating the DC and AC components and then combining them, the circuit becomes simpler and an operating point with less distortion can be selected. Therefore, the circuit of the present invention can be suitably used in communication systems in which the optical DC output is kept constant, particularly in analog signal communication systems.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a configuration example of a conventional AGC circuit, Fig. 2 is a circuit diagram showing a first embodiment of an AGC circuit according to the present invention, and Fig. 3 is an embodiment of a control circuit and a control voltage generation circuit. FIG. 4 is a circuit diagram showing the second embodiment of the present invention, and FIG. 5 is an operating characteristic diagram showing the input/output relationship of the control circuit. 1... Photodiode, 2, 4, 6... Amplifier, 3, 8... Control circuit, 5... Level detection circuit, 7... Broadband direct-coupled amplifier, 9... Control voltage generation circuit, 10, 16, 17... ...Resistor, 11...Voltage control variable resistance element, 12, 13...DC voltage amplification circuit, 14...DC amplifier, 15...AC amplifier.

Claims (1)

[Claims] 1. A received optical signal is converted into a voltage signal in an AGC circuit of an optical receiver installed opposite to an optical transmitter that outputs an optical signal whose optical average power is always kept constant. A light-voltage conversion circuit, a control circuit that attenuates the output of the conversion circuit according to the level of a control signal, detects an output DC voltage of the control circuit, and adjusts the output DC voltage to be equal to a reference DC voltage. and a control voltage generation circuit that generates the control signal. 2. The AGC circuit according to item 1, wherein the control circuit includes an element connected between an input and an output and an element connected between an output and ground, and a voltage-controlled variable resistance element is used for one or both of the elements. 3. The light-to-voltage conversion circuit includes means for converting a received optical signal into an electrical signal, and is connected to the means and separates the electrical signal into a DC signal and an AC signal, and separately amplifies or attenuates the DC signal and AC signal. 2. The AGC circuit according to item 1, comprising a circuit for recombining after combining.