JPS637041A - Agc switching circuit - Google Patents

Abstract

PURPOSE:To attain the setting of the switching level of a full AGC, and an electrical AGC simultaneously, by providing both the full AGC which controls a photodetector, and the electrical AGC which follows the photodetector, and controls a gain variable amplifier, comparing the amplitudes of the outputs of two kinds of AGC voltages with a reference value, and obtaining different AGC voltages each other corresponding to the size of the values. CONSTITUTION:While the reference value Vth of a switching level setting terminal 22 is set at a certain value, and the detecting output Vi of a peak value amplitude detection circuit 5 is smaller than the Vth, a diode D goes to an OFF state, and a constant voltage is outputted from the output terminal 23 of an electrical AGC control voltage. And from the output terminal 24 of a full AGC control voltage, a full AGC control voltage Mc which changes corresponding to the detecting output Vi, is outputted. When the detecting output Vi becomes larger than the Vth, the diode D goes to an ON state, and the constant voltage is outputted from the output terminal 24, and from the output terminal 23, the electrical AGC control voltage Mc which changes corresponding to the detecting output Vi, is outputted. Thus, it is possible to set the switching levels of the full AGC, and the electrical AGC simultaneously corresponding to the reference value Vth of the setting terminal 22 of the switching level.

Description

[Detailed description of the invention] 2 Vane Industrial applications The present invention relates to an AGC switching circuit for optical communication receivers and the like.

BACKGROUND OF THE INVENTION FIG. 2 shows a block configuration of an optical-to-electrical converter generally used in optical communication receivers. In the figure, 1 is an optical transmission line, 2 is an avalanche photodiode (APD), 3 is a preamplifier, 4 is a variable gain amplifier, 5 is a peak amplitude detection circuit, 6 is an automatic gain control voltage switching circuit, 7 is a bias circuit.

With the above configuration, the optical signal from the optical transmission line 1 is received by the APD 2, the output of the APD 2 is taken out as the output 8 via the preamplifier 3 and the variable gain amplifier 4, and the peak value amplitude circuit 5 The output is monitored, and the detected output Vi is input to the automatic gain control voltage switching circuit 6, and the electric AGC control voltage (
Ec) and full AGC control voltage (Mc) are obtained and double feedback is applied to control the gain <a+ of the variable gain amplifier 4 and the multiplication factor (M) of the APD 2 by the bias circuit 7, respectively. It is configured to obtain a constant 3-page output 8 with high accuracy over a wide range of level fluctuations.

FIG. 4 shows a specific configuration of a conventional automatic gain control voltage switching circuit 6, in which the detection output Vi is input to an input buffer 11,
The switching level setting circuit 12 and the control voltage output circuit 3 are configured to output two types of A()C control voltages Ec and Mc. The switching level setting circuit 12 is provided with two comparators 14.15, each with two reference values Vref 1 .

It is designed to be compared with Vref2. The control voltage output circuit 13 has two amplifiers 16 and 1.7, which are connected to comparators 14 and 15, respectively.

Next, the operation of the switching level setting circuit 12 will be explained with reference to FIG. While the detection output Vi corresponding to the output 8 is small, the gain G of the variable gain amplifier 4 due to electricity A()C is constant, and decreases as the detection output Vi increases beyond the switching point P1. In the negative full AGC, the multiplication factor M decreases as the detection output Vi increases within a small range of the detection output Vi, and remains constant after the switching point P2 is exceeded. Therefore, electric AC + C (!: Full AG
By combining C, it is possible to keep the output 8 constant over a wide range of changes in the sensing output Vi, and thus in the input optical signal level. The positions of the switching points P] and P2 can be changed independently by setting the reference voltages Vref 1 and Vref 2 to appropriate values, respectively.

Problems to be Solved by the Invention However, in the conventional AGC switching circuit described above, the full AG
C and electric AGC switching level P1. There is a problem in that the setting of P2 is independent and the switching levels of both cannot be set at the same time.

The present invention solves these conventional problems,
It is an object of the present invention to provide an excellent AGC switching circuit that has a simple circuit configuration and can simultaneously set switching levels for full AGC and electric AGC.

Means for Solving the Problems In order to achieve the above object, the present invention provides both a full A<l)C for controlling an optical receiving element and an electric AGC for controlling a variable gain amplifier following the optical receiving element. The system is configured to provide an automatic gain control voltage switching circuit that compares the output amplitudes and reference values of the two types of AGC voltages, and obtains different AGC voltages depending on the magnitude relationship.

Operation Therefore, according to the present invention, it is possible to easily switch the characteristics of both the full AGC voltage and the electric AGC voltage according to the output based on one reference value.

Embodiment FIG. 1 shows the main structure of an embodiment of the present invention. In Fig. 1, 218 to 21d are two-man powered amplifiers;
R1 to Ro are resistors, D is a diode, and 22 is a switching level setting terminal for inputting a reference value (vth). The amplifier 21a serves as the input buffer 11, the amplifier 21b, the resistor R1, and the diode D serve as the switching level setting circuit 12a; the resistors R2 to R6 and the amplifier 21C serve as the output circuit for electric AGC; and the resistors 7 to R11
The amplifier 21d constitutes a full A (IC output circuit). 23 and 24 are electric AGC
and a full AGC control voltage output terminal.

6 Beige Next, the operation of the above embodiment will be explained with reference to FIG.

The reference value vth of the switching level setting terminal (TH) 22 is set to a certain value, and the detection output Vi of the peak amplitude detection circuit 5 is set to v
While it is smaller than th, the diode D is in the OFF state, and a constant voltage is output from the electric AGC control voltage output terminal 23, and the full A, GC control voltage output terminal 24 changes according to the detection output ■1. Full AGC control voltage Mc is output. When the optical input fluctuates and the detection output Vi becomes larger than vth, the diode D is turned on and full AGC is performed.
A constant voltage is output from the control voltage output terminal 24, and an electric AGC control voltage Me that changes according to the detection output Vi is output from the electric AGC control voltage output terminal 23. In this way, according to the above embodiment, the switching level setting terminal 2
Full according to the reference value vth of 2: AGC,! :Electric AG
This has the advantage that the switching levels of C can be set at the same time.

Effects of the Invention As is clear from the above embodiments, the present invention is configured so that the characteristics of the electric AGC can be changed by using one reference value.
This has the advantage that the mismatch in the switching levels of the characteristics of C and full AGC is eliminated.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the main parts of an AGC switching device according to an embodiment of the present invention, FIG. 2 is a block diagram showing the overall configuration of the device, and FIG. 3 is a characteristic diagram for explaining the operation of the device.
FIG. 4 is a block diagram of a conventional AGC switching device, and FIG. 5 is a characteristic diagram of the device. DESCRIPTION OF SYMBOLS 11... Input buffer, 12a... Switching level setting circuit, 13... Control voltage output circuit, 21a-21d.
...Amplifier, R1...Resistor, D...Diode, V
i...Detection output, Vth...Reference value. Name of agent: Patent attorney Toshio Nakao and one other person

Claims (1)

[Claims] The output of the optical receiving element is taken out as an output via a variable gain amplifier, and the amplitude of the output is taken out by a peak amplitude detection circuit and inputted to an automatic gain control voltage switching circuit to adjust the bias current of the optical receiving element. In obtaining a full AGC voltage for changing the gain of the variable gain amplifier and an electric AGC voltage for changing the gain of the variable gain amplifier, the automatic gain control voltage switching circuit compares the amplitude with a reference value using a switching level setting circuit. , when the output is below a certain value, the full AGC voltage is made variable according to the output, while the electric AGC voltage is kept constant, and when the output is above a certain value, the full AGC voltage is made variable. An AGC switching circuit that controls an output by switching the electrical AGC voltage to be constant or variable in accordance with the output.