JP2723029B2 - Automatic threshold control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic threshold control circuit, and more particularly to a DC-coupled automatic threshold control circuit for optical reception.

[0002]

2. Description of the Related Art Conventionally, various types of automatic threshold control (AT) have been applied to optical receivers in optical communication using burst signals.
C: Automatic Threshold Con
(JP-A-59-14)
No. 8458, JP-A-61-203759, JP-A-3-
No. 35639).

FIG. 3 is a circuit diagram showing an example of a DC-coupled ATC circuit of the conventional ATC circuit. The anode of the light receiving element 1 is connected to the inverting input terminal of the comparator 3 via the equalizing amplifier circuit 2. A resistor 4 is connected between the inverting input terminal and the non-inverting input terminal of the comparator 3. Further, a resistor 5 is connected between the output terminal 7 of the comparator 3 and the non-inverting input terminal. Further, a capacitor 6 is connected between the connection point between the non-inverting input terminal of the comparator 3 and the resistors 4 and 5, and the ground.

In this conventional ATC circuit, light received by the light receiving element 1 is photoelectrically converted into a light receiving signal, and the light receiving signal is input to an inverting input terminal of a comparator 3 through an equalizing amplifier circuit 2, and the non-inverting terminal of the comparator 3 The level is compared with a threshold value connected to the inverting input terminal, and a binarized light receiving signal is output to the output terminal 7.

In this conventional ATC circuit, a capacitor 6
This makes the waveform of the threshold value input to the non-inverting input terminal of the comparator 3 substantially triangular, thereby removing ripples and the like. In FIG. 4, the solid line a0 indicates the light receiving signal input to the inverting input terminal of the comparator 3 at the maximum light receiving level, and the broken line b0 indicates the waveform of the threshold value input to the non-inverting input terminal of the comparator 3.

[0006]

In the conventional ATC circuit, since the output signal amplitude of the equalizing amplifier circuit 2 at the maximum light receiving level increases, the slew rate decreases. Rise time and fall time increase. In that case, the waveform of the threshold value, which is set to be slower so as to become almost a triangular wave, has a further slower speed, so that the tracking by the burst signal is deteriorated,
There is a problem that the pulse width distortion of the output pulse from the output terminal 7 of the comparator 3 increases.

[0007] The present invention has been made in view of the above points,
An object of the present invention is to provide an automatic threshold control circuit capable of reducing pulse width distortion at the time of maximum light reception.

[0008]

According to the present invention, a first resistor is connected between first and second input terminals, and a second resistor is provided between an output terminal and a second input terminal. The variable capacitance element is connected to the second input terminal of the comparator to which the resistor is connected, and the received light signal amplified after receiving light is input to the first input terminal of the comparator, and the threshold is input to the second input terminal. The value is compared with the value.

[0009]

According to the present invention, when the threshold value input to the second input terminal of the comparator increases, the capacitance value of the variable capacitance element decreases as the level of the light receiving signal input to the first input terminal of the comparator increases. With this configuration, the rise and fall of the threshold waveform can be made steeper than in the related art at the maximum light receiving level at which the level of the light receiving signal increases.

[0010]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. 3, the same components as those of FIG. 3 are denoted by the same reference numerals.
In FIG. 1, the anode of the light receiving element 1 is an equalizing amplifier 2
Is connected to the input terminal of In the comparator 3, the output terminal of the equalizing amplifier circuit 2 is connected to an inverting input terminal which is a first input terminal, and a first resistor 4 is connected to a non-inverting input terminal which is a second input terminal. And a second resistor 5 between the output terminal 7 of the comparator 3 and the non-inverting input terminal.
Is connected.

A varactor diode 10 as an example of a variable capacitance element has a cathode connected to a common connection point between the non-inverting input terminal of the comparator 3 and the first and second resistors 4 and 5, and an anode grounded. ing. That is,
In this embodiment, instead of the conventional capacitor 6 having a fixed capacitance value,
It is characterized in that a varactor diode whose capacitance value changes according to the reverse bias voltage is connected.

Next, the operation of this embodiment will be described.
The optical signal is received by the light receiving element 1 and photoelectrically converted to become a light receiving signal. This light receiving signal is supplied to the equalizing amplifier circuit 2, where a predetermined amplification characteristic is given thereto, and then supplied to the inverting input terminal of the comparator 3. The comparator 3 receives the light receiving signal and the non-inverting input terminals,
The level is compared with the threshold voltage determined by the divided voltage. If the received light signal is higher than the threshold voltage, a low level signal is output. Output to terminal 7.

Here, at the time of the maximum light receiving level, the light receiving signal level inputted to the non-inverting input terminal of the comparator 3 becomes large. As a result, the reverse bias voltage of the varactor diode 10 increases, so that the capacitance between the anode and cathode terminals of the varactor diode 10 decreases, and the rise and fall of the threshold voltage waveform become steep. As a result, the threshold voltage waveform approaches from a triangular wave indicated by b0 in FIG. 4 to a square wave as indicated by a broken line b1 in FIG. Note that the solid line a1 in FIG. 2 shows the light receiving signal at the maximum light receiving level.

Further, at the time of the maximum light receiving level, the AC load of the light receiving signal output from the equalizing amplifier circuit 2 can be reduced by decreasing the capacitance between the anode and cathode terminals of the varactor diode 10, so that the burst signal transmission is performed. Even in this case, the deterioration of the followability is reduced. As described above, according to the present embodiment, the pulse width distortion at the maximum light receiving level is improved.

On the other hand, at the time of the minimum light receiving level, the comparator 3
The light receiving signal level input to the non-inverting input terminal of the above becomes low. As a result, the reverse bias voltage of the varactor diode 10 decreases, and the capacitance between the anode and cathode terminals of the varactor diode 10 increases. As a result, it is possible to obtain substantially the same characteristics as when the capacitor 6 shown in FIG. 3 is connected, and to obtain the required ripple elimination characteristics.

[0016]

As described above, according to the present invention,
At the maximum light receiving level, the rise and fall of the threshold waveform can be made steeper than before,
It is possible to improve the followability with the burst signal, and to improve the pulse width distortion, thereby realizing the expansion of the dynamic range of the optical receiver.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention.

FIG. 2 is a diagram showing signal waveforms at an inverting input terminal and a non-inverting input terminal of a comparator at the maximum light receiving level in FIG.

FIG. 3 is a circuit diagram of a conventional example.

FIG. 4 is a diagram illustrating signal waveforms at an inverting input terminal and a non-inverting input terminal of the comparator at the maximum light receiving level in FIG. 3;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light receiving element 2 equalizing amplifier circuit 3 comparator 4 first resistor 5 second resistor 10 varactor diode

Claims (2)

(57) [Claims] 1. A second input terminal of a comparator in which a first resistor is connected between first and second input terminals and a second resistor is connected between an output terminal and the second input terminal. A variable capacitance element is connected to the first input terminal of the comparator, and the amplified light reception signal is input to the first input terminal of the comparator and compared with a threshold value input to the second input terminal. Threshold control circuit. 2. A first input terminal of the comparator is an inverting input terminal, a second input terminal is a non-inverting input terminal, and the variable capacitance element has a cathode connected to the non-inverting input terminal of the comparator. And a varactor diode whose anode is grounded.
Automatic threshold control circuit as described.