JPS63100848A - Optical reception circuit - Google Patents

Abstract

PURPOSE:To contrive to simplify the circuit constitution by constituting the titled circuit by a photoelectric conversion element, a current/voltage converting circuit, an AC amplifier circuit, a waveform shaping circuit and an output buffer. CONSTITUTION:An optical current flows to a photodiode PD in response to an optical signal and converted into a voltage by a resistor R1, but the voltage signal is weak. The signal is amplified into an AC signal with a proper amplitude by 3-stage of AC amplifier circuits W1-W3 and passes through Schmitt circuits W4, K5 to be a rectangular wave. Further, the result is inverted and becomes an output Vout in response to the existence of light. Since the constitution of the block applying photoelectric conversion to the input signal and converting it into a voltage signal is very simple and AC amplification is adopted, the effect of a dark current of a photodetector and of a DC external light are eliminated and the effect of the dark current change due to the temperature change is excluded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to optical receiving circuits used in optical data links.

In a communication system using optical fibers, a transmitter and a receiver are provided on the transmitting side and the receiving side, respectively.

For two-way communication, both stations are provided with a transceiver.

The optical receiving circuit includes a photoelectric conversion element that converts an optical signal into an electrical signal, a circuit that amplifies the photocurrent obtained thereby,
It is equipped with a circuit that compares this with a certain threshold value and binarizes it.

g) In the conventional optical receiving circuit, the binarization circuit converts the received signal into I-
It distinguishes between level and L level.

Previously, the received signal was binarized by comparing it with a DC amplified signal using a fixed threshold value.

However, such a circuit including a direct current component has many problems.

First, the H level signal strength varies depending on the length of the optical fiber and the performance of the light emitting element and light receiving element.

Therefore, it is difficult to set a fixed threshold value.

If a fixed threshold is set as the minimum possible H level,
Although reception errors can be prevented, the width of the H level signal will change depending on the amplitude of the signal.
There is a drawback.

Other problems include changes in dark current and offset due to temperature.

For this reason, a circuit that compares a DC amplified signal with a fixed threshold has the disadvantage that its scope of application is narrow.

Therefore, the inventors invented a circuit that obtains a differential signal and compares it with a threshold value. The threshold is a movable threshold that changes up and down. If the differential signal is positive, a threshold of -Δ is adopted, and if the differential signal is negative, a threshold of +Δ is adopted.

This means that the lever and comparator have a hysteresis of ±Δ. Such an optical receiving circuit is disclosed in Japanese Patent Application Laid-open No. 6O-23.
9138 (released on 560, 11, 28) JP-A-6O-24
0231 (released on S60.11.29) JP-A-6O-24
0232 (released on S60.11.29) JP-A-6O-24
2742 (560, 12, 2 release) JP-A-6O-246
138 (released on 560, 12, 5) JP-A-6O-2479
67 (released on December 7th, 1987) JP-A-61-5655
0 (Published on 561, 3, 22) JP-A-6L-111578
(561, 5, 29 published) etc. in detail. This circuit has excellent high-speed response and low waveform distortion.

Although it is an extremely sophisticated circuit, it has a complex circuit configuration, so if it were manufactured using discrete components, it would require a large number of parts and be expensive. If a dedicated IC were to be manufactured, a large amount of money would be required.

In some cases, high speed is not required, but cost reduction is required. In this case, commercially available SS
I (small 5cale integrated
It is desirable that the configuration be possible using a circuit.

Therefore, the present inventor invented an optical receiving circuit that can be constructed from commercially available parts by using two SS components.

This is explained in Japanese Patent Application No. 129717 (S61.6.3).

This optical receiver circuit uses discrete components only in the part that converts optical signals into photocurrent.

This is because high speed is required. Since the impedance is high and the signal is small, the transistors are connected in Darlington, and the current flowing through the resistor connected in series with the photodiode is amplified by the two transistors mentioned above.

The voltage signal is still amplified, but instead of DC amplification, it is AC amplification. After three stages of AC amplification, the waveform is shaped by a Schmitt trigger circuit.

The AC amplifier circuit is new. The operation will be explained later. The output and input of an inverter are connected through a resistor.

A Schmitt trigger circuit has two inverters connected in series, and the output of the second stage and the input of the first stage are connected through a resistor. This is a commonly used Schmitt trigger circuit.

This circuit consists of two
Since this method uses several transistors, it cannot be said to be sufficiently simplified.

Depending on the application, a simpler circuit configuration may be preferable.

1) Configuration The outline of the optical transmitter/receiver circuit will be explained with reference to FIG.

In the optical connector G, a light emitting diode LED and a photodiode PD are arranged in parallel. Two optical fibers are detachably attached to the optical connector G.

The LEDs are driven by an LED driver J driven by a transmission signal SD. This is ■Rubell and L
Generates a binary level optical signal.

PD receives optical signals. This is considered to be a weak current.

Current-voltage conversion A produces a voltage signal proportional to the weak current.

AC amplification B performs AC amplification of this voltage signal. One feature is that there is no DC amplification.

This is converted into a regular rectangular pulse by waveform shaping E.

The output buffer F sets the voltage value and output impedance to suitable values, and outputs the received signal RD.

A power supply of 1+-SV is supplied from the outside. This serves as a power source for the transmitting section and, through a power filter H, also serves as a power source for the receiving section.

Such a membrane structure is disclosed in the above-mentioned Japanese Patent Application No. 12971/1986.
This is also common to 7.

A specific circuit example will be explained with reference to FIG.

The main part of this circuit consists of six inverters connected in series.

Inverters Wx to W3 constitute an AC amplifier circuit.

Inverters W4 and Ws constitute a waveform shaping circuit.

W6 is an output buffer.

The current-voltage conversion circuit A is a photodiode PD.

It consists of a resistor R1 and a diode Dl.

The cathode of the photodiode PD is connected to the power supply (5). The anode is connected to resistor R1. This point is worth 3 points. The other end of R1 is grounded. A bypass diode D1 is connected in the forward direction between the three points and the ground.

When light enters the photodiode PD, since it is reverse biased, a photocurrent flows through the photodiode PD. Since this also flows through resistor 1, the voltage Va at point a is Va = I R1
(1) becomes. This is current-voltage conversion. If R1 is increased, the voltage signal becomes larger, but then the operation is delayed, so it is set to an appropriate value so as to satisfy the speed requirements and input impedance requirements.

The diode Di is used to keep the voltages at the three points below the voltage drop in the head direction of the diode when the input signal is too large.

When the input signal is large, equation (1) is not followed and the voltage should not exceed 0.6V even when an optical signal is present.

The amplification factor of the subsequent amplifier circuit is sufficiently large so that even a weak optical signal can be amplified to a sufficient magnitude.

Conversely, if the amplitude of the input signal is too large, the amplifier circuit will become saturated. Therefore, the time at H level becomes longer than it actually is.

In other words, the waveform is distorted. Diode D1 is included to prevent this and to pull the input signal back to an appropriate level when it is too large.

Inverter W1 connects point a and coupling capacitor C1
are connected through.

The output C of the inverter W1 and the input are connected through a resistor R2.

Output e and input d of inverter W2 are connected through resistor R3.

Output g and input C of impark W3 are connected through resistor R4.

The output C of the inverter W1 and the input d of the inverter W2 are connected by a coupling capacitor C2.

The output C of the inverter W2 and the input f of Ws are connected by a coupling capacitor C3.

W1 to W3 and associated resistors R2 to R4 constitute an AC amplifier circuit.

An inverter is a digital circuit element used in logic circuits. This has two levels of input: L or [I, and output is [
It is used to take two levels, I or L. This is the normal usage.

Inverters were not designed as analog circuit elements, nor are they used as analog elements.

Inverters, however, are the simplest logic circuits. If this can be used as an amplifier circuit, it will be an extremely inexpensive and simple amplifier circuit.

The inventors of the present invention first realized that an AC amplifier circuit could be constructed by an extremely simple device of connecting the input and output of an inverter with a resistor.

This is patent application No. 6O-218840 (560°10.1
Application).

The feature of the present invention is not in the AC amplifier circuit using an inverter. However, since this AC amplifier circuit is not yet known, it will be explained here.

Connecting the input and output of an inverter with a resistor is contrary to common sense. It is often done to connect the input and output of the two-stage inverter with a resistor, that is, to connect ~Va and 'wVs with R6.

However, connecting the input and output of one inverter with a resistor has never been done before. Since the inverter is used as a logic element, it is unthinkable to connect input and output.

The input and output of the inverter connected to the resistor remain at a voltage Vo that is about half of the power supply voltage Vcc.

It doesn't oscillate.

(2) 0 is neither the L level nor the H level of the input. This is an intermediate voltage, and is a voltage in a region where input is prohibited. In normal usage, this is the only voltage that passes through at the moment of the transition.

However, the state where input=output=Vo is a stable state. If there is no input voltage, the input and output continue to be Vo.

Assume that an AC input signal enters the input of the inverter.

Input goes up or down from VO. The output moves in the opposite direction to the input. Since the output change is larger than the input change, an amplification effect occurs.

However, because there is a resistor that provides negative feedback, the input voltage is pulled toward the output. Therefore, when the resistance is small, the amplification factor is also small. The input voltage is attracted to the output and tries to return to zero. When the input approaches vO, the output also approaches ■0, so the force with which the output attracts the input becomes weaker. Both input and output return to Vo according to the relaxation function.

This relaxation is quite slow, depending on the value of k. Therefore, if the AC input signal is fast enough, the inverter can faithfully AC amplify this signal.

Since the three-stage amplification is W1% and W3, the overall gain is the product of these. The amplification may be performed in four stages or in three stages. It should be three or more steps.

- Give an example.

R2: 50 bil, Ra=1MΩ, R4= 100 kΩ
, C2=0.01 μF, C3=2.2 pF t6o In the example of an IC for amplification d, the first stage is 10 dB, the second stage is 20 dB, and the third stage is 3 dB. Total 33 dB
become.

The reason why the third stage amplification factor is small is because the coupling capacitor C3 is small.

In addition, appropriate constants are determined depending on the magnitude, impedance, and speed of the input signal.

In this way, an AC amplified signal is obtained at point g.

01 to C3 are used to reduce the DC component, all DC components such as the dark current of the photodiode PD, the influence of DC disturbance light, and dark current changes due to temperature are eliminated.

The output i of W4 is connected to the input of W5, and the output j of W5 is connected to the input of W4 through a resistor R6. R6 is a resistor that provides positive feedback. This is often used as a Schmitt trigger circuit. It is well known.

Suppose that the input voltage changes to, for example, I-I level.

Intermediate point i changes to L level. Then, point J changes to H level. Point j, which is at the ■■ level, has a strong initial input (
Since it is raised to the II level, h quickly becomes the II level and the output j also becomes 1 level. In this way, it becomes a Schmitt circuit with hysteresis.

R5 and C4 connect the output g of the AC amplifier circuit B to the input of the Schmitt circuit.

R5 is, for example, 62 kΩ, and C4 is, for example, 0.01 μF.
It is.

The AC signal is transmitted to the Schmitt circuit mainly through C4, and is waveform-shaped there. This results in a regular square wave.

R5 is provided to allow even relatively slow signals to pass through. Also, when there is little change in the signal, the Schmitt circuit's level tends to go out of order, so this has the effect of preventing this.

Inverter W6 is an output buffer. This lowers the output impedance and increases the voltage to S S I L//l/(
Although it is sometimes referred to as rubbing, it is also provided to make the number of inverters an even number so that the level changes at the three points and the three points are in the same direction.

That is, when light is present, the point is at H level, and when there is no light, point k is at L level.

However, there are cases where this is not required.

In that case, W6 may be omitted, or on the contrary, one may be added and W6. It may also be W7.

)) A photocurrent flows through the photodiode in response to the optical signal. This becomes a voltage depending on kl. This voltage signal is weak. The three-stage AC amplifier circuits W1 to W3 generate an AC signal with an appropriate amplitude. This becomes a rectangular wave by passing through the Schmitt circuit of W4 and Ws. Further, it is inverted, resulting in an output Vout corresponding to the presence or absence of light.

e) Circuit example I FIG. 2 shows a second circuit example.

This differs in the connection of the current-voltage converter.

The anode of the photodiode is grounded. The cathode is connected to resistor R1. The other end of the resistor R1 is connected to the power supply. A diode Dl is connected in parallel with R1 in the forward direction.

The voltage Va at the three points is opposite to the previous example, and Va =
Vcc -I R1 (2).

Accordingly, the number of inverters is increased to five, and W6 is excluded. This is to make the presence of light correspond to the H level of Vout.

fhl circuit example ■ FIG. 3 shows a third circuit example.

This uses a phototransistor PTr instead of a photodiode PD as a photoelectric conversion element.

The collector of the phototransistor is connected to a power supply.

The emitter is connected to a resistor Rz. The other end of Rt is grounded.

A diode D1 is connected in parallel with R1 in the forward direction.

Since it is a phototransistor, it has an amplification effect within itself. Therefore, it functions well even for weak input signals.

However, since phototransistors operate slowly, they can only be used for input signals that change slowly.

(+) Effects (1) The configuration of the part that photoelectrically converts input signals and converts them into voltage signals is extremely simple.

(2) Since AC amplification is performed, the dark current of the light receiving element and the influence of DC disturbance light can be removed. The influence of dark current changes due to temperature changes can also be removed.

(3) Since the diode D1 cuts excessive signals, the dynamic range becomes wider. The strength of the signal light varies depending on the length of the optical fiber, but any optical signal can be amplified and binarized.

(4) Amplification and binarization are performed by using an inverter. Eliminates the need for DC amplifiers, comparators, constant current circuits, etc.

Since it has an extremely simple circuit configuration, it can be manufactured easily and inexpensively using SSI.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a first example of the optical receiving circuit of the present invention. FIG. 2 is a circuit diagram showing a second example of the optical receiving circuit of the present invention. FIG. 3 is a circuit diagram showing a third example of the optical receiving circuit of the present invention. FIG. 4 is a configuration diagram of the optical receiving circuit.

Claims (1)

[Claims] In an optical data link receiving circuit that transmits binary digital signals, the optical receiving circuit is composed of a photoelectric conversion element, a current-voltage conversion circuit, an AC amplifier circuit, a waveform shaping circuit, and an output buffer, and the current-voltage conversion circuit is: A photoelectric conversion element, a resistor R_1 connected in series to the photoelectric conversion element, and a diode D_1 connected in parallel to the resistor R_1 in the forward direction,
The AC amplifier circuit is composed of three or more stages of inverters, each with the input and output of the inverter connected through a resistor and mutually connected through a capacitor, and the waveform shaping circuit is connected in series. two inverters W
_4, W_5, a resistor R_6 for feeding back the output of the inverter W_5 to the input of W_4, a resistor R_5 for inputting the amplified output of the AC amplifier circuit to the inverter W_4, and a capacitor C_4 connected in parallel to this, An optical receiving circuit characterized in that the output buffer consists of an inverter for matching output and input signals.