JP3131643B2 - Digital optical signal receiving circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital optical signal receiving circuit, and more particularly to an optical signal which is turned on / off at a predetermined signal speed, and in particular, an input optical signal gradually changes from H to L due to dropping of an optical connector. The switching from H to L in the case of change can be surely reproduced.

[0002]

2. Description of the Related Art Conventionally, a digital optical signal receiving circuit has
The circuit of FIG. 3 having a TC (automatic threshold control) function is used. The input digital optical signal L is incident on a light receiving element 2 such as a photodiode, is converted into an electric signal V0, and is input to an inverted-phase, ie, inverter-type amplifier 3, and the inverted and amplified voltage V1 is inverted by a comparator 4. The threshold voltage Vr1 applied to the input terminal and applied to the non-inverting input terminal is compared with the threshold voltage Vr1. V1> Vr1 or V1 <Vr1
, The comparator 4 outputs a low (L) level or high (H) level output voltage V2 to the outside.

The threshold voltage Vr1 is generated by an ATC (automatic threshold control) circuit 5. In this example, the circuit 5 is
A series circuit of the resistor R1 and the capacitor C connected between the output terminal of the amplifier 3 and the common potential point, and connected between the connection point P of the resistor R1 and the capacitor C and the output terminal of the comparator 4. And supplies the voltage at the connection point P to the comparator 4 as a threshold voltage Vr1. In this example, as shown in FIG. 4, the amplifier 3 and the comparator 4 each output a voltage that changes to L / H with the common potential at the L (low) level. However, the output of the comparator is a saturated output, and the amplitudes V1p and V2p of V1 and V2 usually have V1p ≦
V2p.

Now, in a steady state, V1, V2
Is a DC voltage, and V1 = V1p and V2 = 0. R1,
When R2 is used to represent the resistance value of each resistor, Vr1
Vr1 = V1p × η (1) η = R2 / (R1 + R2) (2) In this example, R1 <R2, and η is a value slightly smaller than 1.

Next, a steady state after V1 changes from the DC voltage V1p to a rectangular wave signal will be considered. When the angular frequency of the rectangular wave is ω, the impedance Zc of the capacitor C
| Zc | = 1 / ωC≪R1, R2 (3) The current due to the AC component included in V1 flows to the common potential point through R1 and C. On the other hand, the current due to the AC component included in V2 flows through R2 and C to the common potential point, provided that the load connected to the output terminal has high impedance. The terminal voltage of C due to the AC component is small because the impedance of C is small, and can be omitted. R1 and R are calculated by the DC component V1dc of V1 and the DC component V2dc of V2.
2, a direct current flows. At this time, the voltage at the connection point P, that is, the threshold voltage Vr1 is as follows: Vr1 = V2dc + (V1dc-V2dc) × η = V2dc (1-η) + V1dc × η (4) For simplicity, set the duty ratio of the square wave signal to 50.
%, V1dc = V1p / 2, V2dc = V2p / 2 (5), so that Vr1 = V2p (1-.eta.) / 2 + V1p.times..eta. / 2 (6) (3) ) Or (5) is substantially equal to the amplitude V1p.
The values of η and V1p, V2p are set to be equal to / 2.

In a transitional period during which the state where V 1 is the DC voltage V 1 p changes to the state where the rectangular wave signal is constantly input, the threshold voltage Vr 1 becomes (1) as shown in FIG. 4B. It gradually changes from the value of the expression to the value of the expression (4) or (6). Consider a steady state after V1 changes from the DC voltage V1p to the L level with a relatively short fall time τ1 as shown in FIG.

When V1 starts to decrease, V1 ≦ Vr1 at t = ta, and the output of the comparator 4 changes from 0 to V2p. In a state where the output of the comparator 4 reaches the steady state after changing to V2p, the threshold voltage Vr1 is Vr1 = V2p + (0−V2p) × η = V2p (1−η) ≡Vr1 min (Vr1 min) 7), and the value of Vr1 at this time becomes the minimum value Vr1 min.

In the transitional period between the two, the threshold value Vr1 gradually changes from the value of the expression (1) to the value of the expression (7). When the input optical signal is in the state described above, the light quantity loss occurs due to, for example, dropping of the optical connector, etc., and V0 gradually changes from H to L, and the light quantity loss recovers after a while. Since V1 changes gently in a DC manner, the threshold voltage Vr1 changes following the change, and Vr1 = V2p + (V1-V2p) η = V1 × η + V2p (1-η) = V1 × η + Vr1 min ………… (8) η is a value relatively close to 1, and is set such that Vr1> V1 during this period as shown in FIG. 5B. The comparator 4 does not invert even when V1 becomes H level. After the loss of light quantity is recovered at td, V2 keeps the H level until t = te when it falls to V0 = 0. For this reason, this optical receiver uses H → L due to a gradual loss of light quantity of the input optical signal.
→ There is a disadvantage that the change in H cannot be reproduced.

On the other hand, in the optical signal receiving circuit of the ATC system shown in FIG. 3, as shown in FIG. 6, an offset voltage (an error voltage appearing at the output when the input is zero) ΔV1 is applied to the output V1 of the amplifier 3 due to a change in the ambient temperature. Occurs and V1 + V1 +
Even if it changes to ΔV1, the threshold voltage also changes from Vr1 to Vr1 + ΔVr1 following the change, so that there is an advantage that malfunction due to the offset voltage can be prevented.

In the ATC circuit, the duty of the input signal is
When the ratio is 50%, the threshold voltage Vr1 becomes the signal V1
9A and 9B, the comparator output V2 can reproduce a signal with a duty ratio of 50% even when the waveform of the amplifier output V1 is distorted due to its frequency band limitation as shown in FIGS. 9A and 9B. On the other hand, in the receiving circuit of the quasi-fixed threshold system, when the amplifier output V1 is distorted as described above, the comparator output V1
2 is distorted as shown in FIGS. 9C and 9D.

[0011]

In a conventional ATC optical signal receiving circuit, in an optical signal transmission system, an input optical signal gradually changes from an H level to an L level due to dropping of an optical connector or the like. When the H level is restored to the H level, the change of H → L → H cannot be reproduced. The present invention aims to solve this drawback.

[0012]

[Means for Solving the Problems]

(1) A digital optical signal receiving circuit according to claim 1, wherein a light receiving element for converting an input digital optical signal to an electric signal, an amplifier for amplifying an output of the light receiving element, and an output of the amplifier for a first threshold value. The first comparator to be compared, the output of the amplifier and the output of the first comparator are input, and have an intermediate value between the H (high) level and the L (low) level of the ON / OFF waveform of the amplifier output. An automatic threshold value control circuit for generating the first threshold value and supplying the same to the first comparator, a second comparator for comparing the output of the amplifier with the second threshold value, and a constant threshold value A threshold voltage generating circuit for generating a voltage; a temperature compensating amplifier for outputting an offset voltage substantially equal to the amplifier in accordance with a change in ambient temperature; an output offset voltage of the temperature compensating amplifier and a threshold voltage generating circuit And the output of the second Adding means for supplying to the second comparator as have value, first, constituted by an AND circuit for outputting to the outside a signal of the logical product of the outputs of the second comparator as an optical signal receiver circuit output.

(2) In the invention of claim 2, in the above (1), the automatic threshold value control circuit is connected between the output terminal of the amplifier and the common potential point with the first resistor and the capacitor ( A series circuit having a capacitor on the common potential point side), an output terminal of the first comparator, and a second resistor connected between a connection point (P) of the first resistor and the capacitor; A voltage at a connection point (P) between the resistor and the capacitor is supplied to a first comparator as a first threshold value.

[0014]

As shown in FIG. 7, if a receiving circuit of a fixed threshold system is used, the input optical signal L gradually changes from H to L due to a loss of light amount due to dropping of an optical connector or the like. Even if the recovery is made, the change of H → L → H can be reproduced. However, in the receiving circuit of the fixed threshold system, if the offset voltage ΔV1 as shown in FIG. 6 is superimposed on the output of the amplifier 3 due to a change in the ambient temperature, a malfunction may occur.

In the present invention, in order to cancel the offset voltage ΔV1 of the amplifier 3, a temperature compensation for generating an offset voltage ΔV1 ′ ≒ ΔV1 substantially similar to that of the amplifier 3 with respect to a change in ambient temperature as shown in FIG. The use amplifier 7 is used. The offset voltage ΔV1 ′ and the output Vr2 ′ of the threshold voltage generating circuit 6 are added by an adder 8, and the added voltage Vr2 ≒ ΔVr2 ′ + ΔV1 (9) is used as a threshold voltage as a comparator 4 To compensate for the offset voltage. The method in which the offset voltage of the amplifier 3 is compensated in this way is called a quasi-fixed threshold method.

In the present invention, as shown in FIG. 1, the above-mentioned circuit of the quasi-fixed threshold method is used in combination with the conventional ATC circuit of FIG. . This solves the conventional problem and also improves the reliability of output data. In FIG. 1, portions corresponding to those in FIGS. 3, 7, and 8 are denoted by the same reference numerals, and redundant description is omitted.

FIG. 2 shows operation waveforms of main parts when a gradual loss of light quantity starts at t = ta and recovers at t = tb. Vr1 and Vr2 are the first and second comparators 4a, 4a, respectively.
4b. The output V2a of the first comparator 4a has no signal on / off switching change due to a gradual loss of light amount (FIG. 2C). However, the second comparator 4
The change appears in the output V2b of b (FIG. 2D).
The output V2 of the first comparator 4a during the light amount dropout period Ta
Since a is held at the H level, the gate of the AND circuit 9 is opened, and the output V2b of the second comparator 4b is output.
Is the output of the AND circuit 9, that is, the reception output V3. Therefore, the information of the change in L / H of the input optical signal due to the light quantity loss is reliably reproduced in the reception output V3 by V2b during this Ta.

[0018]

According to the present invention, a quasi-fixed threshold type receiving circuit with temperature compensation is used in combination with a conventional ATC type receiving circuit, and both outputs are ANDed to obtain a receiver output. As a result, even when the input optical signal gradually changes from H to L due to the loss of the light amount and recovers after a while, the change from H to L to H can be reliably reproduced.

When a signal transmission with a duty ratio of 50% is considered, the signal distortion is as shown in FIG.
2a, a stable reception state can be maintained. The circuit of the ATC method used in the present invention has a feature that it is strong against the fluctuation of the offset voltage of the amplifier due to the change of the ambient temperature, and the circuit of the semi-fixed threshold method also compensates for the offset voltage. According to this, it is possible to perform stable reception with respect to the offset voltage due to the ambient temperature fluctuation.

In the present invention, as described above, the AND of both circuit outputs is taken as the output of the receiving circuit, so that the reliability of the output data can be improved as compared with the case where a single circuit is used.

[Brief description of the drawings]

FIG. 1A is a block diagram showing an embodiment of the present invention, and FIG. 1B is a circuit diagram showing an example of an ATC circuit 5 of A.

FIG. 2 is an operation waveform diagram of a main part of FIG. 1;

FIG. 3 is a circuit diagram of a conventional ATC optical signal receiving circuit.

FIG. 4 is an operation waveform diagram of a main part of FIG. 3;

FIG. 5 is an operation waveform diagram of a main part in FIG. 3 when a light amount loss occurs in an input optical signal.

FIG. 6 is a waveform diagram of an offset voltage generated at an output of an amplifier 3 due to a change in ambient temperature in the circuit of FIG. 3;

FIG. 7A is a block diagram of an optical signal receiving circuit of a fixed threshold system studied before the present invention is obtained, and FIG. 7B is an operation of a main part of A when a light quantity loss occurs in an input optical signal. Waveform diagram.

FIG. 8 is a block diagram of an optical signal receiving circuit of a quasi-fixed threshold system studied in a stage before obtaining the present invention.

9A and 9B are diagrams illustrating another example of the operation waveform of the main part of FIG. 3; and C and D are diagrams illustrating an example of the operation waveform diagram of the main part of FIG.

FIG. 10 is a diagram showing another example of the operation waveform of the main part of FIG. 1;

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-59-221139 (JP, A) JP-A-8-88543 (JP, A) JP-A-1-270408 (JP, A) JP-A-59-221 70342 (JP, A) JP-A-61-10312 (JP, A) JP-A-1-289313 (JP, A) JP-A-2-213216 (JP, A) JP-A-3-46254 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04L 25/00-25/06 H03K 5/08 H04B 10/00

Claims (2)

(57) [Claims] A light receiving element for converting an input digital optical signal into an electric signal; an amplifier for amplifying an output of the light receiving element; a first comparator for comparing an output of the amplifier with a first threshold value; The output of the amplifier and the output of the first comparator are input, and the H (high) level and the L level of the ON / OFF waveform of the amplifier output are input.
An automatic threshold value control circuit for generating the first threshold value having an intermediate value of the (low) level and supplying the first threshold value to the first comparator; A threshold voltage generating circuit for generating a constant threshold voltage; a temperature compensation amplifier for outputting an offset voltage substantially equal to the amplifier in accordance with a change in ambient temperature; Adding means for adding the output offset voltage of the temperature compensation amplifier and the output of the threshold voltage generating circuit, and supplying the added value to the second comparator as the second threshold value; And an AND circuit for outputting a signal, which is the logical product of the outputs of the second comparators, as an output of the optical signal receiving circuit to the outside, and a digital optical signal receiving circuit. 2. A first resistor and a capacitor according to claim 1, wherein the automatic threshold control circuit is connected between an output terminal of the amplifier and a common potential point. And a second resistor connected between an output terminal of the first comparator and a connection point (P) of the first resistor and the capacitor, wherein the first resistor and A digital optical signal receiving circuit, wherein a voltage at a connection point (P) of a capacitor is supplied to the first comparator as the first threshold value.