JPH05145360A - Optical reception circuit and optical transformation circuit - Google Patents

Abstract

PURPOSE:To obtain a signal output of a prescribed level with respect to a broad band signal by providing a combination circuit comprising resistors to an AGC circuit. CONSTITUTION:A buffer amplifier 5 is provided between an output level detection circuit of an optical reception circuit and a detection circuit. A power A1 inputted to the buffer amplifier 5 is multiplied by A and a power of A.P1 is fed to a resistor 17 of the detection circuit. A high frequency signal of V0=(A.P1.Z0)<1/2> appears across the resistor 17, rectified by a diode 16 as a single wave and converted into a DC voltage by a smoothing circuit comprising the resistor 17 and a capacitor 18. Through the constitution above, since a reactance element having a frequency characteristic is not included, the amplitude frequency characteristic of an AGC circuit is made flat. A capacitor 12 is included in an output level detection circuit and is a Dc block capacitor with the buffer amplifier 5 and has only the reactance component sufficiently negligible with respect to the impedance of a capacitor input and output terminal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiver circuit and an optical transmitter circuit provided with an AGC (Automatic Gain Control) circuit for keeping an output level constant.

[0002]

2. Description of the Related Art In general, an optical communication system has an advantage that it can transmit a wide band and high speed signal as compared with a wireless communication system using radio waves. The receiving circuit of such a communication system regardless of the use of radio waves or the use of light includes at least a circuit for keeping the signal output level constant, and an example thereof is an AGC circuit.

FIG. 8 is a block diagram showing a conventional example of an optical receiver circuit having an AGC circuit, and FIG. 9 is a circuit diagram of an output level detection circuit 4 and a detection circuit 6. This optical receiving circuit includes an opto-electric conversion element such as a photodiode 1, a preamplifier 2, an AGC amplifier 3, an output level detection circuit 4, a detection circuit 6, a DC amplification circuit 7, and an output terminal 9. It is composed of a voltage comparison circuit 8 for comparing a reference voltage for setting electric power with the output voltage of the DC amplifier 7.

Here, the AGC amplifier 3, the output level detection circuit 4, the detection circuit 6, the DC amplifier 7 and the voltage comparison circuit 8 constitute an AGC circuit.

The intensity-modulated optical input signal is photo-electrically converted by the photodiode 1, amplified by the preamplifier 2, and further amplified by the AGC amplifier 3 to output the output terminal 9.
Is output to. In the AGC circuit, a part of the output signal of the AGC amplifier 3 is taken out by the output level detection circuit 4, converted into a DC voltage corresponding to the signal level by the detection circuit 6, and further, the DC amplification circuit 7 is provided to the voltage comparison circuit 8 at the subsequent stage. After being amplified to a level suitable for the output voltage, the error voltage with respect to the reference voltage adjusted so as to obtain a preset output is compared with the voltage comparison circuit 8
And the output level is kept constant by controlling the AGC amplifier 3.

In the usual narrow band communication system, the output level detection circuit 4 takes into consideration the influence on the signal output, and as shown in FIG. The detection circuit 6 obtains a DC voltage corresponding to the detected power from the signal obtained from the output level detection circuit 4, rectifies it by the diode 33, and obtains the DC voltage by the smoothing circuit (the resistor 34 and the capacitor 35).
Here, the resistor 32 in FIG. 9 is a load resistor of the output level detection circuit 4.

As in the optical receiver circuit, an AGC circuit for keeping the signal amplitude constant is also present in the optical transmitter circuit. The AGC circuit in this case is provided mainly for the purpose of making the modulation degree of a laser diode or the like for obtaining an optical signal output constant. However, the output level detecting circuit is the same as that of the optical receiving circuit.

[0008]

The above-described conventional optical receiving circuit has a drawback that an output level detecting circuit using a small-capacity capacitor cannot detect a signal at a constant level over a wide band due to the reactance characteristic of the capacitor. Also, if the value of the capacitor is increased to reduce the frequency dependence of the reactance, the impedance seen from the output fluctuates significantly due to the diode of the detection circuit in the subsequent stage turning on and off, and a stable output amplitude cannot be obtained. There are drawbacks. This also applies to the conventional optical transmission circuit.

An object of the present invention is to provide an optical receiving circuit and an optical transmitting circuit which can detect a signal of a constant level even in a wide band signal and can obtain a stable output amplitude.

[0010]

In order to achieve the above object, the optical receiving circuit of the present invention has a coupling circuit formed by a resistor in an AGC circuit. Further, the optical transmission circuit of the present invention has a coupling circuit composed of a resistor in the AGC circuit.

[0011]

As described above, by providing the output signal detecting element of the AGC amplifier with the coupling circuit constituted by the resistor whose impedance does not change with frequency, a wide band AGC signal detecting circuit can be obtained and The impedance seen is also stabilized and the output signal amplitude is also constant.

[0012]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 is a block diagram of an optical receiving circuit according to a first embodiment of the present invention, and FIG. 2 is a circuit diagram of an output level detecting circuit 4, a buffer amplifier 5 and a detecting circuit 6 of FIG.

In this embodiment, a buffer amplifier 5 is provided between the output level detecting circuit 4 and the detecting circuit 6 of the conventional optical receiving circuit (FIG. 9).
It is configured with.

Power supplied from the AGC amplifier 3 to the output terminal 9 and the output level detection circuit 4 is _Pin , and the buffer amplifier 5
The input impedance Z _in, the resistance value of the resistor 11 of the output level detection circuit 4 is as R _1. Assume that the reactance of the capacitor 12 is negligible with respect to R ₁ and Z _{in at} all frequencies using the capacitor 12, and the output terminal 9 is terminated with the impedance Z ₀ , and the impedance of R ₁ is sufficiently high with respect to Z ₀ . To do.

Then, the voltage appearing at the output terminal 9, that is, the voltage V _in input to the output level detection circuit 4 is V _in = [{P _in {Z ₀ * (R ₁ + Z _in )}] ^1/2 Then, “*” of Z ₀ * (R ₁ + Z _in ) is impedance calculation of parallel connection, and Z ₀ * (R ₁ + Z _in ) = Z ₀ * (R ₁ + Z _in ) / {(Z ₀ + (R ₁ + Z _in )} Z ₀ << R ₁ so that V _in ≈ (P _in · Z ₀ ) ^1/2 . Further, the voltage V _A appearing across the input resistance Z _in of the buffer amplifier 5 is V _A. = {Z _in / (R ₁ + Z _in )} · V _in , and the power P ₁ input to the buffer amplifier 5 is P ₁ = _VA ² / Z _in = P _in · Z ₀ · Z _in / (R ₁ + Z _in ) ^{2 where} Z ₀ · Z _in is a pure resistance.

The electric power P ₁ input to the buffer amplifier 5 is multiplied by A, and the electric power A · P ₁ is applied to the resistor 17 of the detection circuit 6. V ₀ = (A · P ₁ · Z ₀ ) across both ends of the resistor 17.
^{A 1/2} high-frequency signal appears, is single-wave rectified by the diode 16, and is converted into a DC voltage by the smoothing circuit including the resistor 17 and the capacitor 18.

Since the reactance element having the frequency characteristic is not included in this embodiment, the amplitude frequency characteristic of the AGC circuit becomes flat. Although the output level detection circuit 4 includes a reactance element (capacitor 12), this is a DC blocking capacitor with the buffer amplifier 5 and has a reactance component that can be sufficiently ignored with respect to the impedance of the condenser input / output terminal. However, in this case, R ₁ + Z _in needs to be sufficiently larger than the characteristic impedance Z ₀ of the output terminal 9.
The reason is that the impedance Z ₁ seen from the output terminal 9 from the AGC amplifier 3 is Z ₁ = 1 / [{1 / (R ₁ + Z _in )} + (1 / Z ₀ )], and R ₁ + Z _in >> Because Z ₀ , Z ₁ ≈Z ₀ , and power matching is obtained.

FIG. 3 is a block diagram of an optical receiving circuit according to a second embodiment of the present invention, and FIG. 4 is a diagram showing a circuit example of the resistance attenuator 10, the buffer amplifier 5, and the detection circuit 6 in FIG.

In this embodiment, the output level detection circuit 4 in the first embodiment is deleted, and a resistor 21, between the AGC amplifier 3 and the output terminal 9 is provided for the purpose of improving the return loss on the output side.
22, 23, 24 (resistance values are R ₁ , R ₂ , R respectively
₃ , R ₄ ) and the input impedance Z _in of the buffer amplifier 5 are arranged as a resistance attenuator 10 of a π-type attenuator, and the AGC voltage is output to the output terminal 20 by detecting a part of the power consumed here. It has gained.

When the output impedance of the AGC amplifier 3, the input impedance of the buffer amplifier 5, and the terminating impedance of the output terminal 9 are all set to Z ₀ ,
The conditions under which impedance matching is obtained are (R ₂ + R ₄ * Z ₀ ) * (R ₁ + R ₃ * Z ₀ ) = Z ₀ R ₂ + R ₄ * Z ₀ = R ₃ R 4 * {R ₂ + Z ₀ * ( R ₁ + R ₃ * Z0)} = Z ₀ . Here, the first expression is a condition that the output side impedance of the AGC amplifier 3 is Z _0, and the second expression is the resistance attenuator 10.
Is a characteristic impedance Z _0, and the third equation is a condition that the impedance viewed from the input of the buffer amplifier 5 is Z ₀ .

When the attenuation amount of the resistance attenuator 10 is BdB, R ₁ = (k ² −1) · Z ₀ / 2k R ₃ = (k + 1) · Z ₀ / (k−1) where k = It becomes log ^-1 (B / 20).

At this time, the power ratio P _a / P _b between the output power P _a of the AGC amplifier 3 and the input power P _b of the buffer amplifier 5 at this time.
Is as shown in FIG.

As another embodiment of the present invention, a method of obtaining an AGC control voltage from the output side of the π-type attenuator as shown in FIG. 6 and a method of detecting as an T-type attenuator as shown in FIG. 7 are considered. Be done.

[0025]

As described above, according to the present invention, since the AGC circuit has the coupling circuit constituted by the resistor, a signal output of a constant level can be obtained for a wide band signal and a stable output amplitude is obtained. There is an effect that can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical receiving circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of an output level detection circuit 4, a buffer amplifier 5, and a detection circuit 6 according to the first embodiment.

FIG. 3 is a block diagram of an optical receiving circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram of a resistance attenuator 10, a buffer amplifier 5, and a detection circuit 6 according to a second embodiment.

FIG. 5 is a graph showing the relationship between the attenuation amount of the resistance attenuator 10, the resistance attenuator input power, and the detection circuit output power in the second embodiment.

FIG. 6 is a circuit diagram of a resistance attenuator 10, a buffer amplifier 5, and a detection circuit 6 according to a second embodiment.

FIG. 7 is a circuit diagram of a resistance attenuator 10, a buffer amplifier 5, and a detection circuit 6 according to a second embodiment.

FIG. 8 is a block diagram of a conventional example of an optical receiving circuit.

9 is a circuit diagram of a conventional output level detection circuit 4 of FIG.

[Description of Reference Signs] 1 Photodiode 2 Preamplifier 3 AGC amplifier 4 Output level detection circuit 5 Buffer amplifier 6 Detection circuit 7 DC amplifier 8 Voltage comparison circuit 9 Output terminal 10 Resistance attenuator 11, 15, 17, 21-24, 32 , 34 resistors 12, 14, 18, 31, 35 capacitors 16, 33 diode 19 input end 20 output end

Claims (6)

[Claims] 1. An optical receiver circuit comprising an AGC circuit for obtaining a constant amplitude output with respect to fluctuations in the intensity of input light and fluctuations in the amplitude of a converted electric signal.
An optical receiver circuit having a coupling circuit composed of a resistor in a C circuit. 2. The optical receiving circuit according to claim 1, wherein the coupling circuit has a combination of a π-type or T-type resistance attenuator and a resistance division circuit. 3. The optical receiving circuit according to claim 1, further comprising a wide-band buffer amplifier between the coupling circuit and the detection circuit. 4. An optical transmitter circuit having an AGC circuit for obtaining a constant amplitude output in response to fluctuations in the amplitude of an input electric signal, wherein the AGC circuit has a coupling circuit composed of a resistor. Optical transmitter circuit. 5. The optical transmission circuit according to claim 4, wherein the coupling circuit has a combination of a π type or T type resistance attenuator and a resistance division circuit. 6. The optical transmission circuit according to claim 4, further comprising a wide-band buffer amplifier between the coupling circuit and the detection circuit.