JPH0993192A - Optical reception circuit - Google Patents

Abstract

(57) Abstract: An optical receiving circuit with less distortion is provided. An optical signal input to an optical receiving circuit is converted into an electric signal by a light receiving element 1, passes through an attenuator 2, and
It is input to the preamplifier 3. The signal amplified by the preamplifier 3 is attenuated by the variable attenuator 4, further amplified by the amplifier 5, and then output. A part of the output is fed back to the AGC circuit 6, and the AGC circuit 6 controls the attenuation amount of the variable attenuator 4 so that the output level of the optical receiving circuit becomes constant. Here, if the output level from the light receiving element 1 is too high, the AG
By controlling the C circuit 6, the level is adjusted by the relay switching type attenuator 2 which does not generate distortion, and the input level to the preamplifier 3 is suppressed, so that the generation of nonlinear distortion can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiving circuit used for CATV, a video surveillance system and the like.

[0002]

2. Description of the Related Art FIG. 3 shows a conventional optical receiving circuit of this type. In FIG. 3, 1 is a light receiving element, 3 is a preamplifier, 4 is a variable attenuator, 5 is an amplifier, and 6 is an AGC circuit.

Next, the operation of the conventional example having the above configuration will be described. First, the optical signal input to the optical receiving circuit is converted into an electric signal by the light receiving element 1 and amplified by the preamplifier 3.
The signal output from the preamplifier 3 is attenuated by the variable attenuator 4, further amplified by the amplifier 5, and then output. A part of the output signal is returned to the AGC circuit 6, and the AGC circuit 6 outputs a control signal so that the output level of the optical receiving circuit becomes constant, and controls the attenuation amount of the variable attenuator 4.

As described above, in the above-described conventional example, even if the input level of the optical signal changes, the AGC circuit 6 can control the attenuation amount of the variable attenuator 4 to keep the output level constant.

[0005]

However, in the optical receiving circuit having the above configuration, when the input level of the optical signal is high, the high level signal is also input to the preamplifier 3 for amplifying the weak electric signal, so that the nonlinear There is a problem that distortion increases and signal quality deteriorates.

The present invention solves the above-mentioned problems of the prior art, and an object of the present invention is to provide an optical receiving circuit for preventing the occurrence of distortion.

[0007]

In order to achieve the above object, the optical receiving circuit of the present invention has a structure in which an attenuator for switching the attenuation amount by a relay is provided between the light receiving element and the preamplifier.

[0008]

According to the above construction, the input signal level to the preamplifier is attenuated when the received light level is high. Thereby, the non-linear distortion can be reduced.

Furthermore, since the distortion characteristic with respect to the preamplifier can be relaxed, an inexpensive element can be used and power consumption can be reduced.

[0010]

Embodiments will be described in detail below with reference to the drawings. FIG. 1 shows a first embodiment of the present invention, and the same components as those in FIG. 3 are designated by the same reference numerals. That is, 1 is a light receiving element, 3 is a preamplifier, 4 is a variable attenuator,
Reference numeral 5 is an amplifier, and 6 is an AGC circuit. An attenuator 2 is provided between the light receiving element 1 for converting an optical signal into an electric signal and the preamplifier 3 for amplifying a weak electric signal and switching an attenuation amount by a relay.

The attenuator 2 has switches SW1 and SW2 interlocking with the input side and the output side thereof, and resistance type attenuator elements A1, ..., An connected between the switches.

Next, the operation of this embodiment will be described. First,
The optical signal input to the optical receiving circuit is converted into an electric signal by the light receiving element 1, is input to the preamplifier 3 via the attenuator 2. The signal amplified by the preamplifier 3 is attenuated by the variable attenuator 4, further amplified by the amplifier 5, and then output. Here, when the level of the optical signal input to the light receiving element 1 increases, the level of the signal output from the amplifier 5 also increases, and the level of the signal input to the AGC circuit 6 also increases. At this time, the AGC circuit 6 controls the attenuation amount of the variable attenuator 4 so that the output level of the optical receiving circuit becomes constant. When the variable range of the variable attenuator 4 is exceeded, the relay of the attenuator 2 is switched to increase the amount of attenuation, and then the output level is further controlled by the variable attenuator 4. On the contrary, when the input optical signal becomes small, the attenuation amount of the attenuator 2 is made small and the output level of the optical receiving circuit is controlled to be constant.

As described above, according to the first embodiment,
When the level of the optical signal input to the light receiving element 1 changes, A
The GC circuit 6 controls the amount of attenuation of the variable attenuator 4 and the attenuator 2 so that the output level of the optical receiving circuit becomes constant over a wide input level of the optical signal, and the input signal level to the preamplifier 3 is suppressed. By doing so, the occurrence of non-linear distortion can be prevented. Further, since the relay switching type attenuator 2 can attenuate the signal without generating distortion, the level control can be performed without degrading the quality of the signal output from the light receiving element 1.

FIG. 2 shows a second embodiment of the present invention, in which the same reference numerals as those in FIG. 1 represent the same elements, and 7 is between the light receiving element 1 and the preamplifier 3. It is an attenuator that is arranged in the switch and switches the attenuation amount by a relay. The attenuator 7 is of a type in which attenuator elements a1, ..., An are connected between the switch SW and the ground to release a part of the signal to the ground. The operation of this embodiment is almost the same as that of the first embodiment, and
The same effect can be obtained.

[0015]

As described above, according to the present invention,
When the signal level output from the light receiving element is high, it is attenuated by a relay switching type attenuator that does not generate distortion and the signal level input to the preamplifier is lowered, so that a high-quality optical receiver circuit with low distortion can be realized. There is an effect that can be.

[Brief description of drawings]

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

FIG. 2 is a block diagram showing a configuration of an optical receiving circuit according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of a conventional optical receiving circuit.

[Explanation of symbols]

1 ... Light receiving element, 2, 7 ... Relay switching type attenuator, 3 ... Preamplifier, 4 ... Variable attenuator, 5 ... Amplifier, 6 ... AGC circuit.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04B 10/04 10/06

Claims (3)

[Claims] 1. An attenuator for switching an attenuation amount by a relay is provided between a light receiving element for converting an optical signal into an electric signal and a preamplifier for amplifying a weak electric signal,
An optical receiving circuit characterized by adjusting an input signal level to the preamplifier. 2. The optical receiving circuit according to claim 1, wherein the relay switching type attenuator is of a type in which a signal is grounded. 3. The optical receiver circuit according to claim 1, wherein the relay switching type attenuator is of a system using a resistor.