A circuit design of optical fiber receiver for telephone voice signal

　　The structure of an actual optical receiver is relatively complex, but overall it consists of three parts as shown in Figure 2. Its front-end circuit, the optoelectronic conversion module, often includes a photodetector and a preamplifier; the AGC part is the main amplifier part, but for low-rate or short-distance applications such as pure voice communication, the decision circuit realizes data regeneration of digital signals. The performance of the entire receiver is largely determined by the optoelectronic conversion module. The characteristics of this part are mainly noise performance, sensitivity, gain, and bandwidth. Generally speaking, dealing with noise is the main consideration in the design of this part of the circuit.

　　Figure 2 Fiber optic receiver design principle block diagram

　　Depending on the specific requirements of communication, the access of the optoelectronic conversion module to the digital logic circuit requires the design of an automatic gain control circuit (AGC) and a judge. The output voltage of the optoelectronic conversion module is proportional to the received optical power. Since the change in optical power is a function of line loss and optical link distance, an amplifier with AGC or a limiting amplifier is required to adapt to the output voltage of various optical links so that it has a larger receiving voltage range. Automatic gain control or amplification circuits are used to increase the amplitude of the output voltage to adapt to different access level standards.

　　Figure 3 shows the circuit diagram of the entire telephone voice optical fiber receiving system designed. HFBR-2416 is used as the optical receiving module to play the role of photoelectric conversion. Its output is an analog signal, which allows the subsequent circuit to be optimized to meet the requirements of various communication rates and communication distances. In the subsequent circuit, various electronic components can be used to convert analog signals into logic signals compatible with various data formats. In Figure 3, the LF357 module plays the role of signal amplification, amplifying the analog signal output by HFBR-2416. The LT1016 module is a voltage comparator, which plays the role of a judge here. The LF357 module and the LT1016 module complete the signal amplification and judgment together, and convert the analog signal into a digital signal.

　　Figure 3 Schematic diagram of telephone voice optical fiber receiving system circuit

　　The 6th pin of the HFBR-2416 photoelectric conversion module is connected to the power supply, the 2nd pin is the output terminal, and the 3rd and 7th pins are the ground terminals. The resistor at the 6th pin must be connected, and a bypass capacitor must be connected between the 5V power supply and the ground.