MC3361+MCU low-speed communication system circuit

As an emerging high-performance serial communication technology, optical fiber communication has been gradually applied in the power field. Most of the current optical fiber communication modules use FPGA or DSP technology to achieve signal demodulation. Although it has fast transmission speed and high efficiency, it is costly and technically complex, and has extremely high requirements for transmission distance, electrical isolation characteristics, reliability, and product cost parameters. The application of optical fiber in the power industry is mainly concentrated in the control of strong electricity, and the field environment has low requirements for the communication speed of optical fiber modules. Therefore, in the actual engineering of power systems, due to the complex field conditions and numerous interference signals, the application of high-cost high-speed optical fiber communication technology is not very ideal. In view of the current application status of optical fiber communication technology in power systems, this paper proposes a low-speed optical fiber communication module design scheme with MC3361+MCU structure. It has low hardware cost, simple software process, stable performance, and the output signal is an industrial standard RS485 signal or RS232 signal, which can be directly connected to various power equipment, and is very suitable for widespread use in power systems.

Modulation principle

The optical fiber communication system is controlled by the MCU through the internal program to complete the modulation through the PWM interface. The external device is connected to the module through a serial interface (including RS232 interface or RS485 interface). After the module receives the data, it first transmits the data to the MCU. The MCU receives the data through the UART interface. The MCU outputs the BFSK modulation signal through program control. The modulated signal is directly sent to the optical fiber transmission interface for transmission. When performing BFSK modulation, the MCU serial interface is used to receive the data sent by the external device. The modulation frequency of BFSK is controlled by the program. The signal "1" corresponds to the 270KHz carrier frequency, and the signal "0" corresponds to the 240KHz carrier frequency. The waveform is shown in Figure 2. The upper waveform is the unmodulated signal, and the lower waveform is the modulated signal. The MCU sends the modulated carrier frequency signal to the optical fiber transmission interface through PWM, and the electrical signal is converted into an optical signal. The modulation hardware principle block diagram is shown in Figure 1.

　　

　　Figure 1 Modulation hardware block diagram

　　

　　Figure 2 Waveform of the signal before and after modulation

Modulation and Demodulation Principles

In order to reduce hardware cost and improve the reliability of hardware circuit, this design uses BFSK modulation and demodulation algorithm. The modulation principle of BFSK is to use the frequency of the carrier to transmit digital messages, that is, to control the frequency of the carrier with the transmitted digital message. The BFSK signal is a modulated waveform in which the symbol "1" corresponds to the carrier frequency f1, and the symbol "0" corresponds to the carrier frequency f2 (another carrier frequency different from f1), and the change between f1 and f2 is completed instantly.

BFSK demodulation is completed using the MC3361 single-chip narrowband FM receiver chip. The MC3361 chip contains an oscillation circuit, a mixer circuit, a limiting amplifier, an integral frequency detector, a filter, a suppressor, a scanning controller, and a noise switch circuit. The schematic diagram of the demodulation circuit is shown in Figure 3. Among them, the 185K network label is the MCU output 185K rectangular wave signal, R1 is a current limiting resistor, C5 and L4 form a filter circuit, and C12 is a resonant capacitor. After the signal passes through R1, C5, L4 and C12, it is input from the first pin of MC3361, forming the second local oscillator level of MC3361 demodulation. The FSK network number in Figure 3 is the rectangular wave signal input by the optical fiber receiving interface. The signal is divided by R4 and R6 to convert the high level of the signal into 500mV, and then filtered by L6 and C25, and filtered by C27, L7, VD1, and VD2 for the second time. After eliminating the interference frequency, it is resonated by C1 and finally converted into a sine wave signal.

　　

　　Figure 3 Schematic diagram of demodulation circuit

Finally, only the standard sine wave signal is input to the 16th pin of MC3361 as the first intermediate frequency IF input signal of MC3361, with a signal amplitude of 0V, a peak value of 500mV, and a frequency of 270KHz or 230KHz. The second mixer stage inside MC3361 performs mixing processing, and the processed signal is a second intermediate frequency signal of 455KHz, which is output from the 3rd pin and selected by the 455kHz ceramic filter, i.e., the Z4 device in Figure 3, and then input to the limiting amplifier of MC3361 through the 5th pin for high gain amplification. The limiting amplifier stage is the main gain stage of the whole circuit. Pin 8 is connected to the demodulation circuit, which is composed of 455kHz demodulator Z3, R2 and C26. The amplified second intermediate frequency signal is demodulated internally and output from pin 9 after a first-level audio voltage amplification. The signal passes through the active filter circuit composed of pins 10 and 11, and then input into pin 12 of MC3361 for carrier frequency detection and control of the electronic switch. Finally, the demodulated signal is output from pin 13 and directly input into the pin of MCU for processing by MCU.

Community comments:

The fiber optic communication module uses FPGA or DSP technology to achieve signal demodulation, which is costly and technically complex. The low-speed fiber optic communication module design using the MC3361+MCU structure has low cost, simple software flow, stable performance, and supports RS485 or RS232 signals, as well as power line connections, making it very suitable for promotion and use.