Optical fiber communication is an emerging high-performance serial communication technology, which has gradually expanded in the field of electricity. At present, most of the fiber communication modules use FPGA or DSP technology to implement signal demodulation, although its transmission speed is fast, high efficiency, high cost, complex technology, and for transmission distance, electrical isolation characteristics, reliability, product cost parameters, etc. Very high demand. The application of the power industry is mainly concentrated in terms of control of power, and the communication speed of the fiber optic module is lower in terms of control. Therefore, in the actual engineering of the power system, due to the complex site, there is a wide interference signal, and the application of high-cost high-speed optical fiber communication technology is not very ideal. In view of the application status of optical fiber communication technology in the power system, a low-speed fiber communication module design scheme of MC3361 + MCU structure is proposed herein. The hardware cost is low, the software process is simple, the performance is stable, the output signal is an industrial standard RS485 signal or RS232 signal, which can be directly connected to a variety of power devices, which is ideal for widely used in the power system.
Modulation principle
The fiber optic communication system is controlled by the PWM interface by the MCU through the internal program. The external device is connected to the module via a serial interface (including the RS232 interface or RS485 interface). After the module receives the data, the data is transmitted to the MCU, the MCU receives the data through the UART interface, the MCU controls the output BFSK modulated signal, modulated The signal is sent directly to the optical fiber transmitting interface. When performing BFSK modulation, use the MCU serial interface to receive the data sent by the external device. The modulation frequency of the BFSK is controlled by the program, and the signal "1" corresponds to the 270 kHz carrier frequency, and the signal "0" corresponds to the 240 kHz carrier frequency, the waveform is shown in FIG. 2 The upper side waveform is an unpaired signal, and the waveform below is a modulated signal. The MCU transmits the modulated carrier frequency signal to the fiber transmitting interface through the PWM manner, and the electrical signal is converted to the optical signal. The modulation hardware principle block diagram is shown in Figure 1.
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Figure 1 block diagram of the modulation hardware principle
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Figure 2 Waveform of the front and rear signal
Modulation principle
In order to reduce hardware costs and improve the reliability of hardware circuits, this design uses BFSK modem algorithm. The BFSK modulation principle is to transmit digital messages with the frequency of the carrier, that is, the frequency of the carrier is controlled by the transmitted digital message. The BFSK signal is a symbol "1" corresponding to the carrier frequency F1, and the symbol "0" corresponds to the modulated waveform of the carrier frequency F2 (another carrier frequency different from F1), and the change between F1 and F2 is instantaneous completion. .
The demodulation of BFSK uses the MC3361 single-chip narrowband FM receiving chip to complete, and the MC3361 contains an oscillating circuit, a mixing circuit, a limiting amplifier, an integral frequency, filter, an suppressor, a scan controller, and a static noise switching circuit. The demodulation circuit schematic is shown in Figure 3. Among them, the 185k network label is the MCU output 185K rectangular wave signal, and R1 is a current limiting resistor, C5, L4 composition filter circuit, C12 resonant capacitor, after R1, C5, L4, and C12, by the MC3361 first foot input, constitutes MC3361 The second local vibration level is demodulated. In Fig. 3, the FSK network is labeled as the rectangular wave signal input input by the optical fiber receiving interface, and the signal is passed by R4, R6, and the signal is converted to 500 mV, and then the L6, C25 is filtered, and C27, L7, VD1, VD2 two After the second pressure filtering, after the interference frequency is eliminated, the final signal is converted to a sine wave signal.
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Figure 3 Schematic diagram of demodulation circuit
Only a standard sine wave signal is input to the 16th pin of the MC3361, as the intermediate frequency IF input signal of the MC3361, the signal amplitude is 0 V, the peak is 500mV, the frequency is 270 kHz or 230 kHz. In the MC3361 internal second mixed frequency, the mixed processing is performed, and the process is 455 kHz second intermediate frequency signal, and is output by the third pin, and is selected from the 455kHz ceramic filter, that is, the Z4 device in Fig. 3, and then The 5th foot input MC3361 limiting amplifier performs high gain amplification, and the limit magnification is the main gain stage of the entire circuit. The 8th foot is configured by the 455kHz Identifier Z3, R2 and C26, and the amplified second intermediate frequency signal is interiively detained, and the signal is output by the 9th pin after the audio voltage is enlarged. The signal passes through the active filter circuit composed of the 10th and 11th pin, and the 12th foot of the MC3361 is input to perform the carrier frequency detection and control the electronic switch, and the demodulated signal is output by the 13th pin, directly input the MCU. The foot is processed by the MCU.
Edit Comment: Fiber Communication Module FPGA or DSP technology implements signal demodulation, high cost, complex technology, low-speed fiber communication module design scheme, low cost, software flow, stable performance, and support RS485 signal or RS232 Signals, there is also a power line connection, which is ideal for promotion.
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Source: Wiku Electronic Market Network
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