EMC tests the composition and design of light guide transmission equipment

Design of EMC test optical transmission equipment

According to the requirements of the project, a kind of equipment is developed to test the electromagnetic radiation of the interference emission level generated by the signal related to the electromagnetic environment inside the aircraft and the power line. The optical transmission equipment of low-frequency analog signal adopts photoelectric conversion technology, combined with corresponding control logic, and integrated with spectrum analyzer, which can accurately measure the real electromagnetic analog signal in the aircraft.

1. Design of EMC test optical transmission equipment

There are many radiation sources inside the aircraft, which will affect the emission level of relevant signal lines and power lines. The design is to ensure the normal operation of the equipment in the aircraft. In the design, when EMC tests the photoconductive transmission equipment, it not only needs to select the high-precision equipment A/D chip to accurately test its electromagnetic signal, so as to evaluate the aircraft electronic system. The carrying capacity of the internal equipment and the interconnection cable to the electromagnetic radiation also needs to ensure that the measured electromagnetic signal is not affected by the electromagnetic environment in the aircraft during the transmission to the spectrum analyzer. Therefore, it is necessary to convert the collected electromagnetic signals into optical signals according to the use of optical fiber transmission to avoid the influence of electromagnetic radiation signals, ensure the accuracy of the electromagnetic signals to be measured, and improve the stability of the machine.

2. Equipment composition and structure block diagram

EMC test low frequency optical transmission equipment consists of optical push module and optical receiving module. The optical push module consists of side panels Controller (including power supply module), test signal generator module and operation circuit unit). 10 MHz channel push board (2 channels) and 1 MHz channel push board (6 channels); The optical receiving module is composed of the side board (including the power module). IEEE488 socket module). The 10 MHz channel receiving board (2 channels) and its 1 MHz channel receiving board (6 channels). As shown in Figure 1.

3. EMC tests the completion of the low-frequency analog signal optical transmitter

3.1 1MHz analog optical channel design

The 1MHz analog optical channel of the EMC test low-frequency optical transmission equipment includes six low-frequency analog signal optical transmission channels, and the signal frequency band of the six low-frequency transmission channels is 100Hz~1MHz. The 1550nm single-longitudinal-mode DFB laser and AM are selected to transmit according to the 6-core single-mode fiber, and the immediate intensity modulation technology is used. The schematic block diagram is shown in Figure 2.

Its principle is to immediately deploy 100Hz~1MHz low-frequency analog signal on a high-performance laser, deploy it as a laser whose light intensity changes with the signal intensity, and carry out remote transmission according to the optical fiber; The receiving end recovers the optical signal to the electrical signal according to the PIN photodetector detection and broadband low-loss frame transport amplification. This analog optical transmission method ensures that the equipment has high signal-to-noise ratio and low distortion.

3.2 10MHz analog optical channel design

The signal frequency band of two 10MHz analog optical transmission channels is DC ～ 10MHz. 131 all-digital modulation method is selected in single-core single-mode fiber for Onm wavelength laser transmission. During the push process, two analog transmission channels are sampled quickly, and A/D is converted according to the photoelectric conversion circuit, and then multiplexed to the optical fiber for transmission; On the contrary, when receiving, the fast data signal on the optical fiber is first demultiplexed into data signal D/A and converted into analog signal. Figure 4 and Figure 5 are 10MHz schematic block diagrams and channels. Because the frequency of the signal transmitted by the 10MHz channel has been very high for a long time. In order to ensure the signal quality, there are two 10 MHz channels in this scheme, which adopt 8-bit A/D.D/A sampling rate of 6OMHz. In essence, the SNR of the 10 MHz channel is ≈ (6.02N1.76) dB, up to 48dB (customer requirement is 36dB).

3.3 Control circuit design

According to the needs of equipment functions, EMC tests the IEEE-488 socket of the optical receiver of the low-frequency analog signal optical transmission device. The main control system of the EMC station sends the operation instructions to the EMC test low-frequency analog signal optical transmission device according to the IEEE-488 socket. The optical receiver sends the operation instructions to the push end (located at the test site) through the special operation optical fiber. In combination with the above provisions, the push end and the receiving end of the EMC test low-frequency analog signal optical transmission machine must have three optical fibers, which are used to transmit 10 optical fiber MHz signals (intelligent, 2-channel multiplexing one optical fiber), 1 MHz signals (intelligent, 6-channel multiplexing one optical fiber) and operation signals (the operation signal of this machine is R8232 data).

3.4 Anti-interference design

In order to accurately measure the real electromagnetic radiation signal in the aircraft, it is very important to improve the electromagnetic interference level of the machine. Therefore, the following aspects should be considered: the optical pushing part and the optical receiving part should be sealed around the chassis, and metal isolation should be used inside to prevent interference with the signal. The AM laser and the PIN photodetector at the receiving end are also protected by metal barriers and control circuits, and pass the filter capacitor according to the electrical connection of the internal baffle. The digital power supply and the analog power supply must be separated at the power anti-interference level to prevent the data signal from affecting the analog signal. At the same time, excellent decoupling and filtering are also effective methods to reduce noise. The common practice is to add decoupling capacitors and bypass capacitors in the power input and output pins. Decoupling capacitor makes the power chip flow back to the ground after removing the AC component; The bypass capacitor can eliminate high-frequency radiated noise and suppress high-frequency interference.

4. Conclusion

In this paper, the real-time deployment of light intensity and photoelectric conversion technology have been selected. At the same time, the precise EMC test of low-frequency electromagnetic radiation signal in the aircraft has been completed in combination with the spectrum analyzer. The technology is easy to use and reliable. According to the practical inspection, the equipment is not only applicable to electromagnetic test in the aircraft, but also can be used for other electromagnetic environment measurements, and has a broad application prospect.