Research on anti-interference of audio and video products

Electromagnetic compatibility, or EMC (ElectromagneTIcCompaTIbility), refers to the ability of a device or system to work normally in the electromagnetic environment where it is located and not to constitute an unsustainable electromagnetic disturbance to other things in the environment. With the development of science and technology, the application of digital circuits is becoming more and more extensive. The anti-interference technology of digital circuits is directly related to the normal operation of digital circuits. Therefore, how to effectively enhance the anti-interference performance of digital circuits has become an increasingly concerned issue. This article takes the DVD player as an example to discuss the anti-interference design of the digital circuit.
1 Interference sources and general classification of interference Interference sources that affect digital circuits are mostly generated in parts where current or voltage changes drastically. The parts where these charges move violently are noise sources, that is, interference sources.
The type of interference is usually divided according to the cause of the interference, the mode of noise interference and the waveform nature of the noise. Among them: according to different causes of noise, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc .; according to the waveform and nature of noise, it is divided into continuous noise, sporadic noise, etc .; according to the different noise interference modes, it can be divided into Common mode interference and differential mode interference. Common mode interference and differential mode interference are a commonly used classification method. Common mode interference is the potential difference between the signal and the ground, which is mainly formed by the superposition of the common state (in the same direction) voltage induced by the grid connection, ground potential difference and space electromagnetic radiation on the signal line. The common-mode voltage can be converted into a differential-mode voltage through an asymmetric circuit, which directly affects the measurement and control signals and causes component damage (this is the main reason for the high damage rate of some system I / O modules). This common-mode interference can be It is DC or AC. Differential mode interference mainly refers to the interference voltage acting between the two poles of the signal, the most important of which is the voltage formed by the coupling induction of the space electromagnetic field between the signals and the conversion of the unbalanced circuit common mode interference, which will be directly superimposed on the signal. Affect the measurement and control accuracy.
2 Features of digital audio and video products The core of digital audio and video (AV) products is the DSP (DigiialSignalProcessing) system, which can perform high-speed digital signal processing on audio and video signals, so that people can enjoy audio-visual enjoyment in a more perfect situation. At the same time, due to the high bit rate of digital signal processing, the sum of the MPEG1 video data rate and audio data rate of the general VCD video disc player is about 1.5Mb / s; the DVD MPEG2 audio and video variable code rate is 4.69Mb / s, the maximum rate is up to 10.7Mb / s, it can be seen that the code rate is high, and the processing system uses high-speed memory to read and write data. With the continuous improvement of the code rate, the speed of digital signal processing is getting faster and faster, so a large number of interference pulses proportional to the speed will be generated, and the frequency will be higher and higher, and the amplitude will be larger and larger, so that the anti-interference design of the product Bringing greater difficulty is also the key to product quality.
3 Common interference of digital circuits The common noises of digital signal processing systems of digital audio and video (AV) products are as follows:
(1) Power supply noise: It is mainly the high-speed change of the logic state of DSP circuits, CPUs, dynamic storage devices and other digital logic circuits during operation, resulting in noise caused by changes in system current and voltage, as well as temperature changes. DC noise and noise generated by the power supply itself;
(2) Ground wire noise: ground noise caused by the potential difference or the presence of ground impedance between the ground wires of each part of the system;
(3) Reflected noise: when the characteristic impedance of each part of the transmission line is different or does not match the load impedance, the transmission signal will be reflected at the terminal (or critical) part, thereby distorting or oscillating the signal waveform.
(4) Crosstalk noise: Due to the electromagnetic induction between the transmission lines such as flat cables or bundled wires, and the parallel printed wires in the printed circuit board and the high-speed switching current, the unwanted signal components are superimposed on the destination signal through parasitic parameters such as distributed capacitance Noise caused by
It can be seen that the basic elements that form interference are the interference source, the propagation path and the sensitive devices. Therefore, the basic principle of anti-interference design is to suppress the interference source, cut off the interference propagation path, and improve the anti-interference performance of sensitive devices.
4 Interference noise suppression measures
4.1 Suppression of power and ground noise Noise suppression of power interference is the first consideration and the most important principle in anti-interference design. While the power supply provides energy to the system, it will add noise to the power supply; the strong interference on the power grid will enter the circuit through the power supply; in addition, even if the battery is powered, the battery itself has high-frequency noise. In fact, the power supply used by DVD has gone through the process from linear power supply, high voltage switching power supply and low voltage switching power supply.
Suppressing power supply interference is to reduce du / dt and di / dt of the interference source as much as possible. Du / dt to reduce the interference source is mainly achieved by paralleling capacitors at both ends of the interference source. Reducing the di / dt of the interference source can be realized by adding series inductance or resistance and adding a freewheeling diode in the interference source loop. Common measures are as follows:

(1) Install a power filter on the front end of the power transformer. This power filter has good ability to suppress common mode noise and series mode noise, and can isolate the interference of external and internal impulsive noise;
(2) Use switching power supply to replace linear power supply to improve the performance of the whole machine;
(3) Choose chip components and shorten the lead length of the components as much as possible to reduce the influence of the distributed inductance of the components; at the same time, digital ICs with large noise tolerances should be used.
(4) Connect the filter capacitors as close to the device as possible at the VDD and VOC power terminals to shorten the flow path of the switching current. 10μF aluminum electrolytic and 0.1μF monolithic capacitors can be connected to the power supply pins. For the main power input terminal of the MPEG board, the power supply terminal of the MPEG decoding chip and DRAM, SDRAM and other high-speed digital ICs can be replaced with tantalum electrolytic capacitors, because at high frequencies, the impedance of tantalum electrolysis to ground is much smaller than aluminum electrolysis .
(5) For a mixed analog-digital circuit, VDD and VOC should be connected to the analog power supply VOC, and AGND and DGND should be connected to the analog ground AGND. According to the experimental results of BB, PHILIPS, ALPINE and other companies, it is recommended to treat the D / A device as an analog device. Therefore, when connecting the MPEG circuit to the D / A device, the D / A device must be placed on AGND, and a digital number must be provided The loop is used to feed these digital noise / energy back to the signal source, thereby reducing the influence of digital device noise on the analog circuit and improving the dynamic characteristics of the D / A device.
According to the measured noise level of the DVD player, MPEG decompression board digital power supply, VDD and analog power supply, and VOC, the noise level superimposed on the power supply is quite small, and the waveform of the VDD noise level and the VOC noise level are basically the same, and The noise level of the digital power supply (VPP = 85mV) is significantly greater than the noise level of the analog power supply, which shows that the interference pulses here are mainly generated by digital signals.
4.2 Suppression of reflected interference noise Adding an appropriate resistor at the output end of the DSP can make it basically consistent with the characteristic impedance of the bundled wire and flat cable, and basically match the impedance of the transmitting end, thereby canceling the rise / fall overshoot of the digital signal pulse. In addition, shortening the length of the bundle wire to less than 1max can also reduce waveform distortion. So that the DSP waveform is significantly improved.

In fact, the termination diode can also be used to replace the matching resistor. This method has been widely used as a matching and protection network for input and output terminals in the manufacture of digital IC chips. This matching method can improve the terminal waveform, and has no effect on the level of the sending end. At the same time, it has the convenience of supplementary setting, the best matching can be achieved when multiple loads on the same machine, and the advantages of overshoot pulses can be effectively suppressed.
The addition of a shaping circuit in the system can also reduce the interference noise caused by the mismatch of the connecting lines. The shaping circuit is usually added before the input terminal, but it should also be noted that the signal cannot be changed in a new phase.
4.3 Crosstalk suppression of digital signals When considering the crosstalk suppression of digital signals, the transmission length of the signal line should be shortened as much as possible. In the transmission of multiple levels of signals, try to divide the level signals of the same level with similar front and back edge times into a group for transmission. A ground wire should be used to isolate DATA, BCK, LRCK signals and the main clock from each other. When necessary, shielded wires can also be used instead of bundled wires to transmit MCLK and BCK clocks to reduce crosstalk and radiation.
When wiring on double-sided printed boards, the front side can be used to transmit high-frequency digital signals and clock signals, and then the ground area should be maximized on the back of the printed circuit. In this way, because the distributed capacitance between parallel wires becomes smaller when the wires are close to the ground plane, crosstalk interference between signal lines will be reduced; when wiring the printed boards of MPEG chips, DRAM, SDRAM and other high-speed digital devices, A large ground wire can be placed on the back of the ground, so that the ground wire can bypass or shield the high-frequency pulse noise generated by the device.

5 Conclusion Electromagnetic compatibility technology is a rapidly developing interdisciplinary subject, involving all aspects of electronics, computers, communications, aerospace, railway transportation, electricity, military and people's lives. Therefore, in the process of digital AV product design and trial production, electromagnetic compatibility design should be regarded as an important part of the design process, from component purchase, circuit board design and overall layout of the machine, in strict accordance with the anti-interference design requirements of digital circuits To design and develop digital AV products with good electromagnetic compatibility and excellent audio and video performance.