Research on Anti-interference of Car Digital AV Products

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Research on Anti-interference of Car Digital AV Products [Copy link]

1. Introduction
Electromagnetic compatibility (EMC) refers to the ability of a device or system to work normally in the electromagnetic environment in which it is located and not to cause unbearable electromagnetic disturbance to any other object in the environment. Electromagnetic compatibility technology is a rapidly developing interdisciplinary subject, involving electronics, computers, communications, aerospace, railway transportation, electricity, military and even all aspects of people's lives. In today's information society, with the development of electronic technology and computer technology, the number of electrical and electronic equipment used in a system has greatly increased, and the frequency band of electronic equipment has become increasingly widened, the power has gradually increased, the sensitivity has increased, and the cable network connecting various devices has become increasingly complex. Especially with the continuous advent of digital products, the design of its electromagnetic compatibility has attracted more and more attention. Because when high-speed digital circuits work, a large number of high-frequency interference signals will be generated. If they are not handled properly, it will not only affect their own performance, but also affect the surrounding environment. For example, the sum of the video data rate and audio data rate of MPEG1 of VCD video disc player is about 1.5Mb/s; the variable bit rate of DVDMPEG2 audio and video is 4.69Mb/s on average, and the maximum rate is 10.7Mb/s. The processing system is used in conjunction with high-speed memory to read and write data. As the bit rate continues to increase, the speed of digital signal processing is getting faster and faster, generating a large number of interference pulses proportional to the speed, with higher and higher frequencies and larger amplitudes. This brings greater difficulty to the anti-interference design of products and is also the key to the quality of products. If handled improperly, it will affect the quality of audio and video and the ability to read and correct discs. In severe cases, high-frequency interference pulses will be radiated through the power supply or space, affecting the normal operation of surrounding electronic equipment. Now take the Car-VCD player as an example to discuss the anti-interference design of digital AV products.

2. Common interference noise in digital circuits

For the digital signal processing system of digital AV products, there are several common noises.
1. Power supply noise: mainly due to the noise caused by the changes in system current and voltage caused by the high-speed change of logic state during the operation of DSP circuits, CPUs, dynamic storage devices and other digital logic circuits, DC noise during temperature changes, and noise generated by the power supply itself.
2. Ground noise: ground noise caused by the potential difference between the ground wires of various parts in the system or the existence of ground impedance.
3. Reflection noise: when the characteristic impedance of each part of the transmission line is different or does not match the load impedance, the transmitted signal is reflected at the terminal (or critical) part, causing the signal waveform to be distorted or oscillate.
4. Crosstalk noise: noise caused by electromagnetic induction between transmission lines such as flat cables or bundled wires, parallel printed wires in printed circuit boards, and high-speed switching currents superimposing useless signal components on the target signal through parasitic parameters such as distributed capacitance.

III. Measures to suppress interference noise

1. Suppression of power supply and ground line noise
CMOS digital devices and digital analog hybrid devices are widely used in automotive CDs and VCDs. When the equipment is working, these devices work simultaneously, which will cause the power supply voltage and ground level in the circuit board to fluctuate, resulting in spike overshoot or attenuation oscillation of the signal waveform, causing the noise tolerance of the digital IC circuit to decrease and cause malfunction. The reason is that the voltage drop formed by the switching current of the digital IC on the power line and ground line and the inductive voltage drop formed by the distributed inductance of the printed strip and the component pins are the result of the two. Since there are many high-frequency digital signal lines in automotive VCDs,
the interference of power supply and ground line is quite serious. Secondly, since part of the CMOS circuit is a digital analog hybrid device, such as D/A converter, according to the basic theory of CMOS, digital and analog circuits are formed on the same type of chip. If only the digital part power supply VDD is powered, even if the analog power supply is not connected, the power of VDD will be converted to the analog part, and the VDD voltage will still appear on the analog power supply VOC pin. Similarly, the noise on VDD will also appear on VOC. Due to the noise on VDD and VOC, the THD+N and dynamic range of the digital-analog hybrid circuit, such as the audio D/A PCM1710, will decrease, affecting the performance of the whole machine.

In order to suppress the power supply and ground line noise, the author believes that the following measures can be taken in the design of automotive VCD: (1) Select chip components and shorten the pin length of the components as much as possible to reduce the influence of the distributed inductance of the components; select digital ICs with large noise tolerance. (2) Connect the filter capacitor as close to the device as possible at the VDD and VOC power supply ends to shorten the flow path of the switching current, and use 10μF aluminum electrolytic and 0.1μF monolithic capacitors in parallel to the power supply pins. Tantalum electrolytic capacitors can be used instead of aluminum electrolytic capacitors for the main power input terminal of the MPEG board and the power supply terminal of the MPEG decoding chip and high-speed digital ICs such as DRAM and SDRAM, because the impedance of tantalum electrolytic capacitors to ground is much smaller than that of aluminum electrolytic capacitors at high frequencies. (3) When laying out the printed circuit board, the analog circuit area and the digital circuit area should be reasonably separated, the power supply and ground wires should be led out separately, and the power supply should be gathered at one point; when wiring the PCB, high-frequency digital signal lines should be short, the main signal lines should be concentrated in the center of the PCB board, the clock generation circuit should be near the center of the board, the clock fan-out should be daisy-chained or parallel wired, and the power line should be as far away from the high-frequency digital signal line as possible or separated by a ground wire. (4) The power line and ground line printed strips of the printed circuit board should be as wide as possible to reduce the line resistance, thereby reducing the interference noise caused by the common impedance. (5) For the analog-digital hybrid 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, and ALPINE, it is recommended to treat D/A devices as analog devices. When connecting MPEG circuits to D/A devices, D/A devices must be placed on AGND, and a digital loop should be provided for these digital noise/energy to be fed back to the signal source to reduce the impact of digital device noise on analog circuits and improve the dynamic characteristics of D/A devices.

According to the noise level of the digital power supply VDD and the analog power supply VOC of the MPEG decompression board of the VCD machine, it is known that the noise level superimposed on the power supply is quite small, the VDD noise level and the VOC noise level waveform are basically consistent, and the digital power supply noise level (VPP=85mV) is significantly greater than the noise level of the analog power supply, which shows that these interference pulses are mainly generated by digital signals.

2. Suppression of reflection interference noise

In the digital signal processing system, the transmission of clock signals and digital signals will generate reflections at the impedance discontinuity due to the mismatch between the impedance at the beginning and the end of the transmission line, causing the transmitted signal waveform to surge, decline and oscillate. Reflection will also reduce the noise tolerance of the device. Increasing the delay time, if the transmission time of the transmission line is roughly the same as the transmitted delay time, the reflection caused will bring serious consequences, some of which will cause errors in the transmitted information, and some of which will cause the voltage to exceed the limit value of the circuit and affect the normal operation of the circuit.

Under normal circumstances, the transmission line is a lossless line, the transmission time per unit length of the transmission line τ=(LC)?, the characteristic impedance ZO=(L/C)?, where C and L are the distributed capacitance and distributed inductance per unit length of the transmission line. The maximum matching line length of the transmission line is lmax=tτv／k, where tτ is the front edge time of the transmission signal, v is the propagation speed of the electromagnetic wave in the transmission line, which is 2×108 m／s when using polyethylene line, and k is an empirical constant, usually 4-5. If the length of the transmission line exceeds lmax, impedance matching should be performed at its beginning and end. Otherwise, the signal will be seriously distorted due to impedance mismatch. Here, the author takes the transmission line between the DSP signal output end of the VCD machine movement and the MPEG board as an example to further illustrate. A comparative experiment was conducted using a 10cm long bundled wire and a 60cm long flat cable as the transmission line. The bundled wire was used for the experiment first. The waveforms at the DSP output end and the MPEG board input end were basically the same, with a rising edge time of tτ10 ns, and lmax=50cm. Therefore, the bundled wire length is less than lmax, so impedance matching is not necessary. If the bundled wire is replaced with a 60cm long flat cable, the waveform distortion becomes significantly larger after the flat cable is replaced, mainly due to the worsening of the rising edge, the increase of the rising time tτ and the increase of the peak-to-valley ratio of the waveform. The reason is that the length of the flat cable is greater than lmax. The transmission cable needs to be treated as a long line and its impedance must be matched. The rise time of the DSP output becomes longer because the reflection coefficient of the reflected wave reflected to the DSP output end is positive and negative, forming peaks and troughs, which makes the rise time longer. The DATA and LRCK waveforms also have similar situations.

The above comparative experiment shows that in order to suppress the reflection interference, it is necessary to try to match the impedance of the transmitting end and the terminal, or to shorten the length of the transmission line as much as possible, that is, l<lmax. Since it is a civilian product, the production cost and the convenience of the production process must also be considered. The main measures taken in the car VCD are:

(1) Add appropriate resistance to the DSP output end to make it basically consistent with the characteristic impedance of the bundled wire and the flat cable, so that the impedance of the transmitting end is basically matched to offset the overshoot of the rise/fall of the digital signal pulse.
(2) Shorten the length of the bundled wire to l<<lmax. Since the wire is very short, the waveform distortion is slight. The actual result is that the waveform of the DSP is significantly improved.
(3) Use terminal diodes to replace matching resistors. This method has been widely used in the chip manufacturing of digital ICs as a matching and protection network for the input and output ends. This matching method has the following advantages: it can improve the terminal waveform; it has no effect on the level of the transmitting end; it is easy to set up, and the best matching is achieved when there are multiple loads on the same machine; it has a protective effect and effectively suppresses overshoot pulses.
(4) Adding a shaping circuit can reduce the interference noise caused by the mismatch of the connecting wire. The shaping circuit is usually added before the input end, but it should be noted that the signal cannot produce a new phase change.

3. Crosstalk suppression of digital signals

The so-called crosstalk refers to the serious interference noise caused by the signal transmission line on its adjacent signal line during the transmission of the signal. It mostly occurs between parallel printed wires on flat cables, bundled wires or printed circuit boards. The strength of the crosstalk is related to the mutual impedance between the two adjacent signal lines and the impedance of the signal itself. The crosstalk problem of flat cables is discussed below.

In modern digital AV products, flat cables are widely used as connecting wires. Although they have many advantages, if they are used improperly, crosstalk is likely to occur, affecting the normal operation of digital products. There is distributed capacitance between each wire of the flat cable. After measurement, the distributed capacitance between adjacent wires of every 10cm is about 3pF. When the frequency is 100MHz, the impedance of 1pF capacitor is 1.6kΩ, and the distributed capacitance of the flat cable wire is proportional to its length. The crosstalk is more serious when the wiring is longer. Taking the VCD machine as an example, the signal is a square wave of hundreds of kilohertz and several megahertz and a clock signal of 10 to 20MHz. It contains dozens of times higher harmonics, and the highest signal spectrum is nearly hundreds of megahertz. This high-frequency component is very easy to crosstalk each other through the distributed capacitance between the wires of the flat cable. Through comparative experiments, 60cm long flat cables and 10cm long bundled cables were used to connect the DSP and MPEG board respectively. It was found that the interference on the 60cm flat cable was significantly greater than the interference on the 10cm long bundled cable, indicating that the distributed capacitance of the flat cable is proportional to the length, and the interference is proportional to the distributed capacitance. If the BCK clock at the DSP output is disconnected, the LRCK interference points will be significantly reduced and the interference pulse amplitude will decrease. This shows that most of the interference comes from the BCK square wave signal. Controlling the distance between the wires can reduce interference. The following measures are taken in car VCDs:

(1) Shorten the transmission length of the signal line as much as possible.

(2) When transmitting signals of multiple levels, the same-level signals with similar front and rear edge times should be grouped together for transmission as much as possible. The DATA, BCK, and LRCK signals are isolated from the main clock by a ground wire. If necessary, use shielded wires instead of bundled wires to transmit MCLK and BCK clocks to reduce crosstalk and radiation.

(3) When wiring on a double-sided printed circuit board, high-frequency digital signals and clock signals are transmitted on the front side, and the grounding area is increased as much as possible on the back side of the transmission printed circuit. In this way, since the distributed capacitance between parallel wires will decrease when the wires are close to the ground plane, the crosstalk interference between signal lines will be reduced; when wiring MPEG chips, DRAM, SDRAM and other high-speed digital device printed circuit boards, a large ground wire is laid on the back side, and the ground wire bypasses the high-frequency pulse noise generated by the shielding device.

IV. Anti-interference design of digital signal processing system

In fact, the inductive voltage drop caused by the change of power line current, the reflected interference of digital signal transmission and the crosstalk between digital signals are closely related and inseparable. Reflected in the digital signal processing system, the most harmful is high-frequency pulse noise. Therefore, suppressing high-frequency pulse noise is an important part of the electromagnetic compatibility

design of digital AV products. For example, during the debugging of the VCD machine, when the whole machine is working, the function error is encountered. The connection between the CPU and the MPEG chip CL680A1 is detected through the built-in detection program. The high-frequency burrs on HRDY and HCK are observed with an oscilloscope. A 51pF capacitor is connected in parallel to HRDY, and HCK is shaped by a trigger. The accuracy of data communication detected by the built-in detection program reaches 100%, and the whole machine works completely normally. In order to improve the anti-interference performance of the system, the following measures can be adopted in digital AV products:

(1) Increase the anti-interference ability of the bus. The three-state gate bus structure is adopted. The bus is added with a pull-up resistor to make the bus in a stable high level at an instant and eliminate the bus in a suspended state of unstable voltage. The bus must be added with a buffer.
(2) Use software to eliminate interference. During system design, although various improvements have been made in hardware, it is impossible to completely eliminate interference. For example, if the system freezes or data transmission errors occur, improvements can be made from the software: use a watchdog timer to detect whether the system is interfered with. Once the system is interfered with, immediately interrupt the system and reinitialize it before restarting to eliminate the interference. Using software fault-tolerant technology means recognizing that faults and errors are objective facts and considering taking measures to eliminate, suppress, and reduce the impact they cause.
(3) Improve the anti-interference ability of system control signals. There are usually control lines such as RESET and STB in the system. The transmission distance between the CPU and its control device is far and the control line impedance is high, which is easily affected by pulse noise. A 20pF capacitor is connected in parallel to the peer control signal end of the controlled device to eliminate interference, and a 0.01μF capacitor is connected in parallel to the control signal such as RESET to solve the interference problem. Adding a buffer driver to the control line to lower the impedance of the control line also has the effect of suppressing interference.
(4) Processing of unused ends of ICs. Unused ends must be properly handled, otherwise noise can easily interfere with the circuit through distributed capacitance. For example, add a 1-10kΩ pull-up resistor to the unused end of the TTL and CMOS circuits, and connect a small ceramic capacitor in parallel to the unused output end of the trigger.

V. Conclusion

The international community attaches great importance to the EMC design of electronic products. The electromagnetic compatibility standards of electronic products in Europe, America, Japan, etc. are mandatory. In the design and trial production of automotive digital AV products, EMC design should be regarded as an important part of the design process. From component selection, circuit board design and overall layout of the whole machine, the anti-interference design requirements of digital circuits should be strictly followed.