This article refers to the address: http://
Author Email:
I. INTRODUCTION Electromagnetic compatibility (EMC) refers to the ability of a device or system to function properly in the electromagnetic environment in which it is located and does not constitute unacceptable electromagnetic disturbances to anything else in the environment. Electromagnetic compatibility technology is a rapidly developing interdisciplinary subject involving electronics, computers, communications, aerospace, railway transportation, electric power, military and even people's lives in today's information society. With the development of electronic technology and computer technology, a system The number of electrical and electronic devices used in the system has increased greatly, and the frequency bands of electronic devices have become wider, the power has gradually increased, the sensitivity has increased, and the cable network connecting various devices has become more and more complicated, especially with the advent of digital products. The design of its electromagnetic compatibility has attracted more and more attention. Because high-speed digital circuits work, a large number of high-frequency interference signals are generated, which are not well handled, which not only affects their performance, but also affects the surrounding environment. For example, the sum of the MPEG1 video data rate and the audio data rate of the VCD video disc player is about 1.5 Mb/s; the DVDMPEG2 audio and video variable bit rate is 4.69 Mb/s on average, and the maximum rate is 10.7 Mb/s, and the processing system is used in conjunction with the high-speed memory. Reading and writing of 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, and the frequency is getting higher and higher, and the amplitude is getting larger and larger, which brings the anti-jamming design of the product. The greater difficulty is also the key to the quality of the product. If not handled properly, it will affect the quality of audio and video and the ability to correct errors. In severe cases, high-frequency interference pulses are radiated through the power supply or space, affecting the normal operation of the surrounding electronic equipment. The Car-VCD machine is taken as an example to discuss the anti-jamming design of digital AV products.
Second, the common interference noise of digital circuits
For digital signal processing systems for digital AV products, the common noises are as follows.
1. Power supply noise: mainly due to the noise generated by the system current and voltage changes caused by the high-speed transformation of the logic state during the working process of the DSP circuit, CPU, dynamic memory device and other digital logic circuits, DC noise when the temperature changes, and the power supply itself. The noise.
2. Ground noise: There is a potential difference between the ground lines of various parts of the system or ground noise caused by grounding 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 will reflect at the terminal (or critical) part, causing the signal waveform to be distorted or oscillated.
4. Crosstalk noise: Due to the electromagnetic induction between the parallel printed conductors in the printed circuit board and the high-speed switching current through the parasitic parameters such as distributed capacitance, the unwanted signal components are superimposed on the destination signal. The noise caused.
Third, measures to suppress interference noise
1. Power and ground line noise suppression CMOS digital devices and digital-analog hybrid devices are widely used in automotive CDs and VCDs. When the devices are working, these devices will work at the same time, causing fluctuations in the power supply voltage and ground level in the circuit board. The signal waveform generates a spike overshoot or attenuates the oscillation, causing the noise tolerance of the digital IC circuit to drop and causing malfunction. The reason is the result of the voltage drop caused by the switching current of the digital IC on the power line and the ground line and the induced voltage drop formed by the distributed inductance of the printed strip and the component leads. Since there are many high-frequency digital signal lines in the VCD for the vehicle, the interference between the power supply and the ground line is quite serious. Second, since some CMOS circuits are digital analog hybrid devices, such as D/A converter devices, according to the basic theory of CMOS, digital and analog circuits are formed on the same type of chip, such as only the digital part of the power supply VDD, although 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 VOC pin. Similarly, the noise present on VDD will also appear on the VOC. Due to the noise on VDD and VOC, the digital-analog hybrid circuit, such as the THD+N and dynamic range of the audio D/A PCM1710, will affect 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 VCD design of the vehicle: (1) Selecting the chip components and shortening the pin length of the components as much as possible to reduce the influence of the component distribution inductance; A digital IC with a large tolerance. (2) Connect the filter capacitor as close as possible to the device at the VDD and VOC power terminals to shorten the switching current. Use 10μF aluminum electrolysis and 0.1μF monolithic capacitor to connect to the power supply pin. For the MPEG board main power input and MPEG decoder chip and DRAM, SDRAM and other high-speed digital IC power supply terminals can use tantalum electrolytic capacitor instead of aluminum electrolytic capacitor, because the high-voltage cesium electrolysis ground impedance is much smaller than aluminum electrolysis. (3) When printing the layout of the printed circuit board, the analog circuit area and the digital circuit area should be reasonably separated, the power supply and the ground line should be separately led out, and the power supply should be gathered to one point; when the PCB is wired, the high-frequency digital signal line should be short-circuited. The main signal lines are preferably concentrated in the center of the PCB. The clock generation circuit should be near the center of the board. The clock fanout should be daisy-chained or parallel-connected. The power lines should be separated from the high-frequency digital signal lines or grounded as far as possible. (4) The power supply line and the ground line printed strip of the printed board are as wide as possible to reduce the line resistance, thereby reducing the interference noise caused by the common impedance. (5), logarithmic mode hybrid circuit, VDD and VOC should be connected to the analog power supply VOC, AGND and DGND are connected to the analog ground AGND. According to the experimental results of BB, PHILIPS, and ALPINE, it is recommended to treat the D/A device as an analog device. In the connection between the MPEG circuit and the D/A device, the D/A device must be placed on AGND, and a digital loop should be provided for these numbers. The noise/energy is fed back to the source to reduce the effects of noise from the digital device on the analog circuitry, improving the dynamics of the D/A device.
According to the measured 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 the same, and the digital power supply noise is electrically The flat (VPP = 85mV) is significantly larger than the noise level of the analog power supply, which means that these interference pulses are mainly generated by digital signals.
2, reflection interference noise suppression
In the digital signal processing system, the clock signal and the digital signal transmission are not matched due to the impedance of the beginning and the end of the transmission line, and the transmitted signal will reflect at the discontinuity of the impedance, causing the transmitted signal waveform to be overshooted, dropped, and oscillated. Reflection also reduces the noise margin of the device. Increase the delay time. For example, the transmission line transmission time is about the same as the transmitted delay time. The reflection will cause serious consequences. Some will cause the transmitted information to generate errors. Some will make the voltage exceed the limit value of the circuit and affect the normal operation of the circuit. .
Normally, the transmission line is a lossless line. The transmission time per unit length transmission line is Ï„=(LC)1â„2, and the characteristic impedance is ZO=(L/C) 1â„2, where C and L are the distributed capacitance and distributed inductance of the transmission line per unit length. The maximum matching line length of the transmission line is lmax=tÏ„v/k, where tÏ„ is the leading edge time of the transmitted signal, v is the propagation speed of the electromagnetic wave in the transmission line, and when using the polyethylene line is 2×108 m/s, k is the empirical constant. Often take 4-5. If the length of the transmission line exceeds lmax, impedance matching should be performed at the beginning and at the end. Otherwise, the signal will be severely distorted due to impedance mismatch. Here I will further illustrate the transmission line between the VCD machine core DSP signal output terminal and the MPEG board. A 10cm long bundle wire and a 60cm long flat cable were used as the transmission line for comparison experiments. The bundle wire was used as the experiment. The waveform of the DSP output and the input end of the MPEG board was basically the same with the YOKOGAWA DL-1540 digital wave. TÏ„10 ns, its lmax = 50cm, so the bundle length is less than lmax, so impedance matching is not necessary. When the bundled wire is changed to a flat cable of 60 cm in length, according to the waveform, after the flat cable is replaced, the waveform distortion becomes significantly larger, mainly because the rising edge is deteriorated, the rise time tÏ„ becomes larger, and the peak-to-valley ratio of the waveform becomes larger. The reason is that the length of the flat cable is greater than lmax, the transmission cable is to be treated as a long line, and its impedance must be matched. The rise time of the DSP output is longer because the reflection coefficient of the reflected wave reflected from the DSP output is positive and negative, and the peak and trough are formed to make the rise time longer. The DATA and LRCK waveforms have similar conditions.
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 shorten the length of the transmission line as much as possible.
(1) The appropriate output of the DSP output is matched 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 rising/falling of the digital signal pulse.
(2) shorten the length of the bundled wire to l<
(4) Adding a shaping circuit can reduce the interference noise caused by the mismatch of the connecting lines. The shaping circuit is usually added before the input end, but care must be taken not to cause a new phase change of the signal.
3. Crosstalk suppression of digital signals
The so-called crosstalk refers to the signal transmission line causing severe interference noise on its adjacent signal lines during the transmission of signals, mostly occurring between flat conductors, bundled conductors or parallel printed conductors on printed circuit boards. The strength of the crosstalk is related to the mutual impedance between the adjacent 2 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 there are many advantages, if they are used improperly, crosstalk is likely to occur, which affects the normal operation of digital products. There is a distributed capacitance between the wires of the flat cable. After measurement, the distributed capacitance between adjacent wires of 10 cm length is about 3 pF. When the frequency is 100MHz, the impedance of the 1pF capacitor is 1.6kΩ, and the distributed capacitance of the flat cable conductor is proportional to its length, and the crosstalk is more serious when the wiring is long. Taking the VCD machine as an example, the signal is a square wave of hundreds of kilohertz, several megahertz and a clock signal of 10 to 20 MHz, which contains dozens of times of higher harmonics, and the signal spectrum is the highest near several hundred megahertz. The high frequency components are easily crosstalked by the distributed capacitance between the wires of the flat cable. Through comparison experiments, the DSP and MPEG boards were connected by a 60cm long flat cable and a 10cm long bundle wire respectively. It is found that the interference on the 60cm flat cable is much larger than the interference on the l0cm long bundle line, indicating the distribution of the flat cable. The capacitance 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 point is significantly reduced and the interference pulse amplitude is reduced. This shows that most of the interference comes from the BCK square wave signal, and controlling the distance between the wires can reduce the interference. The following measures were taken in the car VCD:
(1) Shorten the transmission length of the signal line as much as possible.
(2) When transmitting signals of various levels, the level signals of the same level with similar front and rear time should be classified as a group of transmissions. 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 the double-sided printed circuit board is routed, the high-frequency digital signal and the clock signal 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, so that the distributed capacitance between the parallel wires is close to the ground plane. When the time is smaller, the crosstalk between the signal lines will be reduced. When the MPEG chip, DRAM, SDRAM and other high-speed digital devices are printed on the printed circuit board, a large ground wire and a ground bypass shielding device are disposed on the back surface. High frequency impulse noise generated.
Fourth, the anti-jamming design of digital signal processing system
In fact, the induced voltage drop caused by the change of the power line current, the reflected interference of the digital signal transmission, and the crosstalk between the digital signals are closely related and inseparable. Reflected in digital signal processing systems, the most harmful is high frequency impulse noise. Therefore, suppressing high frequency impulse noise is an important part of the electromagnetic compatibility design of digital AV products.
For example, when the whole machine is working in the VCD debugging process, the function error occurs. The built-in detection program detects the connection between the CPU and the MPEG chip CL680A1, and the oscilloscope observes that the high-frequency glitch on the HRDY and HCK is large, and adopts a parallel connection on the HRDY. The 51pF capacitor uses a flip-flop to shape the HCK. The built-in detection program detects the accuracy of data communication to 100%, and the whole machine works perfectly. In order to improve the anti-jamming performance of the system, the following measures can be taken in digital AV products:
(1) Increase the anti-interference ability of the bus. The three-state gate bus structure is adopted, and the bus plus the pull resistor makes the bus at a stable high level in an instant and eliminates the floating state in which the bus is in an unstable voltage, and the bus must be buffered.
(2) Use software to eliminate interference. In the system design, although the hardware has been improved, it is impossible to completely eliminate the interference, such as system "dead" and data transmission errors, etc., can be improved from the software: use the Watch Dog Timer Detect whether the system is disturbed. Once the system is disturbed, immediately take the interrupt system and re-initialize it before starting to eliminate the interference. The use of software fault tolerance technology is an acknowledgement of faults and errors is an objective fact, and consider measures to eliminate, suppress, reduce the impact.
(3) Improve the anti-interference ability of the system control signal. In the system, there are usually control lines such as RESET and STB. The transmission distance between the CPU and its control device is long and the control line impedance is high, which is susceptible to impulse noise interference. A 20pF capacitor is connected to the peer control signal end of the controlled device. Eliminate interference, and connect the 0.01μF capacitor to the control signal such as RESET. The interference problem can also be solved. Adding a buffer driver to the control line makes the impedance of the control line low, and also has the effect of suppressing interference.
(4), the processing of the unused end of the IC. The unused terminals must be properly handled, otherwise the noise can easily interfere with the circuit through the distributed capacitance. For example, the TTL, CMOS circuit has a pull-up resistor of 1~10kΩ at the unused end, and a small-capacity ceramic capacitor is connected in parallel with the output terminal of the flip-flop.
V. Conclusion
Internationally, the EMC design of electronic products is highly valued, and the electromagnetic compatibility standards of electronic products such as Europe, America, and Japan are enforced. In the design and trial production process of automotive digital AV products, EMC design should be regarded as an important part of the design process. The component selection, circuit board design and overall layout of the whole machine should be strictly in accordance with the anti-interference design requirements of digital circuits.
Speaker Box Co., Ltd. , http://www.nsspeaker.com