PCB Blog - Anti-jamming design of high-speed digital PCB board for train

The problems that should be paid attention to in the design of high-speed digital printed circuit boards are clarified in detail, and what aspects will be affected by high-speed digital circuits in the on-board system of trains, as well as the reasons for these effects. Several approaches should be taken. Practice has proved that the circuit designed by these methods can greatly improve the anti-interference performance of the product. With the continuous development of science and technology, the train is also developing at a high speed, and the high-speed digital circuit is gradually adopted in the on-board system of the train. There are many sources of interference on the train, including electromagnetic interference generated by various transformers, fans, pantographs, air compressors, etc., which affect the normal operation of high-speed digital circuits in the train. In addition, in order to ensure the comfort of the riding environment and working environment, the car is also equipped with various electrical equipment such as air conditioners, electric heaters, ventilators, etc., which also generate electromagnetic radiation externally, which affects the normal operation of high-speed digital circuits. Therefore, in such a complex environment on trains, how to ensure the reliability of high-speed digital signals will become particularly important. If these problems are not handled properly, it will lead to signal distortion, timing errors, system instability and many other situations, which will bring inestimable losses. In order to ensure the normal operation of train communication, control and other systems, the anti-interference design of the equipment is as important as the functional design. At the beginning of the design, the suppression of digital circuit interference must be considered, otherwise it is difficult to meet the anti-interference requirements of high-speed digital circuits. Therefore, it is necessary to improve the anti-interference ability of the digital circuit board and reduce the circuit radiation, and avoid taking the remedial measures for the anti-interference of the circuit board after the design is completed.

1. The way of interference
There are three basic ways of interference formation: interference source, coupling path, and sensitive source. These aspects are described below.
1.1 Interference coupling path of PCB board
The interference on the circuit board mainly includes common mode interference and differential mode interference. Differential mode interference is generated by the signal loop, and common mode interference is generated by the common mode current on the cable. For the printed circuit board, it mainly refers to its differential mode interference, because the frequency range of its differential mode interference is the entire frequency band occupied by the circuit signal. All kinds of interference, affecting the normal working system. The main method to reduce differential mode interference is to minimize the trace length and reduce the signal loop area during wiring.
1.2 The generation method of interference sources on the PCB board
The main reasons for various types of interference in high-speed digital circuits are the inherent noise frequency of the power supply itself and various changes in di/dt and du/dt on external circuits. All kinds of capacitive and inductive loads on the line, so when the signal jumps, a spike will be generated to form noise, and these noises will be conducted along the circuit through the current loop of each circuit, so the inherent noise of the power supply itself and various noise caused by high-speed digital transitions. Decoupling and filtering are the best way to suppress the noise generated by the circuit itself or various types of mutation signals. This not only reduces its own noise, but also absorbs external influences on it, and improves its anti-interference ability.
1.3 Sensitive sources on the PCB board
Sensitive sources of high-speed digital signals mainly refer to objects that are susceptible to external interference, such as: A/D, D/A converters, logic controllers, microcontrollers, crystal oscillators, digital ICs, weak signal amplifiers, etc. The stability of these devices is directly related to the stability and accuracy of the system operation of the circuit board. Therefore, it is necessary to properly protect these sensitive sources and improve their anti-interference ability.

2. Improve the anti-interference measures of the PCB board
2.1 Reduce the coupling loop
The main method to reduce coupling is to reduce the signal loop area, among which the loop area of ground wire, power supply, sensitive signal source and board edge should be mainly solved.
1) Reduce the ground wire and power coupling loop
The ground wire impedance is the main cause of the ground wire noise on the circuit board, so the ground wire impedance should be minimized, and the ground plane or grid ground can be used. For high-speed digital circuit boards, multi-layer boards should be used to reduce the loop area, and the middle layer should be used as the power supply or ground layer, and the distance between the layers adjacent to the power supply and the ground should be as small as possible; let each signal layer have a corresponding ground The area of the ring formed by the signal line and its ground loop should be as small as possible. The smaller the ring area, the less external interference. In view of this feature, when dividing the ground plane, the distribution of the ground plane and important signal traces should be considered to prevent problems caused by ground plane slotting, etc., signal lines cannot cross the separation area between the ground plane and the power plane to prevent the formation of large ground return. At the same time, the power layer should be retracted by about 3 mm from the ground layer, so that more than 70% of the power interference can be suppressed.
2) Reduce the coupling loop of the sensitive source signal
For sensitive signals such as periodic signals, such as clock signals, analog signals, and low-order signals of the address bus, the interference is strong, which is also the key to designing high-speed digital circuits. The wiring of key signals on the printed board should be routed according to the principle of high to low (sorting method: high to low: analog signal - reset signal - I2C - clock signal - read and write signal - high speed, radio frequency signal - data bus - address bus ); the key signal wiring should go to the inner layer as far as possible; and a small capacitor should be connected in parallel for filtering; the signal layer can be routed in parallel only through the two layers separated by the ground plane; the signal line should be as short as possible; The components of the high-frequency connection on the printed board should be as close to the traces as possible to reduce the distribution parameters and electromagnetic interference of high-frequency signals, so as to improve the anti-interference ability of sensitive signal sources.
3) Reduce the coupling loop at the edge of the circuit board
Whether the edge processing of the printed circuit board is reasonable determines whether the external interference of the signal can be suppressed more effectively. In order to prevent the high-speed digital circuit from interfering with the outside through the edge of the board, its wiring position should be strictly controlled, and it should be as close as possible to the inside of the printed board. Signal lines with strong interference such as high frequency should not go to the edge of the board to prevent the lack of a corresponding ground coupling loop, resulting in external interference leakage of the signal.
2.2 Suppress interference sources
Suppressing the interference source is to reduce the influence of the interference source du/dt and di/dt as much as possible. Reducing the du/dt of the interference source is mainly achieved by connecting capacitors in parallel at both ends of the interference source and adding decoupling and filtering. Reducing the di/dt of the interference source is mainly achieved by connecting the inductance in series with the interference source or adding a freewheeling diode. For example, adding a freewheeling diode in the relay can eliminate the back EMF interference generated when the coil is disconnected.
1) Increase the decoupling capacitor
The decoupling capacitor takes the interference of the output signal as the filtering object. Adding a parallel decoupling capacitor to the chip can eliminate the power supply self-excitation and suppress the impact of level changes, which can make the power supply noise and the impact of level changes return to the ground wire with a short path to increase the anti-interference ability. In order to suppress noise well, each chip should be equipped with decoupling capacitors as much as possible, and the decoupling capacitors should be placed as close as possible to the power and ground pins of the chip. The value of the decoupling capacitor is generally 0.01-0.1uf, which can be C=1/F, that is, 10MHZ takes 0.1uf, 100MHZ takes 0.01uf, the higher the frequency, the smaller the decoupling capacitor value should be.
2) circuit filter absorption
Corresponding filtering forms should be adopted for the mutation signals that are prone to burrs to suppress the high-frequency burrs generated by the mutation of high-speed signals. The filtering method generally uses passive element capacitors or inductors with resistors, and uses its energy storage characteristics for voltage and current to achieve the purpose of filtering. The RC filter circuit is often used. When the voltage suddenly rises, the parallel capacitor C can store energy, and when the voltage drops, the energy is released, so that the voltage after load filtering is relatively smooth and high-frequency noise is reduced. However, in order not to affect the normal high-frequency signal waveform, the value should not be too large, and a small capacitor should be used as much as possible. According to the total impedance of the circuit and the bandwidth and rise time of the high-frequency signal, the selection size of the filter capacitor C is obtained according to calculation and experience. Refer to Table 1 below: the higher the system operating frequency, the smaller the value of the filter capacitor should be.

3. Conclusion
The reliability of the anti-interference design of high-speed digital circuits has a profound impact on the overall performance of the entire electronic and electrical equipment. The reliability of any product should start from the source of the design. Only by grasping the reliability design of printed circuits can the reliability of the product be guaranteed. Reliability, truly improve the reliability of printed boards. It can be seen from various diagrams that the improved high-speed digital printed circuit board according to the method can reduce the noise generated by itself and improve its anti-interference ability at the same time. Considering the economical consideration of research and development costs, considering the anti-interference problem of the printed circuit boards at the early stage of design will save a lot of repetitive design costs.