1 Introduction

With the continuous development of science and technology, train toward the high-speed development, gradually train car system with high-speed digital circuits. On the train, there are many sources of interference, electromagnetic interference, including various types of transformers, fans, pantographs, air compressors, etc., affect the normal work of the high-speed digital circuit train. In addition, in order to ensure the comfort of the ride and working environment, the car is also equipped with air conditioning, electric heaters, ventilators and other types of electrical equipment, the same external generate electromagnetic radiation, affect the normal work of the high-speed digital circuit. Such a complex environment in the train how to ensure that the high-speed digital signal reliable, and will become particularly important. These problems if you do not handle will cause signal distortion, timing error, system instability, and many other circumstances, will bring inestimable loss.

Guarantee the train communication , control system normal operation, the anti-jamming design and functional design is equally important. Must be taken into account in the design of the early digital circuit interference suppression issue, it would be difficult to achieve high-speed digital circuits immunity requirements. Should first improve the digital board the anti-jamming ability to reduce the circuit radiation, avoid to go after the design is completed remedies for anti-jamming circuit board .

2 interfere with formation

Three basic ways:

interfere with the formation of the interference source, coupling path, sensitive source. The following described from several aspects.

2.1 PCB board interference coupling path

PCB circuit board interference interference common mode and differential mode interference. The differential mode interference is generated by the signal circuit, common mode interference is generated by the common mode currents on the cable. For printed circuit board mainly refers to the difference mode interference, because the frequency range of the differential mode interference circuit signal occupies the entire band, not only through the wire is coupled to each sensitive to the source device, the current loop will be coupled into the external various types of interference, affecting the normal working system. The main way to reduce the differential mode interference short traces to minimize the length of the wiring to reduce the signal loop area.

2.2 PCB circuit board interference source generates

The main causes

high-speed digital circuit all kinds of interference by the power the inherent noise frequency and external line on all kinds of change di / dt, du / dt generated because there is a power circuit on the circuit board, signal circuits, various types of high-speed signal circuit and ancillary line capacitive, inductive load, so when hopping signal generator will produce a spike impact the formation of noise, which noise conduction through the loop current ring along, and should therefore be Inhibition of power inherent noise and noise caused by a variety of high-speed digital transition. Inhibition of the circuit itself or is the various types of the the mutant signal generating noise, the best suppression mode decoupling and filtering. This will not only reduce the own noise its able to absorb external impact, and improve the anti-jamming capability. Figure 1 schematically illustrates the noise generated in each stage of the circuit.

respective circuit of the Figure 1 stage of the generation of noise

Sensitive source

2.3 PCB circuit board

Sensitive to the source of the high speed digital signal is mainly refers to the object susceptible to external interference, e.g.: the A / D, D / A converter, logic controller microcontroller, crystal, digital IC , the weak signal amplifier. The stability of these devices is directly related to the stability and accuracy of the system board, and to do so for these sensitive source protection and improve the anti-jamming capability.

3 anti-jamming measures to improve the PCB board

3.1 reduced coupling loop

Reducing the coupling method is to reduce the signal loop area, which should solve the edge of the loop in the ground, power, sensitive to the signal source and the board area.

3.1.1 reduce the ground, the power coupling loop

The

ground impedance is the main reason for the circuit board ground noise, and should therefore minimize ground impedance can be taken to the ground plane or grid places.

Use of multilayer

For high-speed digital circuit board should, in order to reduce the loop area, as a power supply or formation of the intermediate layer, and the spacing of adjacent layers try to ensure that the power supply and ground as small as possible; each signal layer has a corresponding to the ground layer, and the annular area constituted by the signal line to its ground loop should be as small as possible, the loop area of ​​the smaller, less external interference. For this feature, the segmentation of the ground plane, to take into account the distribution of the ground plane and the signal traces, to prevent signal line can not be due to the ground plane slotted brought across the ground and power planes separated area, to prevent the formation of large ground loop. Power source layer should be reduced than the ground layer a distance of about 3 mm, and so will be able to inhibit more than 70% of the power supply interference. As shown in Figure 2.

2 power planes than the ground layer shrink schematic diagram

Reduce the the sensitive source signal coupling loop

3.1.2

The

for sensitive signals such as: periodic signal, such as the < FONT color = # 020346> clock signals, analog signals, address bus low signal interference is strong, is also key to the design of high-speed digital circuit. Key signal wiring on the printed circuit board should be in accordance with the principles of high to low traces (Sort: high to low: analog signal - reset signal-I2C-clock signal - read and write signals - high-speed, RF signal - data bus - address bus); critical signal wiring try to walk the inner layer; filtering with small capacitors in parallel; signal layers only through isolated from the ground plane of the two layers can parallel lines; signal line should be as far as possible so that it shortest interconnect; the the element as close as possible to the traces short; sensitive sources in order to reduce the high-frequency signal distribution parameters and electromagnetic interference, so that it can improve the anti-jamming capability .

3.1.3 reduce board edge coupling loop

The board edge processing

printed circuit board is reasonable to determine whether it can be more effective inhibition of the external signal interference. Foreign interference, should strictly control the position of the wiring, and let it as close as possible to the printed circuit board internal side of the plate to prevent high-speed digital circuits. The high-frequency disturbances such as a strong signal line should not come to the edge of the board to prevent no corresponding strata coupling loop, to generate the signal external interference leakage, as shown in Figure 3:

Figure 3 circuit board side of the coupling loop

3.2 sources of interference suppression

Sources of interference suppression is to be reduced as much as possible the source of interference du / dt, di / dt impact. Du / dt to reduce the sources of interference, mainly through capacitor in parallel across the source of interference, to increase decoupling and filtering to achieve. Reduce di / dt of the source of interference, mainly through a series the sources of interference inductance or increase the freewheeling diode, In example: in relay freewheeling diode can eliminate the disconnect coil back EMF generated interference .

The increase in

3.2.1 decoupling capacitance

Decoupling capacitor is to the interference of the output signal as the filtered objects. Additional parallel on-chip decoupling capacitor can eliminate the power self-excitation and inhibition of the impact of the changes in the level, the impact of power supply noise level changes can make the shortest path back to the ground, and to increase the anti-interference ability. For good noise suppression, as far as possible for each chip with decoupling capacitors, and decoupling capacitors as close as possible to the chip power and ground pins placed. Decoupling capacitor value is typically 0.01-0.1uf accordance with C = 1 / F, both 10MHZ take 0.1uf 100MHZ take 0.01uf, the higher the frequency, the smaller the decoupling capacitor value should be.

3.2.2 circuit filter absorption

Prone to glitches mutation signal should take the form of the corresponding filter to suppress high frequency of mutations of the high-speed signal glitch. Filtering methods generally use the capacitance or inductance of the passive element with the resistance, the use of the voltage and current of the energy storage characteristics of the purpose of filtering. Often using a RC filter circuit, when the voltage suddenly increased, the parallel capacitance C is able to store energy, and the energy is released when the voltage drop, so that the filtered load voltage is relatively smooth, and reduce the high frequency noise. But in order not to affect the normal high frequency signal waveform, not value is too large, to use a small capacitor. According to the total impedance of the circuit and the high-frequency signal bandwidth, rise time, according to the calculation and experience come to select the size of the filter capacitor C Reference Table 1: higher system operating frequency filter capacitor value should be as small.

table filter capacitor in the case of a variety of selected

4 Conclusion

Anti-jamming design reliability

high-speed digital circuit having a profound impact on the overall performance of the entire electronic and electrical equipment, the reliability of any of the products should be designed to the source, only take a firm hold of the reliability of the design to printed circuit ensure the reliability of the product actually improving the reliability of the printed circuit board.

Through various icon can be seen that in accordance with the the method improved high-speed digital printed circuit board can be reduced self-generated noise, and improve the anti-jamming capability. Economic considerations from the R & D costs, the anti-interference problems early in the design process to consider the circuit will be able to save a lot of repetitive design costs. This method of promotion within the Technology Center of the Times Electric Company achieved good practice, improve veneer disposable probability of success, which saves design cost, and improve design efficiency.