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At present, electronic equipment is used in various electronic devices and systems, and themain assembly method is still based on PCB. Practice has proved that even if the schematic drawing is correct but PCB design is not proper, it will cause something bad influence on the reliability. For example, if the 2 thin parallel lines are too close to each other, a delay will be in the signal wave, and reflected noise at the end of the  transmission line. Therefore, when designing the PCB, we should take care and use the correct method.

1. GND line design in electronic equipment, grounding is an important method to control the interference

If we can combine the grounding and shielding properly, then most of the interference problems can be solved. The GND structures in the electronic device like system ground, chassis ground (shield ground), digital ground (logically) and simulation ground. Pay attention to the following points in the GND design:

①. Select the single-point GND and multi-point GND in low-frequency circuit correctly,  when the signal frequency is ＜1MHz, the influence between the layout and the device is small while the interference is big due to the circulating current formed by the GND   circuit, so grounding should be adopted a little. When the signal frequency is ＞10MHz,  the ground impedance becomes very big. At this time, the ground impedance should be  reduced and use the GND points around as many as possible. When the frequency is   between 1 and 10 MHz, if we use the GND a little bit, then the length of other GND   should not exceed 1/20 of the wavelength. Otherwise, should use the multi-point GND  method.

②. Separate the digital circuit from the simulated circuit. The circuit board has both high-speed logic circuits and linear circuits， they should be separated as much as possible,   and their GND can not be mixed but should connected to the GND of the power end.   Try to increase the grounding area of the linear circuit.

③. Try to thicker the GND wire as much as possible if it is too thin, and the GND    electric potential changes with the change of the current, which causes the timing signal  of the electronic device to be unstable and the anti-noise performance to be destroyed.   Thus, the ground wire should be as thick as possible so that it can pass the allowable   currents on PCB. If possible, the width of the GND wire should be greater than 3mm. 

④. Make the GND wire to be a closed-loop circuit, when GND wire consist of digital   circuits only connects to the GND system, then it can significantly improve the ability of anti-noise. The reason is that there are many integrated circuit components on PCB,  especially when encounter with power-consuming components, due to the limitation of   the THK of GND wire, a large electric potential difference will be generated on GND  structure causing the noise resistance to be decreased. If turn the ground structure into  closed-loop, then the electric potential difference will be reduced to improve the ability  of noise resistance in the electronic devices.

2. EMC design

EMC refers to the ability of the electronic devices to work in a coordinated and efficient manner in various of electro magnetic environments. The purpose of the EMC design is  to enable the electronic device to suppress various of external interferences, make sure the electronic device to work normally in a specific electro magnetic environment, and at the same time reduce the electro magnetic interference of the electronic device itself to other electronic devices.

①. Choose a reasonable width of the conductor line. Since the interference generated by  the transient current on the printed wire is mainly caused by the inductance component  of the printed conductor line, therefore, the inductance of printed conductor line should be minimized as much as possible. The inductance of printed conductor line is proportional to its length and inversely proportional to its width, so short and fine conductors line are advantageous for suppressing interference. Signal lines for clock leads, row drivers, or    trunk drivers often carry large transient currents, so the printed conductor lines should be short as much as possible. For the discrete component circuit, when the width of printed  conductor line is about 1.5mm, then the requirement can be fully satisfied; for integrated circuit, the width of the printed conductor lines can be selected between 0.2 and 1.0mm.

②. Adopt the correct wiring layout strategy. Use equal traces can reduce wire inductance but the mutual inductance and distributed capacitance between the conductor wires will be increased. If the layout allows, use the # shaped mesh wiring layout structure. The detailed method is that one side is horizontal and the other side is vertical, and use the PTH holes to connect each other at the crossing position.

In order to restrain the crosstalk between the PCB wires, wires with long distance should be avoided when designing the wiring layout, and try to pull out the distance as far as  possible between the wires. Signal wires, GND wires and power wires can not be crossed. Design a ground trace among some signal wires which are very sensitive to interference, it can effectively restrain the crosstalk. 

In order to avoid the electro magnetic radiation generated by the high-frequency signal   when passing through the printed conductor lines, the following points should be noted  when designing the PCB layout:

●Minimize the discontinuity of the printed conductor line. For example, the width of the conductor wire should not be changed, and the corner of the conductor wire should be   greater than 90 degrees to prohibit the loop wires.

●The clock signal lead is easy to cause the interference of electromagnetic radiation. It   should be close to the ground loop when designing the layout, and the driver should be  tightly next to the connector.

●The trunk driver should be close to its trunk that intends to drive. For those leading    wires that leave the PCB, the drive should be next to the connector.

●The wire layout of the data bus should be: insert a signal ground wire between every 2 signal lines. Place the ground loop close to the unimportant wire leads, which often  carry high frequency currents.

●Array the components when placing high-speed, medium-speed and low-speed logical    circuits in PCB

③. Restrain the reflection of interference. In order to restrain the reflection of interference that occurs at the end of the printed line, in addition to special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used. Terminal matching can be added if necessary, that is, matching the resistance with the    same value by adding it at the end of transmission line and the power end. According to experience, for the TTL circuit with faster speed, the terminal matching method should  be adopted when the printed line is longer than 10cm. The resistance of the matching one should be determined according to the output driving current and the Max. value of the absorption current of the integrated circuit.