Electromagnetic compatibility design

1. Choose a reasonable wire width

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

2. Adopt the correct wiring strategy

The use of equal traces can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is better to use a well-shaped mesh wiring structure. The specific method is that one side of the printed board is laterally routed, and the other side is longitudinally routed. Metallized holes are then connected at the intersection holes.

3. In order to suppress the crosstalk between the printed circuit board wires, the long distance equalization should be avoided when designing the wiring. 

Pull the distance between the wires and the wires as much as possible. The signal wires and ground wires and power cables should not be as close as possible. cross. A grounded trace is placed between some signal lines that are very sensitive to interference, which effectively suppresses crosstalk.

4. In order to avoid the electromagnetic radiation generated when the high-frequency signal passes through the printed wiring, the following points should also be noted when wiring the printed circuit board:

(1) Minimize the discontinuity of the printed conductor. For example, the width of the conductor should not be abrupt, and the corner of the conductor should be greater than 90 degrees to prohibit the loop.

(2) The clock signal lead is most likely to cause electromagnetic radiation interference. The wiring should be close to the ground loop, and the driver should be next to the connector.

(3) The bus driver should be close to the bus it is intended to drive. For leads that leave the printed circuit board, the drive should be next to the connector.

(4) The wiring of the data bus should sandwich a signal ground between every two signal lines. It is best to place the ground loop next to the least important address leads, as the latter often carry high frequency currents.

(5) When placing high-speed, medium-speed, and low-speed logic circuits on the printed board, the devices should be arranged in the manner shown in Figure 1.

5. Suppress reflection interference

In order to suppress the reflection 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, a matching resistor of the same resistance value is added to the ground and the power supply end at the end of the transmission line. 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 resistor should be determined according to the output drive current of the integrated circuit and the maximum value of the sink current.

6. Differential signal line routing strategy used in board design

The differential signal pairs that are in close proximity to each other are also tightly coupled to each other. This mutual coupling reduces EMI emissions. Usually (and of course some exceptions) differential signals are also high-speed signals, so high-speed design rules are also generally applicable. This is especially true for the wiring of differential signals, especially when designing signal lines for transmission lines. This means that we must design the routing of the signal lines very carefully to ensure that the characteristic impedance of the signal lines are continuous throughout the signal line and remain constant. During the placement and routing of differential pairs, we want the two PCB lines in the differential pair to be exactly the same. This means that in practical applications, every effort should be made to ensure that the PCB lines in the differential pairs have exactly the same impedance and that the length of the wiring is exactly the same. Differential PCB lines are usually always wired in pairs, and the distance between them remains constant at any position along the direction of the pair. In general, the layout of differential pairs is always as close as possible.