Electromagnetic interference often causes the function of the PCBA board to not be realized, and many output signals will cause relatively large errors due to electromagnetic interference. Therefore, when designing a PCB circuit, we must consider whether this design method and the circuit layout will cause electromagnetic interference to the output signal.

Solving the problem of signal integrity well will improve the electromagnetic compatibility (EMC) of the PCB. It is very important to ensure that the PCB is well grounded.

It is very effective to use a signal layer and a ground layer for complex designs. In addition, minimizing the signal density of the outermost layer of the circuit board is also a good way to reduce electromagnetic radiation. This method can be implemented by using "Build-up" technology to design and manufacture the PCB. "Build-up" is achieved by adding a combination of thin insulation layers and micro-holes used to penetrate these layers on ordinary process PCBs. Resistors and capacitors can be buried under the surface layer, and the trace density per unit area will be nearly doubled. Therefore, the volume of the PCB can be reduced.

The reduction of the PCB area has a huge impact on the topology of the traces, which means a reduced current loop, a reduced branch trace length, and electromagnetic radiation approximately proportional to the area of the current loop. At the same time, the small size means that high-density pin package devices can be used, which in turn reduces the length of the connection, thereby reducing the current loop and improving the electromagnetic compatibility characteristics.

A good PCB designer must have the ability to control the electromagnetic interference of the circuit.

1. The general layout requirements of PCB components.

The layout of circuit components and signal paths must minimize the mutual coupling of unwanted signals.
1) Low-level signal channels should not be close to high-level signal channels and unfiltered power lines, including circuits that can produce transient processes.
2) Separate low-level analog circuits from digital circuits to avoid common impedance coupling between the analog circuits, digital circuits, and the common circuit of the power supply.
3) High, medium and low-speed logic circuits use different areas on the PCB.
4) Arrange the circuit to minimize the length of the signal line.
5) Ensure that there are no excessively long parallel signal lines between adjacent boards, between adjacent layers on the same board, and adjacent wiring on the same layer.
6) The electromagnetic interference (EMI) filter should be as close as possible to the source of electromagnetic interference and placed on the same circuit board.
7) DC / DC converters, switching elements and rectifiers should be placed as close to the transformer as possible to minimize the length of their wires.
8) Place the voltage regulator and filter capacitor as close as possible to the rectifier diode.
9) The printed board is partitioned according to the frequency and current switching characteristics, and the distance between noise components and non-noise components should be further away.
10) The noise-sensitive wiring should not be parallel to high-current, high-speed switching lines.
11) Component layout should also pay special attention to heat dissipation. For high-power circuits, those heat-generating components such as power tubes and transformers should be placed as side-by-side as possible to facilitate heat dissipation. Do not concentrate in one place and do not place high capacitance too close to avoid The electrolyte is prematurely aged.

2. General requirements to avoid PCB layout parameters

1) Increase the spacing of the traces to reduce capacitively-coupled crosstalk.
2) When wiring on two panels, the wires on both sides should be perpendicular, diagonal, or bent to avoid parallel to each other to reduce parasitic coupling. Printed wires used as input and output of the circuit should be avoided as far as possible In order to avoid feedback, it is best to add a ground wire between these wires.
3) Route sensitive high-frequency lines away from high-noise power lines to reduce mutual coupling; high-frequency digital circuit traces are thinner and shorter.
4) Widen the power and ground wires to reduce the impedance of the power and ground wires.
5) Try to use 45° polyline instead of 90° polyline to reduce the external emission and coupling of high-frequency signals.
6) The difference between the length of the address line or the data line should not be too large, otherwise the short line part should be compensated by the artificial curved line.
7) Isolation between high-current signals, high-voltage signals, and small signals (Isolation distance is related to the withstand voltage. Generally, the board should be 2 mm away at 2 kV. To increase, for example, to withstand the 3 kV withstand voltage test, the distance between high and low voltage lines should be more than 3.5 mm. In many cases, to avoid creepage, open between high and low voltage groove).

3. The following circuit measures can be used in actual PCB design

1) It can be used to connect a resistor in series on the PCB trace to reduce the transition rate on the control signal line.
2) Try to provide some form of damping for relays (high-frequency capacitors, reverse diodes, etc.).
3) Filter the signal entering the printed board, and filter the signal from the high-noise area to the low-noise area. At the same time, use a string termination resistor to reduce the signal reflection.
4) The MCU unused terminal should be connected to power or ground through the corresponding matching resistor, or defined as an output terminal. The terminals connected to the power supply and ground on the integrated circuit must not be left floating.
5) The input terminal of the idle and unused gate circuit should not be left floating, but should be connected to power or ground through the corresponding matching resistor. The positive input terminal of the unused op amp is grounded, and the negative input terminal is connected to the output terminal.
6) Set a high-frequency decoupling capacitor for each integrated circuit. Add a small high-frequency bypass capacitor to each electrolytic capacitor.
7) Use large-capacity tantalum or polyester capacitors instead of electrolytic capacitors as charge and discharge energy storage capacitors on the circuit board. When using a tubular capacitor, the case must be grounded.