2.1 The layout of the PCB board

PCB board layout and other logic circuits, should be related to each other as far as possible close to the device, so you can get a better anti-noise effect. Components in the PCB board on the location to fully consider the anti-electromagnetic interference problem. One of the principles is that the leads between the components should be as short as possible. In the layout, the analog signal part, high-speed digital circuit part, the noise source part (such as relays, high current switch, etc.) these three parts are reasonably separated, so that the signal coupling between the smallest.

Clock generator, crystal and CPU clock input are easy to produce noise, to be close to each other. Easy to produce noise devices, small current circuits, high current circuits should be kept away from the logic circuit. If possible, should do other PCB board, which is very important.

PCB board components common layout requirements: circuit components and signal path layout must minimize the mutual coupling of unwanted signals.

1) The low-level signal path can not be close to the high-level signal path and the filterless power supply line, including the circuit that generates the transient process.

2) the low level of analog circuits and digital circuits to separate, to avoid analog circuits, digital circuits and power supply common circuit to generate a common impedance coupling.

3) high, medium and low speed logic circuit in the PCB board to use different areas.

4) When arranging the circuit, make the signal line length minimum.

5) to ensure that between the adjacent board, the same board between the adjacent level, the same level between the adjacent wiring can not have too long parallel signal line.

6) electromagnetic interference (EMI) filter as close as possible to the source of electromagnetic interference, and 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 lead length.

8) Place the voltage regulator and the filter capacitor as close as possible to the rectifier diodes.

9) PCB board by frequency and current switching characteristics of the partition, the noise components and non-noise components to be far between the distance.

10) Noise-sensitive cabling should not be parallel to high current and high speed switching lines.

11) the layout of the components should pay special attention to the issue of heat, for high-power circuit, those who should heat components such as power pipes, transformers, etc. as far as possible to disperse the layout of the layout, easy to heat distribution, do not focus on a place, So that the electrolyte premature aging.

2.2 PCB board wiring

A PCB board is constructed on a vertical stack using a series of laminated, traces and prepreg treated multilayer structures. In the multi-layer PCB board, in order to facilitate debugging, will signal line in the outermost layer.

In the case of high frequency, PCB board alignment, vias, resistors, capacitors, connectors, distributed inductors and distributed capacitance can not be ignored. Resistors produce reflections and absorption of high frequency signals. The distributed capacitance of the trace will also work. When the trace length is greater than 1/20 of the corresponding wavelength of the noise frequency, the antenna effect is generated and the noise is emitted through the trace.

PCB wire connections are mostly done through vias. A vias can bring about 0.5pF of distributed capacitance, reducing the number of vias can significantly increase the speed.

An integrated circuit itself encapsulates the material into a 2 ~ 6 pF capacitor. A connector on a PCB board has a 520nH distributed inductance. A dual inline 24-pin integrated circuit socket, the introduction of 4 ~ 18nH distributed inductance.

To avoid the impact of PCB board distribution parameters should follow the general requirements:

1) Increase the spacing of the traces to reduce the capacitive coupling of the crosstalk.

2) double-sided cabling, the two sides of the wire should be perpendicular to each other, oblique, or curved alignment, to avoid parallel to each other to reduce the parasitic coupling; as the circuit input and output printed wire should try to avoid adjacent parallel , In order to avoid the occurrence of feedback, between these wires is best to add ground wire.

3) the sensitive high-frequency lines in the far away from the high noise power cord to reduce the coupling between each other; high-frequency digital circuit alignment thin, shorter.

4) Widen the power cord and ground to reduce the power line and ground impedance.

5) Try to use a 45 ° polyline without using 90 ° polyline wiring to reduce the high frequency signal from external transmission and coupling.

6) address lines or data lines, the difference between the length of the line should not be too much, otherwise the short-term part of the artificial line for compensation.

7) large current signal, high voltage signal and small signal should pay attention to isolation (isolation distance and bear the pressure to bear, usually 2kV when the board to the distance of 2mm, on the basis of the proportion of the operator also added Large, for example, to withstand the 3kV voltage test, the high and low voltage lines should be between the distance of 3.5mm or more, in many cases to avoid creeping, but also in the PCB board between the high and low pressure slot).

3, PCB board in the circuit design

In the design of electronic circuits, the more consideration is the actual performance of the product, and not too much to consider the product of electromagnetic compatibility (EMC) and electromagnetic interference (EMI) suppression and electromagnetic interference characteristics. In order to achieve the purpose of electromagnetic compatibility, it is necessary to take the necessary measures to increase the necessary additional circuit on the basis of its circuit schematic to improve the electromagnetic compatibility of the product. Actual PCB design can use the following circuit measures:

1) can be used in the PCB alignment on the line in a way to reduce the control signal line up and down along the jump rate.

2) as far as possible for the relay to provide some form of damping (high-frequency capacitors, reverse diodes, etc.).

3) to enter the PCB board signal to filter, from the high noise area to the low noise zone of the signal should also add filtering, while the use of string termination resistor approach to reduce the signal reflection.

4) MCU useless side through the corresponding matching resistor connected to the power supply or ground, or defined as the output. The integrated circuit on the power supply, the ground should be connected, do not vacant.

5) idle Do not use the gate input should not be floating, but through the corresponding matching resistor connected to the power supply or ground. Idle unused op amp is input terminal ground, negative input termination output.

6) Set a high frequency decoupling capacitor for each integrated circuit. Each electrolytic capacitor edge should add a small high frequency bypass capacitor.

7) with a large capacity of tantalum capacitors or polyester capacitors without electrolytic capacitors for the PCB board charge and discharge energy storage capacitor. When using a tubular capacitor, the housing is grounded.