Anti-interference is a very important link in modern circuit design, which directly reflects the performance and reliability of the entire system. For PCB engineers, anti-interference design is the key and difficult point that everyone must master.
The presence of interference in the PCB board
In actual research, it is found that there are four main interferences in PCB design: power supply noise, transmission line interference, coupling and electromagnetic interference (EMI).
1. Power supply noise
In the high-frequency circuit, the noise of the power supply has a particularly obvious influence on the high-frequency signal. Therefore, the first requirement for the power supply is low noise. Here, a clean ground is as important as a clean power source.
2. Transmission line
There are only two types of transmission lines possible in a PCB: strip line and microwave line. The biggest problem with transmission lines is reflection. Reflection will cause many problems. For example, the load signal will be the superposition of the original signal and the echo signal, which will increase the difficulty of signal analysis; reflection will cause return loss (return loss), which will affect the signal. The impact is as serious as that caused by additive noise interference.
3. Coupling
The interference signal generated by the interference source causes electromagnetic interference to the electronic control system through a certain coupling channel. The coupling method of interference is nothing more than acting on the electronic control system through wires, spaces, common lines, etc. The analysis mainly includes the following types: direct coupling, common impedance coupling, capacitive coupling, electromagnetic induction coupling, radiation coupling, etc.
4. Electromagnetic interference (EMI)
Electromagnetic interference EMI has two types: conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiated interference refers to the interference source coupling (interference) its signal to another electrical network through space. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may become radiation interference sources with antenna characteristics, which can emit electromagnetic waves and affect other systems or other subsystems in the system. normal work.
PCB and circuit anti-interference measures
The anti-jamming design of the printed circuit board is closely related to the specific circuit. Next, we will only make some explanations on several common measures of PCB anti-jamming design.
1. Power cord design
According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and the ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability.
2. Ground wire design
Separate digital ground from analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and thick, and the grid-like large-area ground foil should be used around the high-frequency component.
The ground wire should be as thick as possible. If a very thin line is used for the grounding wire, the grounding potential changes with the current, which reduces the noise resistance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the ground wire should be above 2~3mm.
The ground wire forms a closed loop. For printed boards composed only of digital circuits, most of their grounding circuits are arranged in loops to improve noise resistance.
3. Decoupling capacitor configuration
One of the conventional methods of PCB design is to configure appropriate decoupling capacitors on each key part of the printed board.
The general configuration principles of decoupling capacitors are:
① Connect a 10 ~ 100uf electrolytic capacitor across the power input. If possible, it is better to connect to 100uF or more.
②In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If the gap of the printed board is not enough, a 1-10pF capacitor can be arranged for every 4~8 chips.
③For devices with weak anti-noise ability and large power changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.
④The capacitor lead should not be too long, especially the high frequency bypass capacitor should not have lead.
4. Methods to eliminate electromagnetic interference in PCB design
①Reduce loops: Each loop is equivalent to an antenna, so we need to minimize the number of loops, the area of the loop and the antenna effect of the loop. Ensure that the signal has only one loop path at any two points, avoid artificial loops, and try to use the power layer.
②Filtering: Filtering can be used to reduce EMI both on the power line and on the signal line. There are three methods: decoupling capacitors, EMI filters, and magnetic components.
③Shield.
④ Try to reduce the speed of high-frequency devices.
⑤ Increasing the dielectric constant of the PCB board can prevent the high frequency parts such as the transmission line close to the board from radiating outward; increasing the thickness of the PCB board and minimizing the thickness of the microstrip line can prevent the electromagnetic wire from overflowing and also prevent radiation.