How to improve anti-interference ability and electromagnetic compatibility

How to improve anti-interference ability and electromagnetic compatibility when developing electronic products with processors?

1. The following systems should pay special attention to anti-electromagnetic interference:

• (1) A system where the microcontroller clock frequency is extremely high and the bus cycle is extremely fast.
• (2) The system contains high-power, high-current drive circuits, such as spark-producing relays, high-current switches, etc.
• (3) A system containing a weak analog signal circuit and a high-precision A/D conversion circuit.

2. Take the following measures to increase the anti-electromagnetic interference capability of the system:

(1) Choose a microcontroller with low frequency:

Choosing a microcontroller with a low external clock frequency can effectively reduce noise and improve the system's anti-interference ability. For square waves and sine waves of the same frequency, the high-frequency components in the square wave are much more than that in the sine wave. Although the amplitude of the high-frequency component of the square wave is smaller than the fundamental wave, the higher the frequency, the easier it is to emit as a noise source. The most influential high-frequency noise generated by the microcontroller is about 3 times the clock frequency.

(2) Reduce distortion in signal transmission

Microcontrollers are mainly manufactured using high-speed CMOS technology. The static input current of the signal input terminal is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output terminal of the high-speed CMOS circuit has a considerable load capacity, that is, a relatively large output value. The long wire leads to the input terminal with quite high input impedance, the reflection problem is very serious, it will cause signal distortion and increase system noise. When Tpd>Tr, it becomes a transmission line problem, and problems such as signal reflection and impedance matching must be considered.

The delay time of the signal on the printed board is related to the characteristic impedance of the lead, which is related to the dielectric constant of the print circuit board material. It can be roughly considered that the transmission speed of the signal on the printed board leads is about 1/3 to 1/2 of the speed of light. The Tr (standard delay time) of the commonly used logic phone components in a system composed of a microcontroller is between 3 and 18 ns.
 On the printed circuit board, the signal passes through a 7W resistor and a 25cm-long lead, and the delay time on the line is roughly between 4~20ns. In other words, the shorter the signal lead on the printed circuit, the better, and the longest should not exceed 25cm. And the number of vias should be as small as possible, preferably no more than two.

When the signal's rise time is faster than the signal delay time, it must be processed in accordance with fast electronics. Consider transmission at this time
Line impedance matching. For signal transmission between integrated blocks on a printed circuit board, the situation of Td>Trd should be avoided. The larger the printed circuit board, the faster the system speed cannot be.
Summarize a rule of printed circuit board design with the following conclusions:
The signal is transmitted on the printed board, and its delay time should not be greater than the nominal delay time of the device used.

(3) Reduce the cross interference between signal lines:

At point A, a step signal with a rise time of Tr is transmitted to terminal B through lead AB. The delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the signal from point A, the signal reflection after reaching point B and the delay of the AB line, a page pulse signal with a width of Tr will be induced after Td time. At point C, due to the transmission and reflection of the signal on AB, a positive pulse signal with a width of twice the delay time of the signal on the AB line, that is, 2Td, is induced. This is the cross*interference between signals. The intensity of the interference signal is related to the di/at of the signal at point C and the distance between the lines. When the two signal lines are not very long, what you see on AB is actually the superposition of two pulses.

The micro-control made by CMOS technology has high input impedance, high noise, and high noise tolerance. The digital circuit is superimposed with 100~200mv noise and does not affect its operation. If the AB line in the figure is an analog signal, this interference becomes intolerable. For example, the printed circuit board is a four-layer board, one of which is a large-area ground, or a double-sided board, and when the reverse side of the signal line is a large-area ground, the cross* interference between such signals will be reduced. The reason is that the large-area ground reduces the characteristic impedance of the signal line, and the reflection of the signal at the D end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant of the medium from the signal line to the ground, and proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, to avoid the interference of the digital circuit signal line CD to AB, there should be a large area under the AB line, and the distance between the AB line and the CD line should be greater than 2~3 times the distance between the AB line and the ground. It can be partially shielded, and ground wires are placed on the left and right sides of the lead on the side with the lead.

(4) Reduce the noise from the power supply.

While the power supply provides energy to the system, it also adds noise to the power supply. The reset line, interrupt line, and other control lines of the microcontroller in the circuit are most susceptible to interference from external noise. Strong interference on the power grid enters the circuit through the power supply. Even in a battery-powered system, the battery itself has high-frequency noise. The analog signal in the analog circuit is even less able to withstand the interference from the power supply.

(5) Pay attention to the high-frequency characteristics of printed wiring boards and components.

Under high-frequency conditions, the distribution inductance of the leads, vias, resistors, capacitors, and connectors on the printed circuit board is related to
The capacitance etc. cannot be ignored. The distributed inductance of the capacitor cannot be ignored, and the distributed capacitance of the inductor cannot be ignored. The resistance produces the reflection of high-frequency signals, and the distributed capacitance of the lead will play a role. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect will be produced, and the noise will be radiated outward through the lead.

The vias of the printed circuit board cause approximately 0.6pf of capacitance. The packaging material of an integrated circuit itself introduces 2~6pf capacitance. A connector on a circuit board has a distributed inductance of 520nH. A dual-in-line 24-pin integrated circuit skewer introduces 4~18nH distributed inductance. These small distribution parameters are negligible in this line of low-frequency microcontroller systems; special attention must be paid to high-speed systems.

(6) The layout of components should be reasonable.

The position of the components arranged on the printed circuit board should be fully considered for anti-electromagnetic interference. One of the principles is that the leads between the components should be as short as possible. In the layout, the analog signal part, the high-speed digital circuit part, and the noise source part (such as relays, high-current switches, etc.) should be reasonably separated to minimize the signal coupling between them.

G Handle the ground wire on the printed circuit board. The power wire and ground wire are the most important. The most important means to overcome electromagnetic interference is to ground. For double panels, the ground wire layout is particularly particular. Through the use of single-point grounding, the power supply and ground are connected to the printed circuit board from both ends of the power supply. The power supply has one contact and the ground has one contact. On the printed circuit board, there must be multiple return ground wires, which will be gathered on the contact point of the return power supply, which is the so-called single-point grounding. The so-called analog ground, digital ground, and high-power device ground splitting refer to the separation of wiring, and finally all converge to this grounding point. When connecting with signals other than printed circuit boards, shielded cables are usually used. For high frequency and digital signals, both ends of the shielded cable are grounded. One end of the shielded cable for low-frequency analog signals should be grounded. Circuits that are very sensitive to noise and interference or circuits that are particularly high-frequency noise should be shielded with a metal cover.

(7) Make good use of decoupling capacitors.

A good high-frequency decoupling capacitor can remove high-frequency components as high as 1GHZ. Ceramic chip capacitors or multilayer ceramic capacitors have better high-frequency characteristics. When designing a printed circuit board, a decoupling capacitor must be added between the power and ground of each integrated circuit. The decoupling capacitor has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit, which provides and absorbs the charging and discharging energy at the moment of opening and closing the integrated circuit; on the other hand, it bypasses the high-frequency noise of the device. A typical decoupling capacitor of 0.1uf in digital circuits has a distributed inductance of 5nH, and its parallel resonance frequency is about 7MHz, which means that it has a better decoupling effect for noise below 10MHz, and for those above 40MHz Noise has little effect.