How to improve anti-interference ability and electromagnetic compatibility
When developing electronic products with processors, how to improve anti-interference ability and electromagnetic compatibility? This article summarizes some methods for everyone.
1.The following systems should pay special attention to anti-electromagnetic interference:
(1)Systems with extremely high microcontroller clock frequencies and extremely fast bus cycles.
(2)Systems containing high-power, high-current drive circuits, such as spark-generating relays, high-current switches, etc.
(3)Systems containing weak analog signal circuits and high-precision A/D conversion circuits.
2.To increase the system’s anti-electromagnetic interference ability, take the following measures:
(1)Select a low-frequency microcontroller:
Selecting 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 those 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 and become 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 end is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output end of the high-speed CMOS circuit has a considerable load capacity, that is, a considerable output value.
If the output end of a gate is led to the input end with a relatively high input impedance through a very long line, the reflection problem will be very serious, which will cause signal distortion and increase system noise. When Tpd>Tr, it becomes a transmission line problem, and signal reflection, impedance matching and other issues must be considered.
The delay time of the signal on the printed circuit board is related to the characteristic impedance of the lead, that is, it is related to the dielectric constant of the printed circuit board material. It can be roughly assumed that the transmission speed of the signal on the printed circuit board lead is about 1/3 to 1/2 of the speed of light. The Tr (standard delay time) of the commonly used logic telephone components in the system composed of microcontrollers is between 3 and 18ns.
On the printed circuit board, the signal passes through a 7W resistor and a 25cm long lead, and the online delay time is roughly between 4 and 20ns. That is to say, 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
When the rise time of the signal is faster than the signal delay time, it should be processed according to fast electronics. At this time, the impedance matching of the transmission line should be considered. For the signal transmission between integrated blocks on a printed circuit board, it is necessary to avoid the situation of Td>Trd. The larger the printed circuit board, the slower the system speed.
The following conclusion summarizes a rule for printed circuit board design:
The delay time of the signal on the printed board should not be greater than the nominal delay time of the device used.
3.Reduce cross-interference between signal lines:
A step signal with a rise time of Tr at point A is transmitted to end B through lead AB. The delay time of the signal on line AB is Td. At point D, due to the forward transmission of the signal at point A, the reflection of the signal after reaching point B and the delay of line AB, 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, will be induced. This is the cross-interference between signals. The strength 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 is actually seen on AB is the superposition of two pulses.
The microcontroller manufactured 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.
If the printed circuit board is a four-layer board, one of which is a large area of ground, or a double-layer board, and the reverse side of the signal line is a large area of ground, this cross-interference between signals will become smaller. The reason is that the large area of 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 between the signal line and the ground, and is proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, in order to avoid the interference of the digital circuit signal line CD on AB, there should be a large area of ground under the AB line, and the distance from the AB line to the CD line should be greater than 2~3 times the distance between the AB line and the ground. A local shielding ground can be used, and ground wires can be laid on the left and right sides of the lead with the lead.
4.Reduce the noise from the power supply
While the power supply provides energy to the system, it also adds its noise to the power supply it supplies. 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 for battery-powered systems, the battery itself has high-frequency noise. The analog signal in the analog circuit cannot withstand interference from the power supply.
5.Pay attention to the high-frequency characteristics of the printed circuit board and components
In high-frequency conditions, the distributed inductance and capacitance of the leads, vias, resistors, capacitors, and connectors on the printed circuit board cannot be ignored. The distributed inductance of the capacitor cannot be ignored, and the distributed capacitance of the inductor cannot be ignored. The resistor produces 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 is generated, and the noise is emitted outward through the lead.
The vias of the printed circuit board cause about 0.6pf of capacitance.
The packaging material of an integrated circuit itself introduces 2~6pf of capacitance.
A connector on a circuit board has a distributed inductance of 520nH. A dual-row in-line 24-pin integrated circuit socket introduces a distributed inductance of 4~18nH.
These small distributed parameters are negligible for this line of microcontroller systems at lower frequencies; but special attention must be paid to high-speed systems.
6.Reasonable partitioning of component layout
The position of the components arranged on the printed circuit board should fully consider the problem of electromagnetic interference resistance. One of the principles is that the leads between the components should be as short as possible. In terms of layout, the three parts of 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.
7.Handle the grounding wire well
On the printed circuit board, the power line and the ground line are the most important. The most important means to overcome electromagnetic interference is grounding.
For double-sided boards, the grounding layout is particularly particular. By adopting the single-point grounding method, the power supply and the ground are connected to the printed circuit board from both ends of the power supply, one contact for the power supply and one contact for the ground. On the printed circuit board, there should be multiple return ground lines, which will converge on the contact point that returns to the power supply, which is the so-called single-point grounding. The so-called analog ground, digital ground, and high-power device ground separation refer to the separation of wiring, and finally converge to this grounding point. When connected to signals outside the printed circuit board, shielded cables are usually used. For high-frequency and digital signals, both ends of the shielded cable are grounded. For shielded cables used for low-frequency analog signals, it is better to ground one end.
Circuits that are very sensitive to noise and interference or circuits with particularly severe high-frequency noise should be shielded with metal covers.
8.Use decoupling capacitors well.
Good high-frequency decoupling capacitors 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 should be added between the power supply 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, providing and absorbing the charging and discharging energy of the integrated circuit when opening and closing the door; on the other hand, it bypasses the high-frequency noise of the device. The typical decoupling capacitor in the digital circuit is a 0.1uf decoupling capacitor with a 5nH distributed inductance, and its parallel resonance frequency is about 7MHz, which means that it has a good decoupling effect for noise below 10MHz and has almost no effect on noise above 40MHz.
1uf, 10uf capacitors, parallel resonance frequency above 20MHz, the effect of removing high-frequency noise is better. It is often beneficial to add a 1uf or 10uf high-frequency decoupling capacitor where the power supply enters the printed board, even battery-powered systems need this capacitor.
For every 10 or so integrated circuits, a charging and discharging capacitor, or storage capacitor, should be added, and the capacitor size can be selected as 10uf. It is best not to use electrolytic capacitors. Electrolytic capacitors are two layers of thin film rolled up. This rolled-up structure behaves as an inductor at high frequencies. It is best to use a bile capacitor or polycarbonate capacitor.
The selection of decoupling capacitor value is not strict. It can be calculated according to C=1/f; that is, 0.1uf is used for 10MHz. For a system composed of a microcontroller, 0.1~0.01uf can be used.
