Engineer Classroom: Measures to increase the system's ability to resist electromagnetic interference

1. Choose 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 that of 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

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 connected 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, 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 elements in the system composed of microcontrollers

On a printed circuit board, the signal passes through a 7W resistor and a 25cm long lead, and the line delay time is roughly between 4 and 20ns. In other words, the shorter the signal lead on the printed circuit, the better, and the longest should not exceed 25cm. In addition, the number of vias should be as small as possible, preferably no more than 2.

When the rise time of a signal is faster than the signal delay time, it must be processed according to fast electronics. At this time, the impedance matching of the transmission line must be considered. For signal transmission between integrated blocks on a printed circuit board, the situation of Td>Trd must be avoided. The larger the printed circuit board, the slower the system speed.

A rule for printed circuit board design can be summarized as follows:

When a signal is transmitted on a printed circuit board, its delay time should not be greater than the nominal delay time of the device used.

3. Reduce cross-interference between signal lines

At point A, a step signal with a rise time of Tr 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 twice the delay time of the signal on line AB, 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 can be superimposed with 100~200mv noise without affecting 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 ground, or a double-layer board, the back of the signal line is a large-area ground, this cross- interference between signals will become smaller. 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 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, to avoid the interference of the digital circuit signal line CD on AB, there must be a large area of ​​ground under the AB line, and the distance from the AB line to the CD line must 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 are laid on both sides of the lead with a lead.

4. Reduce noise from 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 in battery-powered systems, the battery itself has high-frequency noise. The analog signals in the analog circuit are even more vulnerable to interference from the power supply .