1 Introduction
EMC (Electromagnetic Compatibility) - Electromagnetic compatibility (sex) is a multidisciplinary and marginal discipline. Electromagnetic compatibility technology has been widely used in many fields, and it has received more and more attention in embedded applications. Any electronic device emits electromagnetic energy to the surroundings during operation and may interfere with other devices. At the same time, the device itself may also be interfered by the surrounding electromagnetic environment. The main problem of electromagnetic compatibility research is how to make various devices in the same electromagnetic environment or components in the same device work normally without mutual interference.
2 Characteristics of electromagnetic compatibility in embedded systems
With the development of IC technology, new technologies are constantly emerging. The high-performance single-chip microcomputer system gradually adopts the 32-bit word length RISC architecture, and the running frequency exceeds 100 MHz. The 8-bit single-chip microcomputer also adopts a new technology to improve the system speed expansion function interface. Embedded systems are moving toward high integration, high speed, high precision, and low power consumption. At the same time, due to the wide application of electronic technology, the density of electronic equipment has increased, the electromagnetic environment has deteriorated, and the electromagnetic interference and anti-interference problems of the system have become increasingly prominent.
Electromagnetic interference in embedded systems is mainly transmitted in two ways:
(1) Conductor propagation is directly infiltrated into sensitive equipment through signal lines, control lines, power lines, etc. of the equipment. This method is called conducted interference.
(2) There are electric fields, magnetic fields and electromagnetic fields in the space around the space propagation disturbance source, which will cause interference to nearby electronic circuits, called field interference.
2.1 Conducted interference
2.1.1 Distribution line characteristics of transmission lines
(1) Resistance of transmission line
Any conductor has a certain resistance, and the charge is evenly distributed across the cross section of the conductor when a direct current or low frequency current flows through the conductor. When high-frequency current flows through the wire, the current in the wire is mainly concentrated on the surface of the conductor due to the high-frequency skin effect, and there is almost no current in the center of the wire, so the AC resistance of the wire will be greater than the DC resistance, and the AC resistance and frequency One-half is proportional to each other. The AC resistance of the wire can be reduced by changing the shape of the cross-sectional area. A rectangular wire having the same cross-sectional area has a larger surface than a circular wire, so the AC resistance is smaller than that of the circular wire. Grounding conductors often use flat rectangular conductors instead of round conductors to reduce high frequency resistance.
(2) Characteristic impedance of the transmission line
The transmission line has resistance, inductance and capacitance. For uniform transmission lines, they are evenly distributed in various parts of the transmission line, called distribution parameters. The characteristic impedance describes the distribution parameter characteristics of the transmission line. It is defined as:
Where: s is the interval of parallel double lines; r is the wire radius; μ is the magnetic permeability, and ε is the dielectric constant. The applicable condition of the formula (1) is s>5r.
It can be seen from equation (1) that the characteristic impedance is a physical quantity characterizing the characteristics of the transmission line itself, and is independent of the current and voltage in the transmission line, and is only related to the structure (wire diameter, line spacing) of the transmission line and the medium (ε, μ) around the transmission line. It is important to note that the characteristic impedance describes the distribution parameter characteristics of the transmission line rather than the true impedance. The characteristic impedance of the traces and twisted pairs on the printed board is 100 to 200 Ω, and the coaxial cable is 50 Ω or 75 Ω.
2.1.2 Short-line processing method of transmission line
The distribution parameters of the transmission line necessarily affect the signal transmission in the transmission line, which is closely related to the length of the transmission line. According to the relationship between the length of the transmission line and the signal frequency, the transmission line can be divided into long lines and short lines. When the transmission line length is ≤1/20 signal wavelength or the transmission delay time ≤1/4 digital signal pulse rise time, the transmission line can be regarded as a short line. which is:
The short line can be analyzed by the lumped parameter equivalent circuit, that is, the transmission line is regarded as a network composed of lumped parameter resistors, inductors, and capacitors, and the value thereof is equal to the value of the distribution parameter per unit length multiplied by the length of the transmission line. For example, there is a pair of transmission lines, and the terminal is short-circuited. If it meets the short-line conditional formula (2), it can be regarded as a resistor R and an inductor L connected in series, and the total impedance is: Z=R+j2Ï€fL. For most twisted pair, coaxial cable, printed circuit boards, the resistance plays a major role in the transmission line when the frequency is very low below 3 kHz. When the frequency is greater than 3 kHz, the inductor plays a major role and the resistance is negligible.
Figure 1 is an equivalent circuit of a transmission line, which is set to conform to equation (2), where RS is the source impedance, Ri, Ci is the input impedance of the load, L, C is the inductance and capacitance of the transmission line, then L = L0l, C=C0l, where L0 and C0 are distributed inductance and distributed capacitance, and l is the length of the transmission line. Due to the presence of inductance and capacitance in the transmission line, the digital signal may ring when passing through the transmission line, that is, the attenuation oscillation, and the oscillation frequency is:
The upper and lower impulses of the ringing waveform will reduce the noise margin of the gate circuit. In severe cases, the circuit will malfunction, so it should be tried to overcome the ringing phenomenon caused by the distribution parameters of the transmission line.
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