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How does PCB suppress the spread of electromagnetic interference EMI?

November 18, 2020

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Printed circuit board (PCB) is easy to manufacture, reliable in performance, and low in price, so it is widely used in various electronic devices. In recent years, with the development of electronic technology, the clock rate of microprocessors and logic circuits on printed circuit boards has become faster and faster, and the rise/fall time of signals has become shorter and shorter. At the same time, the density of on-board devices and wiring density With the continuous increase, the electromagnetic compatibility problem of printed circuit boards has become increasingly prominent.


Three basic issues should be considered when analyzing printed circuit boards at the electromagnetic compatibility level:


Ensure the reliable transmission of signals on the board and ensure signal integrity (Signal Integrity);


Suppress the propagation of electromagnetic interference EMI;


Strengthen protection to prevent sensitivity failure (Susceptibility Failure) caused by insufficient immunity.


For relatively low frequency signals (the upper limit of the signal spectrum is 100MHz), the above-mentioned problems can usually be ignored. But when the signal wavelength (λ) and the signal line length (l) are comparable to each other (l≥0.1λ), it is necessary to consider the geometric dimensions of the printed lines, wiring, spacing between lines, and the rise and fall times of the transmission signal. Factors such as pulse width and period make it necessary to use transmission line theory (in some cases microwave theory) to correctly analyze signal propagation.


The traces on the printed circuit board can usually be simulated with microstrip or stripline models. The microstrip line model is composed of a conductor strip on one side of the dielectric substrate and a metal grounding plate on the other side of the substrate. It can be used to simulate printed wires on the surface layer of PCB. The stripline model is composed of upper and lower grounding conductors and intermediate conductor strips. Between the grounding plate and the conductor strips is an insulating medium, which can simulate the printed wires of the middle layer of multilayer PCB.


1. Signal integrity


The signal integrity problems on PCB mainly include time delay, impedance mismatch, ground bounce, and crosstalk. Signal integrity issues will affect the stable operation of electronic devices.


(1) Signal delay: For high-frequency signals, transmission delay should be one of the most basic issues considered by circuit designers. The relationship between transmission delay, signal line length and signal transmission speed is as follows:


Where: c-the speed of light in vacuum;


εreff effective relative conductivity;


lp-the length of the signal line.


εreff is related to the medium surrounding the transmission line. For a microstrip transmission line, ε is between the relative conductivity of the plate and the relative conductivity of air. In most systems, the length of the signal transmission line is the most direct factor that affects the colock skew. The clock pulse phase difference means that the time of two clock signals generated at the same time is not synchronized at the receiving end. The clock pulse phase difference reduces the predictability of signal edge arrival. If the clock pulse phase difference is too large, an error signal will be generated at the receiving end, as shown in Figure 1. Transmission line delay has become an important part of the clock cycle (Clock Cycle).

PCB.png

(2) Impedance mismatch:


Impedance mismatch can be caused by the driving source, the impedance of the transmission line and the load, or it can be caused by the discontinuity of the transmission line? For example, vias, stubs? In addition, due to changes in local inductance and capacitance on the return path, discontinuity in the return path will also cause discontinuity in impedance. This impedance mismatch will cause reflections and damped oscillations. Can reflections cause signal ringing? Ringing? Phenomenon, that is, the voltage overshoot and undershoot that occur up and down the steady-state signal, as shown in Figure 2. In order to limit the voltage overshoot/undershoot within a reasonable range (not exceeding 10%~15% of the steady-state value), the following principle should be followed: the rise time of the signal should be less than the signal caused by the back and forth on the printed wire The transmission delay. Namely: tr≤2lp/tppd


Where: tr—refers to the rise time of the signal;


lp-the length of the signal line;


tppd—The delay caused by the unit length of the signal line.


The ringing phenomenon may cause false triggering. In order to eliminate the influence of the ringing phenomenon, one of the methods is to wait for the signal to stabilize, but this will reduce the maximum possible clock rate of the system.


(3) Ground bounce: The so-called ground bounce refers to the fact that the ground wire of PCB and the ground lead of the integrated circuit have a certain inductance when a certain integrated circuit is switched, which will cause a brief impact on the internal ground potential of the device. Or drop. The input drive signal from other devices, or other devices driven by the output signal of this device, are all referenced to the external system ground. The inconsistency of this reference ground potential may lead to changes in the input threshold or output level of the device, thereby causing problems for the design of high-speed PCBs. For power supplies, similar problems exist.


(4) Crosstalk: Usually can be divided into two parts, namely common impedance coupling and electromagnetic field coupling. Common impedance coupling is caused by different signals sharing a common return path. This coupling usually plays a decisive role at low frequencies. Electromagnetic field coupling can be divided into inductive coupling and capacitive coupling. Crosstalk is a near-field problem. On PCB0, crosstalk is related to the length of the line, the spacing of the lines, the direction of the transmission signal in the line, and the condition of the reference ground plane. For example, a split on the ground plane will increase the crosstalk of adjacent lines crossing the split, causing signal waveform distortion.


2 Reduce conducted emission and radiated emission


Electromagnetic interference problems mainly include conducted emission and radiated emission. The so-called emission in electromagnetic compatibility refers to "the phenomenon that electromagnetic energy is emitted from the source". Unlike the artificial emission of electromagnetic waves in the general communication field, the emission in PCB is often unintentional. Radiation emission standards usually cover the range of 30 MHz to 1 GHz, and will be expanded to 5 to 40 GHz in the near future. For conducted emission, FC


C limit the scope


The system is in the range of 0.45 to 30 MHz, while CISPR extends the lower limit by 0.15 MHz. Filtering is an important method to suppress conducted emissions. Filtering the signal lines leaving the PCB board can suppress the propagation of conducted emissions.


At high frequencies, the printed lines on the PCB are like a mono-pole antenna (mono-pole antennas) or loop antennas (Loop Antennas), radiating energy outward. Radiation emission can be divided into two basic types: differential mode radiation and common mode radiation.


Is the differential mode radiation due to the current in the closed loop? The so-called differential mode current? caused. The intensity of radiation is proportional to the area of the ring, the size of the current, and the square of the frequency. By reducing the above factors, especially the frequency, the intensity of radiation can be reduced. In addition, the radiation of the ring is directional. The electric field radiation value of a small current ring is the largest in the plane where the ring is located, and the smallest in the axial direction of the ring.


Common mode radiation is caused by parasitic effects, such as induced current on the ground plane, power plane, or cable? The so-called common mode current? caused. Common mode radiation is similar to a monopole antenna. The intensity of radiation is related to the current and frequency per unit line length, but it is not sensitive to direction.


Since the radiation generated by the differential mode current is subtractive, and the radiation generated by the common mode current is additive, even if the common mode current is much smaller than the differential mode current, a comparable radiation field will be generated. For example, a cable with a length of 1m, in which a differential mode current of 30MHz and 20mA flows between two wires with a distance of 1.27mm, will produce a 100μV/m radiation electric field at a distance of 3m, while for common mode current, only 8μA is required. Electric current can produce the same degree of radiation. When performing far-field analysis, common mode radiation must be considered.


3 Strengthen protection


The strength of protection depends on the use of the product. For insignificant electronic products, no special


Protection. For military electronic products and electronic equipment used for power plant and grid control, the highest level of protection is required, because even in extreme cases, these equipment must be guaranteed to work normally.


4 Conclusion


When designing a printed circuit board, electromagnetic compatibility must be considered to ensure the realization of the design function. At high frequencies, simple circuit simulation may no longer be applicable, and transmission line theory or microwave theory is needed to analyze the problems encountered.