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Electromagnetic compatibility considerations in PCB design
Nov 29, 2018

Electromagnetic compatible PCB design

With the large-scale improvement of system design complexity and integration degree, electronic system designers are engaged in circuit design above 100MHZ, and the working frequency of the bus has reached or exceeded 50MHZ, and some even exceeded 100MHZ.When the system works at 50MHz, transmission line effect and signal integrity problems will be generated.When the system clock reaches 120MHz, PCB based on traditional methods will not work unless high-speed circuit design knowledge is used.Therefore, the high speed circuit design technology has become the electronic system designer must take the design method.Only by using the design technology of high-speed circuit designers can the design process be controlled.

It is generally believed that if the frequency of digital logic circuit reaches or exceeds 45MHZ ~ 50MHZ, and the circuit working above this frequency has taken up a certain proportion of the entire electronic system (say, 1/3), it is called high-speed circuit.In fact, the harmonic frequency of the signal edge is higher than the frequency of the signal itself, which is the unexpected result of signal transmission caused by the rapidly changing rising and falling edges of the signal (or the jump of the signal).In order to achieve high frequency PCB design conforming to EMC standards, the following technologies are usually required: bypass and decoupling, grounding control, transmission line control, wiring terminal matching, etc.

(1) bypass and decoupling

Decoupling refers to the removal of RF energy from high-frequency devices into the distribution network during device switching, while bypass transfers unwanted common mode RF energy from components or cables.

All capacitors are made up of LCR circuits, where L is the inductance, which is related to the length of the wire, R is the resistance in the wire, and C is the capacitance.At a certain frequency, the LC series combination will produce resonance.In resonant state, LCR circuit will have very small impedance and effective RF bypass.When the frequency is higher than the self-resonance of the capacitor, the capacitor gradually changes to the inductive impedance, while the bypass or de-coupling effect decreases.Therefore, the bypass and decoupling effect of capacitors is affected by the lead length, the wiring between capacitors and devices, and the dielectric filler.An ideal decoupling capacitor can also provide all the current required for a logic device to switch state. In fact, it is the impedance between the power supply and the ground layer that determines how much current the capacitor can provide.

When bypass and decoupling capacitors are selected, the self-resonant frequency of the required capacitors can be calculated through the logic series and the clock speed used, and the capacitance value can be selected according to the frequency and the capacitive reactance in the circuit.In choosing package size, the capacitor with lower lead inductance should be selected as far as possible, which is usually manifested as SMT (Surface Mount Technology) capacitor rather than through hole capacitor (such as DIP encapsulated capacitor).In addition, parallel decoupling capacitors are often used in product design to provide a larger working frequency band and reduce the unbalance of grounding.In shunt capacitor system, when the frequency is higher than the self-resonant frequency, the large capacitance shows the inductive impedance and increases with the increase of frequency.Small capacitors exhibit capacitive impedance and decrease with increasing frequency, and the impedance of the entire capacitor circuit is smaller than that of a single capacitor.

(2) ground system

Grounding is required for most electronic products.Grounding is an important method to minimize noise interference and divide circuits.Grounding is mainly manifested in providing reference connections between analog and digital circuits and providing high-frequency connections between PCB layers and metal shells.

PCBS often contain dangerous voltages.It is included in power supply components, communication circuits, delayed-drive instrument control, power exchange modules, and similar devices.In order for the product to comply with safety rules and electromagnetic compatibility, these dangerous voltages must be removed, and a common strategy is to use a ground or ground plane system.The ground line (or ground plane) is essentially the low impedance path of the signal backflow source.Due to this effect of the ground wire, there may be a large current in the ground wire.Because the impedance of the ground wire is not zero, this current creates a potential difference.When there is a potential difference in the local line, the impact on the system is obvious: the potential difference can cause the wrong action of the circuit, making the system work abnormal.

Because the ground potential difference exists in the grounding system, the corresponding grounding method must be selected according to the characteristics of PCB in the grounding process of the design product, and cannot be used at will.Common grounding methods include single-point grounding, multi-point grounding, mixed grounding, etc.Single-point grounding means that in product design, the grounding line is connected to a single reference point. This grounding setting is designed to prevent the common impedance coupling caused by the same return path of current and rf current in subsystems from two different reference levels.This grounding method is suitable for low frequency PCB and can reduce the influence of distributed transmission impedance.However, in high-frequency PCB, the inductance of the return path becomes the main part of the line impedance at high frequency, so in high-frequency PCB, multi-point grounding method is usually adopted to minimize the grounding impedance.The most important point in multi-point grounding is that the length of the grounding lead is required to be minimum, because the longer lead represents a greater inductance, thus increasing the ground impedance and causing the ground potential difference.The hybrid grounding structure is a combination of single-point grounding and multi-point grounding.This structure is often used when high and low mixing frequencies are present in PCB, that is, single-point grounding is presented at low frequency and multi-point grounding is presented at high frequency.Figure 1 below shows capacitive coupled hybrid grounding.In the corresponding inductive coupling mixed ground model, C1 ~ C3 can be changed into the appropriate inductance.