EMC problems - ground skills and PCB engineers Notes

EMC problems
When the layout should also pay attention to EMC suppression Oh!  This is not good grasp of the existence of distributed capacitance at any time! 
How to ground!
The original PCB design must consider many factors, a different environment need to consider different factors.
Land division and aggregation
Suppression of electromagnetic interference grounding is to improve the EMC performance of electronic devices is one important means. Proper grounding for electromagnetic interference suppression can not only improve the ability of products, and reducing external EMI product launches.
Grounding the meaning of
Electronic equipment "to" usually has two meanings: one is the "Earth" (safe), the other is the "system reference to" (signal ground). Grounding refers to base-level system with a potential conductive path between the low resistivity. "Access land" is a reference potential of the earth, and to the earth as a zero potential of the electronic equipment, metal enclosures, circuit reference point connected with the earth.
The ground plane and the earth connection, usually for the following considerations:
A, the circuit system work to improve the stability of equipment;
B, electrostatic discharge;
C, to provide security for staff.
The purpose of grounded
A, safety considerations, that the protection of ground;
B, the signal voltage to provide a stable reference point of zero potential (the signal or systematic);
C, shield grounding.
Basic grounding
There are three basic electronic equipment grounding method: single-point grounding, multi-point grounding, floating.

Note PCB Engineer
Single point ground:
Single-point grounding of the entire system, only one physical point is defined as the ground reference points, each of the other points need to connect to the ground on this point.
Single point ground to the low frequency circuits (1MHZ below). If the system frequency is high, so that wavelength and the system ground lead length can be compared, the single-point grounding have a problem. The length of the local line is close to 1 / 4 wavelength, it is like a terminal short circuit transmission line, ground current, voltage was standing wave distribution, ground into a radiating antenna, and can not play "to" role . To reduce the ground impedance, to avoid radiation, the length of the ground should be less than 1 / 20 wavelength. The handling of the power circuit can generally be considered a single point ground. The extensive use of digital circuit PCB, due to its rich high harmonics are generally not recommended a single point grounding.
Multi-point grounding:
Multi-point grounding of each device is connected directly to locations away from its nearest ground plane, so that the length of the shortest ground lead. Simple circuit structure and multi-point grounding, the grounding line of high-frequency standing wave phenomenon may be significantly reduced for high frequency (> 10MHZ) occasions. But the multi-point grounding may result in the formation of many ground loops inside the device, thereby reducing the equipment's resilience to external electromagnetic fields. In the case of multi-point grounding, ground loops should pay attention to the problem, especially in different modules, when the network between devices. Electromagnetic interference caused by ground loops: the ideal should be a zero potential ground zero impedance of the physical entity. But the actual ground itself, there are both components of reactance component of resistance, when a current through the ground, it is necessary a voltage drop. Ground with other connections (signal, power lines, etc.) constitute the loop, when the electromagnetic field coupled to the loop, on the ground loop induced electromotive force generated by ground loops coupled to the load, a potential EMI threat.
Floating ground system is the equipment and the earth in electrical insulation on a grounded way. Since floating its own weaknesses, not suitable for large-scale systems in general, the grounding is rarely used
Selection on the general principles of grounding:
For a given device or system, the highest frequency of interest (corresponding to a wavelength) into the above, when the transmission line length L> income, are considered high-frequency circuits, on the contrary, it is considered low-frequency circuits. Rule of thumb, the lower 1MHZ the circuit, using single-point ground is better; the above 10MHZ, multi-point ground is better. For frequencies in between, as long as the length of the longest transmission line L is less than / 20 in, single-point ground can be used to avoid common impedance coupling.
The general principle of selecting the ground as follows:
(1) low-frequency circuit (<1MHZ), single-point ground is recommended;
(2) high-frequency circuits (> 10MHZ), multi-point ground is recommended;
(3) high frequency hybrid circuits, hybrid ground.