Shielding and grounding should be used together to achieve good shielding effect. Mainly considering electromagnetic compatibility, the typical two kinds of shielding are electrostatic shielding and alternating electric field shielding, which are introduced as follows:
Electrostatic shielding: when the charged conductor is surrounded by a complete metal shield, the same charge as the charged conductor will be induced on the inside of the shield, and the same charge will appear on the outside, so there is still an electric field on the outside. If the metal shielding layer is grounded, external charges will flow into the earth, and there will be no electric field outside the metal shell, which is equivalent to the electric field of the charged body in the shielded shell.
Alternating electric field shielding: In order to reduce the coupling interference voltage of alternating electric field to sensitive circuits (such as multistage amplifier circuits, RAM and ROM circuits), a metal shield with good conductivity can be set between the interference source and sensitive circuits, or the interference source and sensitive circuits can be shielded separately, and the metal shield can be grounded. As long as the metal shield is well grounded, the coupling interference voltage of alternating electric field to sensitive circuit can be greatly reduced, so that the circuit can work normally.
Shielding of circuit: Shielding should be provided for circuits that are susceptible to electromagnetic radiation, such as various signal sources and amplifiers. Because there is parasitic capacitance between the signal circuit and the shield, the grounding terminal of the signal circuit should be connected to the shield to eliminate the influence of parasitic capacitance, and the shield should be grounded to eliminate the * * * mode interference.
The shielding layer of the cable is grounded: in some weak signal transmission cables of communication equipment, in order to ensure the safety and stability of signal transmission, cables with shielding nets are used to stabilize signal transmission and prevent interference with other equipment and themselves. For example, coaxial cable for closed-circuit television, and the metal mesh outside is used to shield signals. For example, eight thin metal wires are wound in the network cable, four of which play a shielding role to ensure the digital signal of the signal is correct.
Grounding of cable shield of low-frequency circuit: low-frequency circuit <1MHz >; The shielding layer of the cable should be grounded at a single point, and the grounding point of the shielding layer should be consistent with the grounding point of the circuit, usually the negative pole of the power supply. For multi-layer shielded cables, each shielding layer should be grounded at one point, but each shielding layer should be insulated from each other.