In order to restrain the electromagnetic interference effectively, we must take the measures of grounding, shielding and filtering.
The necessary condition for electrostatic shielding is that the shield body is grounded. In order to shield both magnetic and high-frequency electric fields, the shield body should also be grounded. Electromagnetic shielding is to use shielding body to prevent electromagnetic wave in space propagation of a measure, in order to avoid electromagnetic induction caused by shielding effectiveness decline, shielding body should be connected to the ground. In addition, in order to avoid the interference of ground voltage in the shielding body, a single point of ground should be used.
Shielding cable is to wrap a layer of metal film outside the insulated conductor, that is, shielding layer. The shielding efficiency of the shielding layer is not mainly due to reflection and absorption, but is generated by the grounding of the shielding layer. That is to say, the shielding layer of the shielded cable can only play a shielding role after grounding. For example, the interference of the conductor of the interference source circuit to the single-core shielded wire of the sensitive circuit is realized through the coupling capacitance between the interference source conductor and the shielding layer of the single-core shielded wire, as well as the coupling capacitance between the shielding layer and the core wire. If the shield is grounded, the interference is short-circuited to the ground and can no longer be coupled to the core. The shield plays a shielding role. However, when the cable is used for electromagnetic shielding, both ends of the shielding layer should be grounded. For low-frequency circuits, single-end grounding is possible. For example, if an ungrounded signal source is connected to a grounded amplifier at a common end through a cable, the cable shield shall be connected at that common end. When the common end of the signal source is grounded and the amplifier is not, the shielding layer shall be connected to the common end of the signal source. For high frequency circuits, both ends should be grounded, and when the cable is longer than 1/20 wavelength, it should be connected at 1/10 wavelength distance. The method of grounding the shield layer is to make the shield layer and the connector shield shell have a good welding of 360° to avoid "braiding". Cable core and connector pin or socket welding; At the same time, the shielding shell of the connector is closely connected with the shielding case, so that the shielding cable becomes an extension of the shielding case, and good shielding effect can be achieved. Thus, shielding and grounding are closely related.
The main ways for emi to invade the shield are I/O interface and drive power line input port. In fact, electromagnetic interference inside the shield can be coupled to wires, cables, or drive power lines connected to the I/O interface, and can generate interference current that can be transmitted outside the shield, causing conduction interference and radiation interference. Similarly, external electromagnetic interference can enter the shield through wires or cables and power lines connected to the I/O interface, or through electromagnetic induction to generate interference current into the shield, while causing radiation interference to the shield body. In order to suppress the inflow or outflow of interfering current and maintain a higher shielding efficiency for the shielding body, filter connectors or feed-through filters can be used in the I/O interface and the input port of the power line, respectively. In addition, the honeycomb ventilation board installed on the shield body is a high-pass filter composed of cut-off waveguide. When the panel needs to pass through the non-metallic shaft of adjustable components, the shaft can also pass through the cut-off waveguide. A shielded window made of conductive glass is essentially a high-pass filter. Thus, in order to ensure the effectiveness of shielding, shielding and filtering are closely related.
All filters must be grounded except those that are specifically allowed to be ungrounded. Because the common-mode bypass capacitance in the filter is only useful when grounded. In particular, the PI filter, when the ground is not good, is equal to the capacitor and inductance parallel, completely lost the filter function. In addition, when installing the filter, the input end and the output end should be completely isolated with the aid of shielding, in order to play the suppression role of the filter. Therefore, filtering has a close relationship with grounding and shielding.
The electromagnetic compatibility design of the product looks very complex from the surface, do not know how to start, but if you can pay attention to the above several problems, the correct use of protective measures, any complex electromagnetic compatibility design problems can be readily solved.