Introduction：
By today's increasing variety of electronic products, a variety of charged equipment and products are constantly generating various frequencies of radio frequency interference to the outside radiation, resulting in a complex EMC electromagnetic compatibility problems. Although a variety of national standards, IEC standards, CE and other electronic products in various industries have been standardized EMC limits, but because of the abstract and random characteristics of the EMC makes it difficult to directly conclude that a product is qualified. So many EMC engineers in the rectification of electromagnetic compatibility problems will be used in a one-size-fits-all approach to curb the product's outward interference, while also preventing other equipment from interfering with their own equipment. How to curb it across the board? This is what this article says to talk about two effective ways to attenuate EMI radiation interference: metal chassis cabinet shielding method and its shielding effectiveness test method.
Two effective ways to attenuate EMI radiation interference
Interference propagation path analysis:
The propagation of electromagnetic interference signals is similar to the propagation of sound, and we know that noise can be reduced when there is noise around us by the following three methods:
- attenuation from the sound source, (direct hooding)
- Weakening from propagation, (emitting acoustic signals by planting trees, etc.)
- attenuate from the receiver. (Wearing earplugs, etc.)
Propagation diagrams for sound sources and electromagnetic wave radiation sources from electronic products
As can be seen from the above figure, electronic products emit electromagnetic wave signals to the outside in the field of EMC testing by the EMI receiving antenna from a fixed distance to collect and test specific data, and then with the national standard and other EMC standard requirements to determine whether the product complies with the standard, the propagation path in the above figure in the EMI wave dark room is a calibration over the uniformity of the field of space, there will not be any obstruction, the back end of the EMI antenna is also the standard Requirements of the fixed antenna, so in summary, it is not possible to propagation path and receiving antenna to attenuate the EMI signal, we can only start from the beginning of the radiation source.
The method of adding a metal shielding box to the exterior of the product:
Increase a metal shielding box is the best way to contain the source of radiation in reference to EUTTEST agent sales of EMC laboratories in the shielded room and radio wave dark room and other architectural structures can be known, welded and have a certain thickness of the metal shell can be shielded from EMI noise. At this point, the shielding effectiveness of the metal shield depends on the quality of welding and material thickness, according to the dB voltage formula, as long as the signal down 6dB can make the EMI noise is reduced by a factor of two. So this method is very effective in rectifying EMC problems.
Shielded box shape as shown in the figure below, shielded box built by the metal shell, electronic products can work freely inside will not be emitted to the outside, and will not be interfered with by other equipment. However, most of the electronic products are auxiliary equipment, they will be put outside the shielding box, then you need to punch holes in the shielding box installed on the interconnecting cable to maintain communication between the equipment, shielding box holes will result in shielding leakage, this time you have to do shielding of interfaces, cable shielding treatment, and other operations, please check out the other introduction of my company, this article does not look into the issue.
Metal shielding box on the outside of the product to reduce EMI interference.
Predicting effects:
When the PCB of an electronic product is placed in a steel box, the radiated interference of the product is trapped inside the metal box. Note that the box can be large or small, depends entirely on the size of the PCB circuit board, like most of the military or avionics are placed inside a metal box to effectively attenuate the EMI radiation interference.
Method of adding shield on PCB:
Above we introduced the use of cabinets to shield the method, in fact, most of the PCB circuit boards are loaded into a clinker shell encapsulated as a product for sale, we can not ask the user to do their own shielding, so we need to use the metal material cover the source of interference emissions. The following figure for a market cell phone circuit board, from the figure you can see most of the chips and modules are covered by black material, which is the PCB quoted cabinet shielding method to quickly improve the EMI interference. Some of these materials are soft, some are metal sheets, they all have a common feature is that they can conduct electricity, and some materials are the soft material of the metal layer into the middle of the two layers of insulating material and coated with glue to use, so it is essentially still a metal shell, only it is not so hard. So we can collectively call them for EMI noise improvement metal shield.
Shield on PCB to attenuate EMI interference
Predicting effects:
The PCB shield does not need to cover the entire PCB, you canTesting PCBs with near-field probesOn some of the chips or modules that may have a high radiation component on the radiation emission curve, you can also test the entire PCB in turn once to get the radiation distribution map of the entire PCB, and then add PCB shielding to the sensitive areas. This method effectively solves the problem of the shielding box introduced in the previous section needs to contain the entire PCB, but also allows the PCB to communicate with external modules will not be interrupted.
Testing:
After using the above two effective methods to attenuate EMI radiation interference, such as adding a shield for the circuit board, or adding a shielded box on the outside of the product, the final R & D personnel should be in the laboratory to test the electronic products to improve the EMI interference? If you need to compare, you need to test once before rectification as well. These can be done using the EUTTEST suppliedEMC test systems measure and record EMI interference dataThe