This product is a camera device, using MIPI interface camera, the signal line is a 10-core twisted pair with shielding layer, the shielding layer is used as a ground wire by the supplier, after soldering to the motherboard, to do EMC test, found that the radiation is quite The high, 10db exceeding the standard.
The main board is a four-layer board, the second layer is a ground layer, and there is also a ground on the top layer, which is connected to the ground layer with via holes. The top layer layout is shown in the figure. Box 1 is an island, and ring 2 is the rest of the layout. It can be determined that the two are connected through the GND layer.
At the beginning, the shielding layer was welded to the position 2 and it could work normally. However, due to excessive radiation, it was found that the frequency multiplier was indeed the working frequency of the 180M module MIPI signal, so it must be related to the module or the shielded wire.
After analysis, it is found that the analog ground corresponding to all MIPI signals is connected to 1, except that the shielding wire is soldered to 2. At the beginning, it is all GND, and the shielding wire is very thick, and the soldering pad of 1 is not easy to solder; but radiation After exceeding the standard, the shielding layer was changed to the position of welding 1, and the result was quite satisfactory.
From the figure, all noises are greatly reduced, and there is no 180M frequency doubling interference problem. Only a small peak appears at the 360M position, but the margin is sufficient.
The reason is the ground path problem. Although they are all GND, since 1&2 are not directly connected, but are connected to the GND layer through vias, the signal loop becomes longer and there will be radiation leakage; After changing to 1 welding, the signal loop is greatly reduced and the radiation is reduced.
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