In EMC design, the purpose of cable shielding is twofold:
- The common mode interference current injected into the cable is guided to the earth (grounding equipment) or the working ground GND (floating equipment) in the PCB through the shielding layer, so that the signal and the cable interface circuit in the shielding layer are protected from external interference;
- Enclose the EMI signal in the signal line in the shielding layer to ensure that there is no EMI common mode current on the cable shielding layer.
List several common grounding methods of cable shielding layer, as follows:
In the application of shielded cables, sometimes for the convenience of connection, the braid of the shielding layer is often twisted into a section, which is twisted into a "pig tail"-shaped braid. A long section of the core wire exposes the shielding layer, as shown in the following figure. At this time, a "pig's tail effect" will occur, which greatly reduces the shielding effect of the shielding layer. At the same time, this kind of cable cannot well suppress common-mode radiation. Similarly, when the cable shielding layer and the metal chassis have a 360° complete overlap, but there is no guarantee of good electrical continuity, the same effect will be caused.
In the EMC design, the shielding layer of the shielded cable must be lapped 360°, and the "pig tail" method must not be used for overlap.
If the design of the cable shielding layer connection is evaluated from the concept of risk, then according to experience, at a frequency above 30MHz, the shielded layer cable has a zero-length "pig's tail", which means there is no risk. The DB connector shown in the figure below is When used in computer products, the ESD level can reach 15kV;
A "pig tail" with a length of 1cm has a 30% risk; a "pig tail" with a length of 3cm has a 50% risk. When the DB connector shown in the figure below is used in computer products, the ESD level can only reach 4kV;
A "pig tail" with a length of 5cm has a 70% risk. When the DB connector shown in the figure below is used in computer products, the ESD level is only 2kV.
