When dealing with EMI, problems must be analyzed in detail. In the PCB design, EMI can be controlled from the following aspects.
1. Device selection
When doing EMI design, first you should consider the speed of the selected device. In the circuit, if a device with a rise time of 5ns is replaced with a device with a rise time of 2.5ns, EMI will increase by about 4 times. The radiation intensity of EMI is proportional to the square of the frequency. The highest EMI frequency is also called the EMI emission bandwidth. It is a function of the signal rise time rather than the signal frequency:
The frequency range of this radiated EMI is 30MHz to several GHz. In this frequency band, the wavelength is very short, and even very short wiring on the circuit board may become a transmitting antenna. When the EMI is high, the circuit easily loses its normal function. Therefore, in the selection of devices, under the premise of ensuring the performance requirements of the circuit, low-speed chips should be used as much as possible, and a suitable driving/receiving circuit should be adopted
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