Electromagnetic Interference Sources
Electromagnetic interference sources include microprocessors, microcontrollers, transmitters, electrostatic dischargers and instantaneous power actuators such as electromechanical relays, switching power supplies, and lightning. In the microcontroller system, the clock circuit is the largest broadband noise generator, and this noise is spread to the entire spectrum. With the development of a large number of high-speed semiconductor devices, this circuit will generate up to 300 MHz harmonic interference.
The noise is most likely to be transferred when coupled to the circuit. If a wire passes through a noisy environment, the wire will sense the ambient noise and pass it to the rest of the circuit. Noise is transferred through the power cable into the system, and then carried by the power cord is to the entire circuit, which is a coupling situation.
Coupling also occurs in a circuit with a shared load (impedance). For example, two circuits share a power supply wire or a ground wire. If one of the circuits requires a burst of a large current, and the two circuits share the power cable, when use the same power supply resistance, the current imbalance will lead to the voltage drop of another circuit power supply. The effect of this coupling can be reduced by reducing the common impedance. But the power supply resistance and ground wire are fixed. If the ground is unstable, the return current in one circuit will change the ground potential in the other circuit. The change of the ground potential will seriously reduce the Analog circuit performance, such as A / D converter, the operational amplifier and the sensor.
In addition, the radiation of electromagnetic waves exists in each circuit, which forms the coupling between the circuits. When the current changes, it will produce electromagnetic waves. These electromagnetic waves can be coupled to nearby conductors and interfere with other signals in the circuit.
All electronic circuits are subject to electromagnetic interference. Although a portion of the electromagnetic interference is directly accepted by radio frequency radiation, most of the electromagnetic interference is accepted by transient conduction. In digital circuits, critical signals such as reset, interrupt and control signals are most susceptible to electromagnetic interference. Control circuits, analog low-level amplifiers and power supply adjustment circuits are also susceptible to noise.