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Radiation EMI interference can come from an undirected source and an unintentionally formed antenna.Conductive EMI interference can also be caused by a radiation EMI source or by some circuit board assembly.Once your circuit board receives conductive interference, it is inserted into the PCB trace of the application circuit.Common sources of radiation EMI interference include components mentioned in previous articles, as well as switching power supplies, connections and switching or clock networks on PCB boards.
Conductive EMI interference is a result of the interaction between the normal operation of the switching circuit and the parasitic capacitance and inductance.Figure 1 shows some of the EMI interference sources that will get into your PCB trace.Vemi1 originates from switching networks, such as clock signals or digital signal lines.The coupling modes of these interference sources are parasitic capacitance between the lines.These signals carry the current spike pulse into the adjacent PCB trace.Similarly, Vemi2 comes from a switching network, or from an antenna on a PCB.The coupling modes of these interference sources are parasitic inductance between the lines.The signal brings the voltage disturbance into the adjacent PCB trace.Each of the three EMI sources comes from adjacent wires within the cable.Signals traveling along these wires can produce crosstalk effects.
During normal operation, the switching power supply (SMPS) circuit provides an opportunity for the formation of conductive EMI.Switching between "on" and "off" in these power supplies produces a strong discontinuous current.These discontinuous currents exist at the input end of the buck converter, the output end of the boost converter, and the input and output ends of the flyback and boost topology.Discontinuous current caused by switching action produces voltage ripple, which is transmitted to other parts of the system through the PCB trace.Input and/or output voltage ripple caused by SMPS will endanger the operation of the load circuit.Figure 2 shows an example of frequency composition of a DC/DC buck SMPS input working at 2 MHz.The basic frequency component of SMPS conduction interference ranges from 90 to 100 MHz.
Here are a few examples of the many different types of sensors:
In a mercury-based glass thermometer, the input is temperature. The liquid contained expands and contracts in response, causing the level to be higher or lower on the marked gauge, which is human-readable.
An oxygen sensor in a car's emission control system detects the gasoline/oxygen ratio, usually through a chemical reaction that generates a voltage. A computer in the engine reads the voltage and, if the mixture is not optimal, readjusts the balance.