Microwave Tech - The main points of electromagnetic compatibility design of high frequency board

The main points of electromagnetic compatibility design of high frequency board

High frequency equivalent characteristics of magnetic ring and magnetic beads

However, the high-frequency equivalent characteristics of the magnetic ring and the magnetic beads have to be mentioned. Because the magnetic ring's absorbing effect on high-frequency pulsation is similar to the performance of inductance, it is often considered as an inductive characteristic, but in fact it is wrong. The magnetic ring is a resistance characteristic, but this resistance is a bit special. Its resistance value is a function of frequency R(f). In this case, when a signal with high-frequency fluctuations passes through the magnetic beads, the high-frequency fluctuations will be Because of the heat generated by the I2R, the fluctuations will interfere with the conversion process of electrical energy-magnetic energy-thermal energy. Therefore, when the fluctuations on the wire are relatively strong, the magnetic ring will feel warm to the touch.

High frequency equivalent characteristics of cable or PCB wiring

The second is the high-frequency equivalent characteristics of the cable or PCB wiring. No matter the high and low frequencies, the trace resistance exists objectively, but for the trace inductance, it can only be revealed at the higher frequency. In addition, there is also a distributed capacitor. However, when there is no conductor near the wire, this distributed capacitor is useless. Just like a woman can’t give birth to a child without a man, it can only be played by two conductors. effect.

EMC design technology under high frequency

The above is the basic knowledge of EMC professional middle and high frequency thinking. With these, a series of design experience can be easily solved. For example, why two capacitors, one electrolytic capacitor and one ceramic capacitor, are installed on the VCC terminal of the IC? It is because of the high-frequency equivalent characteristics of the capacitor. At the frequency point of WL= (1/WC), it is the point with the smallest impedance (as shown in the figure). In addition, the two capacitors have their own minimum impedance points, corresponding to different frequency points, so as to provide current for the power supply requirements of the IC in different frequency ranges.

The ground wire of the electrostatic workbench uses a wide copper belt and a metal wire mesh snake skin tube instead of a yellow-green round ground cable. The wiring inductance of the round ground cable is too large, which is not conducive to high-frequency electrostatic charges. Vent. The distance between the cable and the cable should not be too close, otherwise it will cause crosstalk between the signal cables due to the distributed capacitance of the wires. Of course, the coupling of the signal wire to the ground wire is better to be closer. In this way, The fluctuating interference on the signal line can be easily vented to the ground line.

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