EMC problems often occur at high frequency, except for some problems (voltage sag and instantaneous interruption). High frequency thinking, in a word, is the characteristics of devices and circuits. In high frequency, it is different from that in conventional medium and low frequency. If we still judge and analyze according to the common control thinking, we will go into the wrong area of design. For example:

  High frequency equivalent characteristics of capacitors

Capacitance, in the case of medium and low frequency or DC, is an energy storage component, which only shows the characteristics of a capacitor, but in the case of high frequency, it is not only a capacitor, it has the characteristics of an ideal capacitor, with leakage current (in the In the high frequency equivalent circuit, there is ESR (equivalent series resistance) with lead inductance and heating under the condition of voltage pulse fluctuation (as shown in the figure). From the analysis of this diagram, we can get a lot of useful design ideas for designers. First, according to the conventional idea, 1 / 2 π FC is the capacitive reactance of the capacitor. The higher the frequency is, the smaller the capacitive reactance is, the better the filtering effect is. That is to say, the higher the frequency of the clutter is, the easier it is to be discharged. But this is not the case. Because of the existence of the lead inductance, a capacitor is only in its 1 / 2 π FC = 2 π F When the l-equation is established, the filtering effect is the best when the overall impedance is the smallest. When the frequency is high or low, the filtering effect will decline. Therefore, we can analyze and draw a conclusion that why two capacitors, one electrolytic and one ceramic, are added to the VCC end of the IC, and the capacitance difference is generally more than 100 times. That is to say, the resonant frequency of two different capacitors diverges for a certain distance, which is beneficial to the filtering of higher frequency and lower frequency.

  

 High frequency equivalent characteristics of cable or PCB wiring

The second is the high-frequency equivalent characteristics of the cable or PCB wiring (as shown in the figure). No matter high or low frequency, the wiring resistance is objective, but for the wiring inductance, it can only be shown at higher frequency. In addition, there is a distributed capacitance. However, when there is no conductor near the conductor, the distributed capacitance is useless, just like women can’t have children without men. This is a function that needs two conductors.

  High frequency equivalent characteristics of magnetic ring and magnetic bead

However, the high-frequency equivalent characteristics of magnetic rings and magnetic beads have to be mentioned. Because the absorbing effect of magnetic rings on high-frequency pulsations is similar to that of inductors, it is often considered to be the characteristics of inductors. But in fact, it is wrong that magnetic rings are electrical Resistance characteristic, but this resistance is a little special. Its resistance value is a function of frequency R (f). In this way, when a signal with high-frequency fluctuation passes through the magnetic bead, the high-frequency fluctuation will occur due to the effect of I2R Heat, the fluctuation interference through the electric energy – magnetic energy – thermal energy conversion process, so when the fluctuation on the wire is relatively strong, the magnetic ring will feel warm.

  EMC design technology at high frequency

The above is the basic knowledge of high frequency thinking in EMC specialty. With these, a series of design experience can be easily solved. For example, why two capacitors, one electrolytic capacitor and one ceramic chip capacitor are installed on the VCC end of IC is due to the high-frequency equivalent characteristics of the capacitor, and the series connection of lead inductance and capacitor causes its comprehensive impedance to change with frequency, while the frequency point with WL = (1 / WC) is the point with the smallest impedance (as shown in the figure). Moreover, the two capacitors have their own minimum impedance points, corresponding to different frequency points, so as to provide current for power supply demand of IC in different frequency range.

The wide copper belt and metal mesh snake skin tube are used for the grounding wire of the electrostatic workbench instead of the yellow and green round grounding cable. The inductance of the round grounding cable is too large, which is not conducive to the discharge of high-frequency electrostatic charges. The distance between cables should not be too close, otherwise crosstalk will occur between signal cables due to the existence of distributed capacitance of wires. Of course, the coupling between signal line and ground wire should be closer, so that the fluctuation interference on signal line can be easily discharged to the ground line.

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