User question: We know that the most important point of the capacitor we use is filter and bypass. I use it in the design. For high-frequency clutter, my experience is not to use too large a capacitor, because I personally think that the excessive capacitance may be better for low-frequency clutter filtering, but for high-frequency clutter due to its resonant frequency. The drop is so that the filtering effect on high frequency clutter is not ideal. So the choice of capacitor is not as large as possible. Questions: 1. All of the above are my experience. There is no theoretical confirmation. I hope that anyone can help explain whether it is correct in theory. Or recommend a web page or website. 2. Is it more than the resonant frequency, its impedance will be greatly increased, so the effect of the high-frequency filtered signal is relatively reduced? 3. Is the ideal filter point at the resonant frequency? ? ? (Not understanding) 4. In the past, I only knew that the bypass function of the capacitor was through the direct communication. Now, in the PCB design, what is the bypass function of the capacitor? I hope that everyone can discuss it together. One person may not answer all of them, but I believe that the wisdom of everyone will be ok. Thank you for your help~~~~~~ hunter188 reply: When using electromagnetic capacitance to suppress electromagnetic disturbance, the most easily overlooked problem is the effect of the capacitor lead on the filtering effect. The capacitive reactance of the capacitor is inversely proportional to the frequency. It is this characteristic that uses the capacitor in parallel between the signal line and the ground line to bypass the high frequency noise. However, in practical engineering, many people find that this method does not have the effect of filtering out noise, and it is helpless in the face of stubborn electromagnetic noise. One reason for this is to ignore the effect of the capacitor leads on the bypass effect. The circuit model of the actual capacitor is a series network of equivalent inductance (ESL), capacitance and equivalent resistance (ESR). The impedance of the ideal capacitor decreases with increasing frequency, and the impedance of the actual capacitor is the impedance characteristic of the network shown in Figure 1. At lower frequencies, the capacitance characteristic appears, that is, the impedance decreases with increasing frequency. Resonance occurs at a point where the impedance of the capacitor is equal to the equivalent series resistance ESR. Above the resonance point, due to the action of ESL, the capacitance of the capacitor increases with increasing frequency, which is the impedance characteristic of the capacitor exhibiting inductance. Above the resonance point, since the impedance of the capacitor increases, the bypass effect on the high frequency noise is weakened or even disappeared. The resonant frequency of the capacitor is determined by the combination of ESL and C. The larger the capacitance or inductance value, the lower the resonant frequency, that is, the worse the high frequency filtering effect of the capacitor. In addition to the type of capacitor, the length of the lead of the capacitor is a very important parameter. The longer the lead, the larger the inductance and the lower the resonant frequency of the capacitor. Therefore, in practical engineering, the lead of the capacitor should be as short as possible. According to the principle of LC circuit series resonance, the resonance point is not only related to the inductance, but also related to the capacitance value. The larger the capacitance, the lower the resonance point. Many people think that the larger the capacitance of the capacitor, the better the filtering effect, which is a misunderstanding. The larger the capacitance, the better the bypass effect of the low-frequency interference, but since the capacitance resonates at a lower frequency, the impedance starts to increase with the increase of the frequency, so the bypass effect on the high-frequency noise is deteriorated. Table 1 shows the self-resonant frequency of capacitors of different capacities. The lead length of the capacitor is 1.6mm (do you use the lead of the capacitor so short?). Table 1 Self-resonant frequency (MHz) Capacitance value Self-resonant frequency (MHz) 1m F 1.7 820 pF 38.50.1m F 4 680 pF 42.50.01m F 12.6 560 pF 453300pF 19.3 470 pF 491800 pF 25.5 390 pF 541100pF 33 330 pF 60 Although the resonance of the capacitor is undesirable from the point of view of filtering out high frequency noise, the resonance of the capacitor is not always harmful. When the noise frequency to be filtered is determined, the capacitance can be adjusted to make the resonance point just fall on the disturbance frequency.
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About the filtering and bypassing of capacitors