Large capacitance filter low frequency, small capacitance filter high frequency filter capacitor selection (III)

The specific capacitance can be chosen by formula C=4Pi*Pi / (R * f * f).

How to select the power filter capacitor and grasp its essence and method is not difficult at all.

1. theoretically, the impedance of the ideal capacitor decreases with the increase of frequency (1/jwc), but due to the inductance effect of the pins at both ends of the capacitor, the capacitance should be considered as a LC tandem resonant circuit, and the self resonant frequency is the FSR parameter of the device, which indicates that the capacitance becomes an inductor when the frequency is greater than the FSR value, if the capacitance is filtered to the ground, When the frequency exceeds FSR, the suppression of interference is greatly reduced, so a smaller capacitor is needed to connect to the ground. Why?

The reason lies in the small capacitance, the SFR value is big, the high frequency signal provides a ground path, so in the power filter circuit, we often understand that the large capacitance is low frequency, the capacitance is high frequency, the fundamental reason is that the SFR (self resonant frequency) value is different, of course, why do you think of it? If you think from this angle, you can understand why the capacitor in the power filter is as close to the ground as possible.

2. then in practical design, we often have doubts. How do I know the SFR of capacitance? Even if I know the SFR value, how do I select the capacitance values of different SFR values? Is it a capacitor or two capacitors?

The SFR value of the capacitance is related to the capacitance value, which is related to the pin inductance of the capacitor, so the SFR value of the 04020603 or the straight plug capacitance of the same capacitance will not be the same, and of course there are two ways to obtain the SFR value.

(1) device Data sheet, for example, the SFR value of 22pf0402 capacitance is about 2G.

(2) directly measure its self resonant frequency through network analyzer, and think about how to measure S21?

After knowing the SFR value of the capacitor, using software simulation, such as RFsim99, selecting one or two circuits is whether there is enough noise suppression ratio in the working band of the power supply circuit. After the simulation, it is the actual circuit test, such as when debugging the sensitivity of the mobile phone, the LNA power filter is the key and the good power filter is often possible. Improve several dB.

The common point is to use the capacitance as a leaking water, when the peak of the alternating current is taken as the water of the arms, when the amount of water is equal, then the frequency of the water is high with the smaller arms, so that the water level can be kept high. On the contrary, the baby is small at low frequency of adding water. The water level in the water has dropped much better when the two water comes into being, so we need to use large water to ease the water level drop due to water leakage in Huailai.

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