Detailed analysis of the selection and calculation of power supply filter capacitor(III)

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

1.In theory, the impedance of the ideal capacitance 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. 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 the FSR, the suppression of interference is discounted, so a small capacitance parallel to the ground is needed. The reason is that the small capacitance, the SFR value is large, and the high frequency signal provides a ground path. So in the power filter circuit, we often understand that the large electric capacity filter low frequency, the small capacitance filter the high frequency, fundamental The reason is that the value of SFR is different. Why? 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.In actual 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, if the sensitivity of the receiver is debugged, the LNA power filter is the key and the good power filter is often available. Improve several dB.

The essence of capacitance is through AC, DC, and theoretically speaking, the greater the capacity of the power filter, the better the better. But because of the lead and PCB wiring reasons, the capacitance is actually the shunt circuit of the inductance and the capacitance (and the resistance of the capacitor itself, sometimes can not be ignored). This introduces the concept of the resonant frequency: Omega =1/ (LC) 1/2

The capacitance below the resonant frequency is capacitive, and the capacitance above the resonant frequency is inductive. Therefore, a large capacitor can be used to filter low frequency waves and small capacitance to filter high-frequency waves. This also explains why the capacitance filter frequency of the same value STM package is higher than that of the DIP package. As for how large the capacitance is, this is a reference capacitor resonant frequency:

Capacitance value DIP (MHz) STM (MHz)

1 mu F 2.55

0.1 mu F 816

0.01 mu F 2550

1000pF 80160

100 pF 250500

10 pF 8001.6 (GHz)