Why are large capacitors used in low-frequency channels and small capacitors Used in High-frequency circuits?

 

The capacitor around 10 PF is used to filter out high-frequency interference signals, and the capacitor around 0.1uf is used to filter out low-frequency ripple interference.
Can play the role of regulator

The specific capacity of the filter capacitor depends on the main operating frequency on your PCB and the harmonic that may affect the system.
Frequency. You can check the capacitor information of the relevant manufacturer or refer to the database software provided by the manufacturer.
Select. As for the number, it may not be necessary. It is nice to add one or two more items according to your specific needs. It is useless for the time being.
Do not post it first. Select the capacity value based on the actual debugging conditions. If your PCB works at a low frequency, add two
The capacitor is enough. One considers the ripple and the other considers the high-frequency signal. If a large Instantaneous Current occurs, we recommend that you
Add a relatively large ta capacitor.

In fact, filtering should also contain two aspects, namely, the large and small values, that is, decoupling and bypass.
I will not talk about the principle, practical point, generally digital circuit decoupling 0.1uf can be used for 10 m or less; 20 m or more with 1
10 UF, to remove high-frequency noise better, probably according to C = 1/f. The bypass is usually relatively small, generally based on the resonance frequency
Generally 0.1 or 0.01 UF

When it comes to capacitors, a variety of calls will make people dizzy, bypass capacitors, decoupling capacitors, filtering capacitors, etc.
The principle is the same no matter how it is called, that is, it uses the characteristics of low impedance for the AC signal.
According to the equivalent impedance formula of the capacitor, the higher the operating frequency, the larger the capacitance value, the more
The lower the impedance is .. In a circuit, if the main function of a capacitor is to provide a low-impedance channel for the AC signal
Circuit capacitor. If the main purpose is to increase the AC coupling between the power supply and the ground and reduce the influence of the AC signal on the power supply, you can
It is called a decoupling capacitor. If it is used in a filter circuit, it can also be called a filter capacitor. In addition, for DC
Voltage, capacitor can also be used as circuit energy storage, and the use of Chong discharge plays the role of the battery. In actual situations
There are many functions. We don't have to spend too much time thinking about how to define them. In this article, we integrate these
Capacitors Used in High-Speed PCB design are called bypass capacitors.

The essence of a capacitor is communication and DC-separated. Theoretically, the larger the capacitor used for power filtering, the better.

However, due to the wiring of the lead and PCB, the capacitor is actually a parallel circuit of the inductance and capacitor,

(Also the resistance of the capacitor itself, and sometimes it cannot be ignored)

This introduces the concept of resonant frequency: ω = 1/(LC) 1/2

The capacitance below the resonant frequency is capacitive, And the capacitance above the resonant frequency is inductive.

Therefore, the low-frequency wave and the high-frequency wave are filtered by small capacitors.

This also explains why STM packages with the same capacity have a higher capacitance filtering frequency than dip packages.

This is a reference for how much capacitor is used.

Capacitor resonance frequency

Capacitance value Dip (MHz) STM (MHz)

1.0 μF 2.5 5

0.1 μF 8 16

0.01 μF 25 50

1000pf 80 160

100 pF 250 500

10 pf 800 1.6 (GHz)

It is just a reference. In the words of an old engineer, we rely mainly on experience.

A more reliable approach is to connect one small two capacitors in parallel,

Generally, the difference must be more than two orders of magnitude to obtain a larger filtering frequency band.

In general, the large Capacitor Filters out low-frequency waves, and the small Capacitor Filters out high-frequency waves. The capacitance value is inversely proportional to the square of the frequency to be filtered out.
.

For specific capacitor selection, use the formula C = 4pi * PI/(R * f * F)

It is not difficult to master the essence and method of how to select the power supply filter capacitor.

1) theoretically, the impedance of the ideal capacitor decreases with the increase of the frequency (1/JWC). However, due to the inductance effect of the pins at both ends of the capacitor
The capacitor is regarded as an LC series resonance circuit. The self-resonance frequency is the FSR parameter of the device, which indicates that the frequency is greater
When the FSR value is used, the capacitor becomes an inductor. If the capacitance filters the ground, when the frequency exceeds the FSR, the interference suppression will increase.
Discount, so a small capacitor is required to be connected in parallel. Why?

The reason is that small capacitance and high SFr value provide a ground-to-ground path for high-frequency signals. Therefore, in the power supply filter circuit
It is often understood as follows: Large capacitors consider low frequencies, while small capacitors consider high frequencies. The root cause is that SFr (self-resonance frequency) values are different.
Why? From this point of view, we can understand why the capacitor is used to filter the power supply.
As close as possible.

2) In actual design, we often have questions. How do I know the SFR of a capacitor? Even if I know the SFR value
How can I select the capacitance values of different SFr values? Select a capacitor or two capacitors?

The SFr value of the capacitor is related to the capacitance value and the pin inductance of the capacitor. Therefore, the SFR value of the capacitor is 0402, 0603, or directly inserted.
The SFr value of the capacity will not be the same, of course, there are two ways to obtain the SFR value, 1) device data sheet, such as 22pf0402 Capacitor
The SFr value is about 2 GB. 2) use a network analyzer to directly measure the auto-resonance frequency. How can this problem be measured? S21?

After knowing the SFR value of the capacitor, use software simulation, such as rfsim99 to select one or two circuits for the work of the power supply circuit.
Whether the frequency band has sufficient noise suppression ratio. After the simulation, it is the actual circuit test, such as when the mobile phone receives the sensitivity,
LNA power filtering is the key, and good power filtering can often improve several DBS.