Magnetic circuit design of the PCB selection

 1, the bead is in ohm, not Hunt, which is to pay special attention. Because the unit is in accordance with its beads in a certain frequency to produce the nominal impedance, the impedance of the unit is ohm. Beads DATASHEET will provide the general characteristics of the frequency and impedance curves, generally 100MHz as standard, such as the 1000R @ 100MHz, 100MHz frequency means that when the bead in the impedance of the equivalent of 600 ohms.
  2, the general filter is lossless reactance component composition, it is the role of the line in the stop-band frequency signal reflected back to the source, so this type of filter called the reflection filter. When the reflection filter and source impedance match, there will be a part of the energy is reflected back to the source, resulting in an enhanced level of interference. To solve this drawback, can be used in the filter into the line ferrite bead or bead set, the use of Zi ring or bead of high-frequency signals of eddy current losses, the high frequency components into a heat loss. So actually ring and bead from the absorption of high frequency components, it is sometimes also known as absorption filters.
  Different ferrite suppression components, have different optimal inhibition of the frequency range. Usually the higher the permeability, the lower the frequency of suppression. In addition, the greater the volume of ferrite, the better the inhibition effect. In a certain volume, the long, thin shape of the short and thick than the inhibitory effect of good, the smaller the diameter the better inhibitory effect. However, there are AC or DC bias in the case, there is the problem of saturation ferrite, inhibition element cross-section, the more easily saturated, the bias can withstand greater. EMI absorbing bead / bead differential mode interference suppression, through its current value proportional to its size, both disorders cause saturation, reducing the device performance; suppress common mode interference, the power of the two wires (positive and negative) At the same time through a magnetic ring, the effective signal for the differential mode signal, EMI absorbing bead / bead it has no effect, and for the common mode signal will show a larger inductance. The use of magnetic ring there a better way is to make repeated passes through the magnetic wire around a few, to increase the inductance. According to the principle of its suppression of electromagnetic interference, the rational use of its inhibition.
  Ferrite suppression components should be installed in areas near the sources of interference. For the input / output circuit should be as close to the shield shell into the outlet. Ferrite bead and bead form of the absorption filter, should be used in addition to the high permeability of lossy materials, but also pay attention to its applications. They line the high-frequency components of the resistance presented by about ten to several hundred Ω, so it is in high impedance circuit is not obvious, on the contrary, in low impedance circuits (such as power distribution, power supply or RF) in use will be very effective.
  Since ferrite can attenuate the higher frequency while allowing the low frequency is almost unimpeded through, so the EMI control has been widely applied. EMI absorption for magnetic / magnetic beads can be made into various shapes, are widely used in various occasions. If the PCB, can be added to the DC / DC module, data cable, power cord, etc.. It absorbs the high frequency interference signals on the line, but not in the system generate a new zero-pole, will not disrupt the stability of the system. It is used in conjunction with the power filter, the filter can be a good complement the lack of high frequency performance, improved system filter characteristics.
Beads specifically inhibit the signal line, power line noise and spikes of high frequency interference, but also has the ability to absorb static pulse.
Bead is used to absorb the ultra-high frequency signal, as some RF circuits, PLL, oscillator circuits, including ultra-high frequency memory circuit (DDR SDRAM, RAMBUS, etc.) need to increase the power input part of the bead, and the inductor is a build to components used in the LC oscillator circuit, the low-frequency filter circuit, the application frequency range rarely exceeds wrong 50MHZ.
  The main function is to remove beads present in the transmission line structure (circuit) in the RF noise, RF energy is superimposed on the DC transmission voltage level of the AC sine wave components, DC components of the useful signal is needed, and radio frequency RF energy is no along the lines of electromagnetic interference with the transmission and radiation (EMI). To eliminate these unwanted signal energy, the use of chip beads play the role of high-frequency resistance (attenuator), the device allows the DC signal, and filtering the AC signal. Usually high-frequency signal of 30MHz or more, however, low-frequency signal will be the impact of chip beads.
   By the ferrite chip bead material, forming a high volume resistivity of the monolithic structure. With eddy current loss is inversely proportional to the resistivity of the ferrite material. Eddy current loss with the signal frequency proportional to the square. The benefits of using chip beads: small and lightweight radio frequency noise in the frequency range of high impedance transmission lines to eliminate electromagnetic interference. Closed magnetic circuit structure, the better to eliminate signal crosstalk. Excellent magnetic shielding structure. Lower DC resistance, so as not to produce too much useful signal attenuation. Significant high-frequency characteristics and impedance characteristics (better to eliminate the RF energy). In the high-frequency amplifier circuit to eliminate parasitic oscillations. Work effectively in a few MHz to several hundred MHz frequency range.
Beads to the right choice, we must note the following:
1, no signal of the frequency range of the number;
2, the noise sources are;
3, require much noise attenuation;
4, What is the environmental conditions (temperature, DC voltage, structural strength);
5, circuit and load impedance is;
6, if there is room to place beads on a PCB;
  The first three manufacturers to provide resistance by observing the frequency curve can be judged. Three curves in the impedance curve is very important that the resistance, inductance and total resistance. The total impedance by ZR22πfL () 2 +: = fL to describe. With this curve, select the frequency you want the noise attenuation with the largest range of impedance in low frequency and DC signal attenuation as small as possible under the bead model. Chip Beads in excessive DC voltage, impedance will be affected, in addition, if the work temperature is too high, or an external magnetic field is too large, bead impedance will be adversely affected. Using a chip beads and chip inductors reasons: the use of chip beads or chip inductors mainly lies in the application. Required in the resonant circuit chip inductor. The need to eliminate unwanted EMI noise, the use of chip beads is the best option. Chip Bead and Chip Inductor applications:
  Chip Inductors: Radio Frequency (RF) and wireless communications, information technology equipment, radar detectors, car, cellular phones, pagers, audio equipment, PDAs (personal digital assistants), wireless remote control system and low voltage power supply module.
  Chip Beads: clock circuits, analog circuits and digital circuits between the filter, I / O Input / Output Internal connectors (such as serial, parallel, keyboard, mouse, long-distance telecommunications, local area network), radio frequency (RF) circuits and logic devices susceptible to interference between the power supply circuit filter out high frequency conducted interference, computers, machines, video recorders (VCRS), television systems and mobile phones in the EMI noise rejection.