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Characteristics Of Impedance PCB

- Jul 03, 2017 -

Impedance PCB Definition:

At a certain frequency, the electronic device transmission signal line, relative to a reference layer, its high-frequency signal or electromagnetic wave in the transmission process of resistance is called the characteristic impedance, it is resistance, inductance resistance, capacitance resistance ... The sum of a vector.

When the signal in the PCB wire transmission, if the length of the wire near the signal wavelength of 1/7, at this time the wire will become a signal transmission line, the general signal transmission lines are required to do impedance control. PCB production, according to customer requirements to determine whether the need for control impedance, Impedance PCB if the customer requires a certain line width need to do impedance controls, production needs to control the wire width impedance.

When the signal is transmitted on the PCB, the characteristic impedance of PCB board must match the electronic impedance of the head and tail element, but the impedance value exceeds the tolerance, the signal energy will appear reflection, scattering, attenuation or delay, Impedance PCB which leads to incomplete signal and distorted signal.

Impedance PCB calculation:

The proportional relationship between the signal's rising time and the time required to transmit the signal to the receiving end determines whether the signal connection is considered as a transmission line. The specific proportion of the relationship by the following formula can be explained: If the PCB board wire connection length is greater than L B can be the signal between the connecting wire as a transmission line. By the formula of the equivalent impedance of the signal, the impedance of the transmission line can be expressed in the following formula: In the case of high frequency (dozens of MHz to hundreds of MHz), the Wl (of course, Impedance PCB in the range of the signal frequency greater than 109Hz, taking into account the skin effect of the signal, the relationship needs to So for a certain transmission line, its characteristic impedance is a constant. The reflection of the signal is caused by the characteristic impedance of the driving end and the transmission line, and the impedance inconsistency of the receiving end. For CMOS circuits, the output impedance of the signal driver is small, for dozens of euro. The input impedance of the receiving end is relatively large.

Impedance PCB Control:

The characteristic impedance of the conductor on the printed circuit board is an important index of the circuit design, especially in the PCB design of the high-frequency circuit, it is necessary to consider whether the characteristic impedance of the wire and the characteristic impedance required by the device or signal are consistent. Therefore, Impedance PCB there are two concepts that must be paid attention to in the design of PCB design reliability.

Printed Circuit board Impedance control:

The conductor in the circuit board will have a variety of signal transmission, in order to improve its transmission rate and must improve its frequency, the line itself, if due to etching, laminated thickness, wire width and other factors, will cause impedance worthy of change, so that its signal distortion. Therefore, the conductor on the high-speed PCB, its impedance value should be controlled within a certain range, called "Impedance control." The main factors that affect the impedance of PCB are the width of copper wire, the thickness of copper wire, dielectric constant, the thickness of the medium, the thickness of the solder plate, Impedance PCB the path of the ground wire, the line of walking around the line and so on. Therefore, in the design of PCB must be on the board to control the impedance of the line, in order to avoid the reflection of the signal and other electromagnetic interference and signal integrity problems, to ensure the actual use of the PCB board stability. Impedance PCB The calculation method of Microstrip line and strip line impedance on PCB board can refer to the corresponding empirical formula.

Printed circuit board Impedance matching:

In the circuit board, if there is signal transmission, it is hoped that the power supply from the end, in the case of the smallest energy loss, can be successfully transmitted to the receiving end, and the receiving end will be fully absorbed without any reflection. To achieve this transmission, the impedance of the line must be equal to the impedance inside the emitting end, called "Impedance matching". When designing high speed PCB circuit, Impedance PCB impedance matching is one of the elements of design. But the impedance value and the line way have the absolute relation. For example, is walking in the surface layer (MICROSTRIP) or the inner layer (Stripline Double Stripline), with reference to the power layer or the formation of distance, the line width, PCB material, etc. will affect the characteristic impedance of the line. In other words, to be able to determine the impedance after wiring, at the same time different PCB manufacturers produced by the characteristic impedance also have a small difference. General simulation software will be due to the line model or the limitations of the mathematical algorithm used to take into account some of the impedance of the wiring is not continuous, at this time in the schematic diagram can only be reserved for some of the termination (Temninators), such as series resistance, to ease the line impedance discontinuity effect.

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