Definition of impedance PCB board

- Mar 22, 2017-

One,Printed circuit board impedance characteristics:

According to the theory of signal transmission, the signal is a function of time and distance variables, so every part of the signal on the wire is likely to change. Therefore, to determine the impedance of the line is the characteristic impedance of the transmission line voltage variation changes and current ratio (Characteristic Impedance): the characteristic impedance of transmission line and signal line itself characteristics. In the actual circuit, the resistance value of the wire itself is less than that of the system, which is the characteristic impedance of the high frequency circuit. The characteristic impedance of an ideal transmission line depends only on the unit distributed capacitance and the unit distributed inductance of the line.

Two, printed circuit board characteristic impedance calculation:

The proportional relationship between the rising time of the signal and the time required for the transmission of the signal to the receiving end determines whether the signal line is regarded as a transmission line. The specific proportion of the relationship can be explained by the following formula: if the length of the wire connection on the PCB board is greater than l/b, the connecting wire between the signals can be regarded as a transmission line. According to the calculation formula of signal equivalent impedance, impedance of the transmission line can be represented by the following formula: in high frequency (tens to hundreds of megahertz MHz) under the condition of satisfying wL>>R (range in the course of the signal frequency is greater than 109Hz, while considering the skin effect, the signal need to carefully study the relationship). For a given transmission line, the characteristic impedance is a constant. The reflection of the signal is caused by the characteristic impedance of the driver and the transmission line and the impedance of the receiving end. For the CMOS circuit, the signal output impedance of the driver side is relatively small, for dozens of europe. The input impedance of the receiver is relatively large.

Three, printed circuit board characteristic impedance control:

Printed circuit board wire characteristic impedance is an important index of the circuit design, especially in the design of PCB high frequency circuit, must consider the characteristic impedance of the conductor impedance requirements and device or signal is consistent, whether matching. Therefore, there are two concepts in the reliability design of PCB design.

Four, printed circuit board impedance control:

The conductor of the circuit board in a variety of signal transmission, and when must improve its frequency to increase its transmission rate, the line itself due to etching, laminate thickness, width and other factors, will cause the change of impedance is, signal distortion. As a result, the impedance of a conductor on a high speed circuit board shall be controlled within a certain range". The main factors that affect the impedance of the PCB line are the width of the copper wire, the thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the pad, the path of the earth wire and the route of the line. Therefore, in the design of the PCB must be on the board to control the line impedance, as far as possible to avoid signal reflection and other electromagnetic interference and signal integrity issues, to ensure the stability of the actual use of PCB board. The calculation method of microstrip and stripline impedance on the PCB board can refer to the corresponding empirical formula.

Five, printed circuit board impedance matching:

In the circuit board, if the signal is transmitted by the sending end, hope power, in the case of the minimum energy loss, can be smoothly transmitted to the receiving end, and the receiving end will be completely absorbed without any reflection. In order to achieve this kind of transmission, the impedance in the circuit must be equal to the impedance of the terminal". Impedance matching is one of the key elements in the design of high speed PCB circuits. And the impedance value has a positive relationship with the route.

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