Define the PCB impedance board

- Aug 05, 2016-

A good laminated structure will be able to play on the printed circuit board characteristic impedance control, the traces can be formed easily controlled and predictable transmission line structure is called impedance plate.

      A printed circuit board impedance characteristics:

According to the theory of signal transmission, the signal is a function of time, distance variables, so the signal in each part of the connection are likely to change. So make sure to connect the AC impedance, the ratio of the change in the voltage and current variations of the characteristic impedance of the transmission line (Characteristic Impedance): characteristic impedance of the transmission line is only inherent to the signal wiring. In the actual circuit, the resistance value is less than the wire itself distributed impedance of the system, which is still a high-frequency circuit, the characteristic impedance depends on the unit to connect distributed capacitance and distributed inductance impedance distribution brings. Ideal characteristic impedance of the transmission line to connect the unit depends only on the distributed capacitance and distributed inductance.

Second, the characteristic impedance of the printed circuit board is calculated:

Rise time and signal transmission signal to the receiving end of the ratio between the time required to determine whether the signal connection is seen as a transmission line. The ratio between the specific by the following formula may be: If the PCB conductor wiring length is greater than l / b can be connected between the signal wire is regarded as a transmission line. By a signal equivalent impedance calculation formula shows that the impedance of the transmission line can use the following formula: meet wL >> R at high frequency (tens of MHz to several hundred MHz) case (of course, in the range of signal frequencies above 109Hz, the Taking into account the skin effect signals the need to study this relationship carefully). Then for determining the transmission line, the characteristic impedance is a constant. Reflection phenomenon is because the signal characteristic impedance of the transmission line and the driving end of the signal at the receiving end and an impedance inconsistencies caused. For CMOS circuits, the output impedance of the driving end of the signal is relatively small, several tens of ohms. And the receiver input impedance is relatively large.

Third, the printed circuit board characteristic impedance control:

Printed wire board is an important indicator of the characteristic impedance of the circuit design, especially in the high frequency circuit PCB design must be considered and the characteristic impedance of the wire or signal devices required are the same characteristic impedance, match. Thus, there are two concepts is important to note that the reliability design PCB design.

Fourth, the printed circuit board impedance control:

Circuit board conductors have a variety of signal transduction, when to increase its transmission rate must increase its frequency, the line itself Ruoyin etching, lamination thickness, wire width and other factors differ, it will cause changes in impedance worth to signal distortion. Therefore, in the high-speed circuit board conductor, the impedance value should be controlled within a certain range, known as the "impedance control." Factors affecting impedance PCB traces the main thickness, the thickness of the pad, the copper ground path width, thickness of the copper, the dielectric constant of the medium, medium, surrounding traces traces like. So in the design of PCB must be on board trace impedance is controlled in order to avoid as far as possible reflection and other electromagnetic interference signals and signal integrity problems and ensure the stability of the actual use of the PCB board. Calculation PCB board microstrip and stripline impedance can refer to the corresponding empirical formula.

Five, printed circuit board impedance matching:

In the circuit board, if the signal transmission time, I hope by the power of porting issue in the case of minimal energy loss, it can be smoothly transferred to the receiving end and the receiving end will be completely absorbed without any reflection. To achieve this transmission line impedance must end the internal impedance equal to the job issue is called "impedance matching." In the high-speed PCB design circuit, impedance matching is one of the elements of design. The impedance value and alignment methods are absolute relationship. For example, walking in the surface layer (Microstrip) or inner (Stripline / Double Stripline), the distance between the reference power supply layer or stratum, trace width, PCB material, and affection characteristic impedance value traces. In other words, in order to determine the impedance value after routing, while PCB manufacturers produced different characteristic impedance is also a slight difference. General simulation software due to restriction line model or mathematical algorithms used and can not take into account some of the wiring of impedance discontinuities, which can only be set aside some time termination (Temninators) on the schematic, such as series resistance to mitigate trace impedance discontinuity effects. The method of the real fundamental solution to the problem or try to pay attention when wiring to avoid impedance discontinuities occur.


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