Four elements of PCB impedance board

Four elements of PCB impedance board

The definition of PCB impedance board is: a good laminated structure can control the characteristic impedance of the printed circuit board, and its wiring can form an easily controllable and predictable transmission line structure called impedance board. Now that we know what a PCB impedance board is, do you know some elements about a PCB impedance board?

1. printed circuit board characteristic impedance control

The characteristic impedance of the conductor on the printed circuit board is an important indicator of circuit design, especially in the design of high-frequency circuit PCB circuit board, it is necessary to consider whether the characteristic impedance of the conductor is consistent with the characteristic impedance required by the device or signal, and whether it matches .

Second, the impedance characteristics of the printed circuit board

According to signal transmission theory, a signal is a function of time and distance variables, so every part of the signal on the connection may change. Therefore, determine the AC impedance of the connection, that is, the ratio of the voltage change to the current change as the characteristic impedance of the transmission line: the characteristic impedance of the transmission line is only related to the characteristics of the signal connection itself. In the actual circuit, the resistance of the wire itself is less than the distributed impedance of the system, especially in high-frequency circuits, the impedance of the characteristic PCB impedance board mainly depends on the distributed impedance brought by the unit distributed capacitance and unit distributed inductance of the connection. The characteristic impedance of an ideal transmission line depends only on the unit distributed capacitance and unit distributed inductance of the connection.

Three, printed circuit board impedance control

There are various signal transmissions in the conductors in the circuit board. When the frequency must be increased in order to increase its transmission rate, if the circuit itself is different due to factors such as etching, stack thickness, wire width, etc., the impedance value will change. Its signal is distorted. Therefore, the impedance of the conductor on the high-speed circuit board should be controlled within a certain range, which is called "impedance control". The main factors affecting the impedance of the PCB circuit board traces 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 ground wire, and the wiring around the wire. Therefore, when designing the PCB, the impedance of the traces on the circuit board must be controlled to avoid signal reflection and other electromagnetic interference and signal integrity problems as much as possible, and to ensure the stability of the actual use of the PCB board. The calculation method of the impedance of the microstrip line and strip line on the PCB can refer to the corresponding empirical formula.

Four, printed circuit board impedance matching

In the PCB circuit board, if there is a signal transmission, it is hoped that from the sending end of the power supply, it can be smoothly transmitted to the receiving end under the condition of minimum energy loss, and the receiving end will completely absorb it without any reflection. To achieve this kind of transmission, the impedance in the line must be equal to the internal impedance of the transmitter to be called "impedance matching". When designing high-speed PCB circuits, impedance matching is one of the design elements. The impedance value has an absolute relationship with the wiring method. For example, whether it is walking on the surface layer or the inner layer, the distance from the reference power layer or ground layer, the trace width, and the PCB material will affect the characteristic impedance value of the trace. In other words, the impedance value can only be determined after wiring, and the characteristic impedances produced by different PCB manufacturers are also slightly different. Generally, simulation software cannot take into account some wiring conditions with discontinuous impedance due to the limitation of the circuit model or the mathematical algorithm used. At this time, only some terminations, such as series resistance, can be reserved on the schematic diagram to ease the wiring. The effect of impedance discontinuity. The real solution to the problem is to try to avoid impedance discontinuities when wiring.