What is an impedance board?
I believe that many friends who are engaged in circuit boards are familiar with the name of impedance board. So what exactly is an impedance board and what is the role of an impedance board? This has stumped many friends who are engaged in circuit boards. Today we will learn what an impedance board is? What are the characteristics of an impedance board? How to tell if it is an impedance board! The definition of an impedance board is: a good laminated structure can control the characteristic impedance of a printed circuit board, and its routing can form an easily controlled and predictable transmission line structure called an impedance board.
1.Impedance characteristics of printed circuit boards
According to the transmission theory of signals, signals are functions of time and distance variables, so every part of the signal on the connection line may change. Therefore, the AC impedance of the connection, that is, the ratio of the change in voltage to the change in current, is determined 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 actual circuits, the resistance of the wire itself is less than the distributed impedance of the system, especially in high-frequency circuits.
The characteristic impedance 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.
2.Calculation of Characteristic Impedance of Printed Circuit Board
The ratio between the rising edge time of the signal and the time required for the signal to be transmitted to the receiving end determines whether the signal connection is regarded as a transmission line.
The specific ratio 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. From the signal equivalent impedance calculation formula, it can be seen that the impedance of the transmission line can be expressed by the following formula: Under high frequency (tens of megahertz to hundreds of megahertz), wL>>R is satisfied (of course, in the range of signal frequency greater than 109Hz, considering the skin effect of the signal, this relationship needs to be carefully studied).
Then for a certain transmission line, its characteristic impedance is a constant. The reflection phenomenon of the signal is caused by the inconsistency between the characteristic impedance of the signal driving end and the transmission line and the impedance of the receiving end. For CMOS circuits, the output impedance of the signal driving end is relatively small, which is tens of ohms. The input impedance of the receiving end is relatively large.
3.Characteristic impedance control of printed circuit boards
The characteristic impedance of the conductors on printed circuit boards is an important indicator of circuit design. Especially in the PCB design of high-frequency circuits, it is necessary to consider whether the characteristic impedance of the conductors is consistent with the characteristic impedance required by the device or signal, and whether they match. Therefore, there are two concepts that must be paid attention to in the reliability design of PCB design.
Impedance control of printed circuit boards
There will be various signal transmissions in the conductors in the circuit board. When the frequency must be increased to increase its transmission rate, if the line itself is different due to factors such as etching, stacking thickness, and conductor width, it will cause the impedance value to change, causing its signal to be distorted. Therefore, the impedance value of the conductors on high-speed circuit boards should be controlled within a certain range, which is called “impedance control”.
The factors that affect the impedance of PCB traces mainly include 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 traces around the traces. Therefore, when designing PCBs, it is necessary to control the impedance of the traces on the board to avoid signal reflection and other electromagnetic interference and signal integrity problems as much as possible, and ensure the stability of the actual use of the PCB board. The calculation method of the impedance of microstrip and stripline on PCB can refer to the corresponding empirical formula.
4.Impedance matching of printed circuit board
In the circuit board, if there is a signal transmission, it is hoped that it can be smoothly transmitted from the power supply end to the receiving end with minimal energy loss, and the receiving end will completely absorb it without any reflection. To achieve this transmission, the impedance in the line must be equal to the impedance inside the sending end 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 routing method.
For example, whether it is on the surface layer (Microstrip) or the inner layer (Stripline/Double Stripline), the distance from the reference power layer or ground layer, the routing width, the PCB material, etc. will affect the characteristic impedance value of the routing. In other words, the impedance value can only be determined after wiring, and the characteristic impedance produced by different PCB manufacturers also has slight differences.
Generally, simulation software cannot take into account some impedance discontinuity wiring situations due to the limitations of the line model or the mathematical algorithm used. At this time, only some terminations (Temninators), such as series resistors, can be reserved on the schematic diagram to alleviate the effect of impedance discontinuity. The real fundamental solution to the problem is to try to avoid impedance discontinuity during wiring.
