Impedance plate

The definition of the impedance plate is: a good laminated structure can play a role in controlling the characteristic impedance of the printed circuit board, and its wiring can form an easily controlled and predictable transmission line structure called the impedance plate.If you want to order PCB product, please check and custom your order online.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 may change on the connection. Therefore, determine the AC impedance of the connection, that is, the ratio of the change in voltage to the change in current as the characteristic impedance of the transmission line (Characteristic Impedance): the characteristic impedance of the transmission line is only related to the characteristics of the signal connection itself. In the actual circuit, the resistance value 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 caused 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.Impedance calculation The proportional relationship between the rising edge time of the signal and the time required for the signal to reach the receiving end determines whether the signal connection is regarded as a transmission line. The specific proportional relationship can be explained by the following formula: If the length of the wire on the PCB is greater than l/b, the connecting wire between the signals can be regarded as a transmission line. From the calculation formula of the signal equivalent impedance, the impedance of the transmission line can be expressed by the following formula: at high frequencies (tens of megahertz to hundreds of megahertz), wL>>R is satisfied (of course in the range of signal frequency greater than 109 Hz, then 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 driving end of the signal and the transmission line and the impedance of the receiving end. For the CMOS circuit, the output impedance of the driving end of the signal is relatively small, which is tens of ohms. The input impedance of the receiving end is relatively large.Impedance control There are various signal transmissions in the conductors in the circuit board. When the transmission rate must be increased, the frequency must be increased. If the circuit itself is different due to factors such as etching, stack thickness, and wire width, it will cause the impedance value to change, making it The signal is distorted. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, called "impedance control". The factors that affect the impedance of the PCB traces 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 trace. Therefore, when designing the PCB, the impedance of the traces on the board must be controlled to avoid signal reflection and other electromagnetic interference and signal integrity problems as much as possible to ensure the stability of the actual use of the PCB board. The calculation method of the impedance of the microstrip and stripline on the PCB board can refer to the corresponding empirical formula.Impedance matching In the circuit board, if there is signal transmission, it is hoped that from the sending end of the power supply, in the case of minimum energy loss, it can be smoothly transmitted to the receiving end, and the receiving end absorbs it completely 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 wiring method. For example, whether it is walking on the surface layer (Microstrip) or inner layer (Stripline/Double Stripline), the distance from the reference power layer or ground layer, the width of the trace, the PCB material, etc. will affect the characteristic impedance value of the trace. In other words, the impedance value must be determined after wiring, and the characteristic impedance produced by different PCB manufacturers also has slight differences. Generally, the simulation software will not be able to consider some wiring with discontinuous impedance due to the limitations of the line model or the mathematical algorithm used. At this time, only some terminations (Temninators), such as series resistance, etc., can be reserved on the schematic. Alleviate the effect of trace impedance discontinuity. The real fundamental solution to the problem is to try to avoid impedance discontinuities when wiring.definition:Definition of characteristic impedance: At a certain frequency, in the transmission signal line of an electronic device, relative to a certain reference layer, the resistance of its high-frequency signal or electromagnetic wave during propagation is called characteristic impedance, which is electrical impedance, inductance A vector sum of reactance, capacitance reactance...Classification of characteristic impedance:Common characteristic impedances are divided into: single-ended (line) impedance, differential (dynamic) impedance, coplanar impedance, etc.Single-ended (line) impedance: English single ended impedance refers to the impedance measured by a single signal line.Differential (dynamic) impedance: English differential impedance, refers to the impedance measured in two transmission lines of equal width and equal interval during differential driving.Coplanar impedance: English coplanar impedance, which refers to the impedance measured when the signal line is transmitted between its surrounding GND/VCC (the distance between the signal line and the GND/VCC on both sides is equal).Determining conditions for impedance control requirements: when the signal is transmitted in the PCB wire, if the length of the wire is close to 1/7 of the signal wavelength, the wire at this time becomes a signal transmission line, and generally the signal transmission line needs to be impedance controlled. During PCB production, it is determined whether the impedance needs to be controlled according to the customer's requirements. If the customer requires impedance control for a certain line width, the impedance of the line width needs to be controlled during production.Three elements of impedance matching: output impedance (original active part) characteristic impedance (signal line) input impedance (passive part) (PCB board)Impedance matchingWhen the signal is transmitted on the PCB, the characteristic impedance of the PCB must match the electronic impedance of the head and tail components. Once the impedance value exceeds the tolerance, the transmitted signal energy will be reflected, scattered, attenuated or delayed, resulting in The signal is incomplete and the signal is distorted.Wanna know PCB knowledge? Check and read for more.