First, impedance characteristics of printed circuit board
According to the theory of signal transmission, the signal is a function of time and distance variables, so the signal can change in each part of the line. Therefore, the AC Impedance of the wire, that is, the ratio of the change in voltage to the change in current, is determined to be the characteristic impedance of the transmission line: the characteristic impedance of the transmission line is only related to the characteristics of the signal line itself. In the actual circuit, the resistance value of the wire itself is less than the distributed impedance of the system, especially in the high frequency circuit, the characteristic impedance mainly depends on the distributed impedance brought by the unit distributed capacitance and unit distributed inductance of the wire. The characteristic impedance of an ideal transmission line depends only on the unit distributed capacitance and unit distributed inductance of the line.
Two, printed circuit board characteristic impedance calculation
The proportional relationship between the rising edge time of the signal and the time required for the signal to travel to the receiving end determines whether the signal line is considered a transmission line. The specific proportional relationship can be explained by the following formula: If the wiring length of wires on PCB board is greater than l/b, the connecting wires between signals can be regarded as transmission lines. From the calculation formula of signal equivalent impedance, the impedance of a transmission line can be expressed by the following formula: it meets wL> at high frequency (tens of megahertz to hundreds of megahertz); > R(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 given transmission line, the characteristic impedance is a constant. The reflection of the signal is caused by the inconsistency between the characteristic impedance of the signal driver and the transmission line and the impedance of the receiving end. For CMOS circuits, the output impedance of the driving end of the signal is relatively small, which is tens of euros. The input impedance at the receiving end is larger.
Three, printed circuit board characteristic impedance control
The characteristic impedance of the wire on the printed circuit board is an important index of circuit design, especially in the PCB design of high frequency circuit, we must consider whether the characteristic impedance of the wire is consistent with the characteristic impedance required by the device or signal, whether it matches. Therefore, there are two concepts that must be paid attention to in the reliability design of PCB design.
Four, printed circuit board impedance control
There will be a variety of signal transmission in the conductor of the circuit board, when the transmission rate must be increased to improve its frequency, the line itself if the etching, laminated thickness, wire width and other factors are different, will cause the impedance value change, so that the signal distortion. Therefore, the impedance value 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 PCB wiring 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, the routing around the wire and so on. Therefore, when designing PCB, we must control the impedance of the line on the board, so as to avoid signal reflection and other electromagnetic interference and signal integrity problems as far as possible, and ensure the stability of the actual use of PCB board. The calculation method of microstrip line and strip line impedance on PCB board can refer to the corresponding empirical formula.
Five, printed circuit board impedance matching
In the circuit board, if there is a signal transmission, it is hoped that from the source end of the power supply, in the case of minimum energy loss, can be smoothly transmitted to the receiving end, 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 originating terminal for this to be called "impedance matching". Impedance matching is one of the key factors in designing high speed PCB circuits. And the impedance value has absolute relationship with the route mode.
For example, whether you walk on the surface layer (Microstrip) or the inner layer (Stripline/Double Stripline), the distance from the reference power layer or stratum, the width of the line, and the PCB material all affect the characteristic impedance of the line. That is, impedance values can only be determined after wiring, and the characteristic impedance produced by different PCB manufacturers also varies slightly. General simulation software cannot take into account some impedance discontinuity wiring conditions due to the line model or the mathematical algorithm used. In this case, only some Temninators, such as series resistors, can be reserved on the schematic diagram to mitigate the effect of impedance discontinuity. The real fundamental solution to the problem is to avoid impedance discontinuity when wiring as much as possible.