PCB Tech - Impedance matching of circuit board PCB

Impedance matching impedance matching is a suitable matching mode between signal source or transmission line and load. According to the access mode, the impedance matching can be divided into two modes: serial and parallel; according to the frequency impedance matching of signal source, it can be divided into low frequency and high frequency.


1. Serial impedance matching is usually used for high frequency signals. The resistance value of the serial resistance is 20 ~ 75 Ω, which is directly proportional to the signal frequency and inversely proportional to the PCB wiring width. In the embedded system, when the frequency is more than 20m and the PCB wiring length is more than 5cm, the serial matching resistance should be added, such as clock signal, data and address bus signal in the system. There are two functions of serial matching resistor


1.1. Reduce high frequency noise and edge overshoot. If the edge of a signal is very steep, it contains a large number of high-frequency components, which will radiate interference, in addition, it is easy to produce overshoot. The series resistance, the distributed capacitance of the signal line and the load input capacitance form an RC circuit, which reduces the steepness of the signal edge.


2. Reduce high frequency reflection and self-excited oscillation. When the frequency of the signal is very high, the wavelength of the signal is very short. When the wavelength is short enough to match the length of the transmission line, the shape of the original signal will be changed when the reflected signal is superimposed on the original signal. If the characteristic impedance of the transmission line is not equal to the load impedance (i.e. mismatch), reflection will occur at the load end, resulting in self-excited oscillation. The low frequency signal of the wiring in PCB board can be connected directly without adding serial matching resistance.


2. Parallel impedance matching, also known as "terminal impedance matching", is generally used in input / output interface, mainly refers to impedance matching with transmission cable. For example, if the resistance of the cable used is 150 Ω / 485, the resistance of the cable used is 120 Ω / 485, and the cable used is 150 Ω. The resistance value of parallel matching resistance is related to the medium of transmission cable, and has nothing to do with the length. Its main function is to prevent signal reflection and reduce self-excited oscillation.


It is worth mentioning that impedance matching can improve the EMI performance of the system. In addition, in addition to using series / parallel resistance, transformer can also be used to transform impedance, such as Ethernet interface, can bus, etc.