Impedance matching in PCB design and impedance matching of 0 ohm resistance according to the access method
Impedance matching Impedance matching refers to an appropriate matching method between the signal source or transmission line and the load. According to the access mode, there are two ways of impedance matching: serial and parallel; according to the frequency of the signal source, impedance matching can be divided into two types: low frequency and high frequency. High-frequency signals generally use series impedance matching. The resistance value of the series resistor is 20~75Ω, and the resistance value is proportional to the signal frequency and inversely proportional to the trace width in the PCB design. In embedded systems, serial matching resistors are generally required for signals with frequencies greater than 20M and PCB trace lengths greater than 5cm, such as clock signals, data and address bus signals in the system. The series matching resistor has two functions: reducing high frequency noise and edge overshoot. If the edge of a signal is very steep, it contains a lot of high-frequency components, which will radiate interference. In addition, it is also prone to overshoot. The series resistance and the distributed capacitance of the signal line and the load input capacitance form an RC circuit, which will reduce the steepness of the signal edge. Reduce high-frequency reflections and self-oscillation. When the frequency of the signal is high, the wavelength of the signal is very short. When the wavelength is as short as the length of the transmission line, the reflected signal superimposed on the original signal will change the shape of the original signal. If the characteristic impedance of the transmission line is not equal to the load impedance (that is, it does not match), reflections will occur at the load end, causing self-oscillation. The low-frequency signal of the wiring in the PCB board can be directly connected, and it is generally not necessary to add a series matching resistor.