Impedance matching in PCB design and impedance matching of 0 ohm resistors according to access mode
Impedance matching Impedance matching is a suitable way to match a signal source or transmission line to a load. According to the access mode, there are two impedance matching modes: serial and parallel; according to the source frequency impedance matching, it can be divided into low frequency and high frequency.
Series impedance is typically used to match high frequency signals. The resistance of the series resistor is 20~75Ω, and the resistance value is proportional to the signal frequency, which is inversely proportional to the trace width in the PCB design. In embedded systems, when the signal is greater than 20M and the PCB trace length is greater than 5cm, the system adds serial matching resistors (such as clock signals, data and address bus signals) to the system. The serial matching resistor has two functions:
Reduce high frequency noise and edge overshoot. If the edge of the signal is very steep, it will contain many high frequency components, which will cause radiated interference and also tend to overshoot. The series resistors and signal lines, as well as the distributed capacitance of the load input capacitors, form an RC circuit to reduce the steepness of the signal edges.
Reduce high frequency reflection and self-oscillation. When the frequency of the signal is high, the wavelength of the signal is very short. When the wavelength is shorter than the length of the transmission line, the superposition of the reflected signal 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 (ie, it does not match), then reflection will occur at the load end, causing self-oscillation. The low frequency signals of the traces on the PCB can be directly connected, and it is usually not necessary to add a series matching resistor.