King Sun PCB

Impedance matching is a suitable matching method between the signal source or the transmission line and the load.According to the impedance matching of the access mode, there are two ways of serial and parallel. According to the frequency impedance matching of the signal source, it can be divided into two kinds of low frequency and high frequency.

(1.) High frequency signals generally use serial impedance matching. The resistance value of the serial resistance is 20~75 Ω, and resistance value is proportional to the frequency of the signal, and is inversely proportional to the width of the PCB line.In the embedded system, the serial matching resistance should be added when the frequency is larger than 20M and the length of PCB line is more than 5cm.For example, the clock signal, the data and the address bus signal in the system. There are two functions of the serial matching resistance in the design of the circuit board.

• 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 interfere with radiation.In addition, it is easy to produce overshoot.The series resistance, the distributed capacitance of the signal line and the load input capacitance form a RC circuit, which will reduce the steep edge of the signal.
• 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 length of the wavelength is comparable to the length of the transmission line, the reflection signal will change the shape of the original signal on the original signal.If the characteristic impedance of the transmission line is not equal to the load impedance (that is, the mismatch), the reflection will be produced at the load end, causing the self excited oscillation.The low frequency signal in the PCB board is directly connected, and the serial matching resistance is not required.

(2.) Parallel impedance matching is called “terminal impedance matching”,It is usually used at the end of the input / output interface, which mainly refers to the impedance matching with the transmission cable.For example, LVDS and RS422/485 use the input – end matching resistance of the 5 kind of twisted pair lines to be 100~120Ω,The matching resistance of a video signal using a coaxial cable is 75 Ωor 50 Ω and an article flat cable is 300 Ω. The resistance value of the parallel matching resistance is related to the medium of the transmission cable, which is independent of the length, and its main function is to prevent the reflection of the signal and reduce the self excited oscillation.

It is worth mentioning that impedance matching can improve the EMI performance of the system.Besides,not only series/parallel circuit can solve impedance transformation but also use transformer shunt resistor, typical examples include Ethernet interface, CAN bus and so on.