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Solve impedance matching. In addition to using serial/parallel resistance, transformers can be used for impedance transformation.

2019-11-02 22:43:23 Juding Circuit Technology 355


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


High frequency signals typically use serial impedance matching. The resistance of the serial resistor is 20~75Ω, and the resistance value is proportional to the signal frequency, which is inversely proportional to the PCB trace width. In an embedded system, a serial matching resistor is added to a signal with a frequency greater than 20M and a PCB trace length greater than 5 cm, such as a clock signal, data, and address bus signals in the system.


There are two functions of serial matching resistors:


1. Reduce 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 cause radiation interference, and it is also prone to overshoot. The series resistor and 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-oscillation:

   When the frequency of the signal is high, the wavelength of the signal is very short. When the wavelength is short compared with the length of the transmission line, the reflected signal is superimposed on the original signal, which 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), reflection will occur at the load end, causing self-oscillation. The low frequency signal of the traces in the PCB board can be directly connected, and generally no serial matching resistor is needed.


Parallel impedance matching is also called terminal impedance matching:

    Generally used on the input / output interface, mainly refers to the impedance of the transmission cable. For example: LVDS and RS422/485, the input matching resistance of the type 5 twisted pair is 100~120Ω, the matching resistance of the video signal using the coaxial cable is 75Ω or 50Ω, and the flat cable is 300Ω. The resistance of the parallel matching resistor is related to the medium of the transmission cable, and has nothing to do with the length. Its main function is to prevent signal reflection and reduce self-oscillation.


It is worth mentioning that impedance matching can improve the EMI performance of the system. In addition to solving the impedance matching, in addition to using series/parallel resistance, transformers can also be used for impedance transformation. Typical examples are Ethernet interface, CAN bus, etc. Wait.


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