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1. Impedance matching impedance matching refers to a suitable matching mode between signal source or transmission line and load. There are two kinds of impedance matching: serial and parallel according to the access mode, and two kinds of impedance matching are divided into low frequency and high frequency according to the frequency of the signal source.
(1) high frequency signals usually use serial impedance matching. The resistance of the serial resistor is between 20 and 75_. The resistance is proportional to the frequency of the signal and inversely proportional to the widths of the PCB. In embedded system, when the frequency is higher than 20M and the length of PCB is longer than 5cm, the serial matching resistor should be added, such as clock signal, data and address bus signal. There are two functions of serial matching resistance.
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 resistor and the distributed capacitance of the signal line and the load input capacitance form a RC circuit, which reduces the steepness of the signal edge.
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 comparable to the length of the transmission line, the reflection signal will change the shape of the original signal when 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 in the PCB board is directly connected to the low frequency signal. Generally, serial matching resistance is not required.
_(2) Parallel impedance matching, also known as "terminal impedance matching", is generally used in the input/output interface, mainly refers to the impedance matching with the transmission cable. For example, LVDS and RS422/485 use five types of twisted pair input matching resistances of 100-120_; video signals use coaxial cables with matching resistances of 75_or 50_, and flat cables with 300_. The resistance of the parallel matching resistor is related to the medium of the transmission cable and is independent of the length. Its main function is also 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 order to solve impedance matching, besides series/parallel resistors, transformers can be used for impedance transformation, such as Ethernet interface, CAN bus and so on.
The role of_2, 0 Euro resistance (1) the simplest is to do jumper, if a section of the line is not used, directly do not weld the resistance can (do not affect the appearance).
(2) When the matching circuit parameters are uncertain, 0 ohm is used instead. When the actual debugging is carried out, the parameters are determined, and then replaced by specific numerical elements.
(3) When you want to measure the working current of a certain part of the circuit, you can remove 0 ohm resistance and connect the ammeter, so that it is convenient to measure the current.
(4) In wiring, if the cloth can not go over, you can also add a 0 Euro resistance to play the role of jumper.
(5) acts as an inductor or capacitor in a high-frequency signal network (impedance matching, 0-ohm resistance also has impedance!). When acting as inductor, it mainly solves the EMC problem.
(6) single point grounding, for example, single point docking between analog and digital ground.
(7) the configuration circuit can replace jumpers and dialing switches. Sometimes the user will disorderly settings, easy to cause misunderstanding, in order to reduce maintenance costs, the use of 0 Euro resistance instead of jumpers and other welding on the board.
(8) System debugging, for example, the system is divided into several modules, the power between the modules and the use of 0 ohm resistance separated, the debugging phase found power or ground short circuit, remove 0 ohm resistance can reduce the scope of search.
The above functions can also be replaced by "magnetic beads". The 0-Euro resistance is similar to the magnetic bead in function, but there is essential difference. The former is impedance characteristic, the latter is inductance characteristic. Magnetic beads are usually used in power and ground networks, and have filtering effect.

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