Match the impedance of the transmission line or load source. According to the access mode, impedance matching can be divided into series and parallel; according to the frequency impedance matching of the signal source, it can be divided into low frequency and high frequency.
Serial impedance matching is typically used for high frequency signals
The resistance value of the series resistor is 20~75Ω, which is proportional to the signal frequency and inversely proportional to the width of the PCB trace. In an embedded system, when the frequency is greater than 20m and the length of the PCB trace is greater than 5cm, series matching resistors should be added, such as clock signals, data and address bus signals in the system. The series matching resistor has two functions
1. Reduce high frequency noise and edge overshoot. If a signal has steep edges, it contains a lot of high-frequency components that radiate interference and are prone to overshoot. The series resistance, 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-excited oscillation. When the frequency of the signal is high, the wavelength of the signal is very short. When the wavelength is short enough to match the length of the transmission line, the original signal changes shape when the reflected signal is superimposed on it. If the characteristic impedance of the transmission line is not equal to the load impedance (that is, there is a mismatch), reflections will occur at the load end, causing self-oscillation. The low-frequency signal traced in the PCB board can be directly connected without adding series matching resistors.
Parallel impedance matching is also called "terminal impedance matching"
Generally used for input/output interface, mainly refers to the impedance matching with the transmission cable. For example, LVDS and RS422/485 use Category 5 twisted-pair cables with an input matching resistance of 100-120Ω; video signal coaxial cables have a matching resistance of 75Ω or 50Ω, and flat cables have a matching resistance of 300Ω. The resistance value 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-excited oscillation.
It is worth mentioning that impedance matching can improve the EMI performance of the system. In addition, in addition to using series/parallel resistors, transformers can also be used to transform impedance, such as Ethernet interfaces, CAN buses, etc.
Zero Ohm Resistor
The simple thing is to make a jumper. If a section of the line is not used, do not solder the resistor directly (does not affect the appearance).
When the parameters of the matching circuit are uncertain, use zero ohms instead. In actual debugging, parameters are determined and replaced by components with specific values.
When it is necessary to measure the working current of a part of the circuit, the zero-ohm resistor can be removed, and an ammeter can be connected to facilitate the current measurement.
If the wiring is not connected to zero, it can also play the role of the past.
In high frequency signal networks it acts as an inductor or or capacitor (acts as an impedance match, a zero ohm resistor also has impedance). When used as an inductor, it mainly solves EMC problems.
Single-point grounding, such as analog ground and digital ground single-point grounding.
Configuration circuits can replace jumpers and DIP switches. To reduce maintenance costs, zero-ohm resistors are used instead of jumpers soldered on the board.
For example, the system is divided into several modules, and the power supply and ground between the modules are separated by zero-ohm resistors. When a short to power or ground is found during the debug phase, removing the zero-ohm resistor can narrow the search.