1, the impedance matching between refers to the signal source or transmission line and load impedance matching a suitable way of collocation. According to the access impedance matching serial and parallel two ways; According to the signal frequency impedance matching can be divided into two kinds of low frequency and high frequency.
(1) the high frequency signal is generally use serial impedance matching. Serial resistance of the resistance of 20 ~ 75 Ω, resistance size is proportional to the signal frequency, go line width is inversely proportional to the PCB. In embedded systems, the signal frequency more than 20 m and PCB walk the line length is greater than 5 cm to add serial matching resistance, such as the system clock signals, data and address bus, etc. There were two serial matching the effect of resistance,
A. In pieces to reduce the high frequency noise and edge overshoot. If the edge of a signal is very steep, it contains a lot of high frequency components, will be radiated interference, in addition, also prone to overshoot. Series resistance and signal lines of distributed capacitance and load capacitance formed an RC circuit, this will reduce the signal edge of steep.
B. To reduce the high frequency reflection and self-excited oscillation. When the signal frequency is very high, the wavelength of the signal is very short, as short wavelength with transmission line length can be compared, reflected signal superimposed on the original signal will change the shape of the original signal. If the characteristic impedance and load impedance of a transmission line is not equal to (ie does not match), can produce a reflection at the load end, cause self-excited oscillation. In the PCB line low frequency signal can be connected directly, matching generally do not need to add the serial resistance.
(2) parallel impedance matching is also called "terminal impedance matching", commonly used in the input/output interface, mainly refers to the matching with the impedance of the transmission cables. An LVDS and RS422/485, for example, using five types of twisted-pair cable input matching resistance is 100 ~ 120 Ω; Video signal using coaxial cable matching resistor of 75 Ω or 50 Ω, use flat cable for 300 Ω. Parallel matching resistance value associated with the medium of transmission cables, has nothing to do with the length, its main function is to prevent signal reflection, reduce the self-excited oscillation.
It is worth mentioning that the impedance matching can improve the EMI performance of the system. In addition, solve the impedance matching in addition, the use of series/parallel resistor transformer CAN also be used to do impedance transformation, typical examples such as Ethernet interface and CAN bus.
2, The role of 0 the resistance:
(1) the most simple do jump line, if a particular section of the line no, not directly the resistance can be welded (does not affect the appearance.
(2) in matching circuit parameters are uncertain, with 0 ohm, actual debugging, to determine the parameters, with specific numerical element to replace again.
(3) to measure a part of the working current of the circuit, can remove zero resistance, ammeter is connected, so convenient measuring current.
(4) when the wiring, if is not passed, can also add a zero resistance of the jumper.
(5) in the high frequency signal network, ACTS as the inductor or capacitor (impedance matching role, 0 the resistance have impedance!) . ACTS as the inductor, mainly solve the problem of EMC.
(6) single point grounding, such as analog and digital ground to a single point of docking altogether.
(7) configuration circuit, can take the place of jump line and dial the code switch. Sometimes users touch Settings, easy cause misunderstanding, in order to reduce the maintenance cost, application of 0 the resistance welding instead of jump line on the board.
(8) system debugging, such as the system is divided into several modules, modules of power supply and ground with zero resistance to separate, when he found the power or ground short circuit debugging stage, remove the search range 0 the resistance be narrowed.
The above function can also use "magnetic beads" alternative. 0 the resistance on the magnetic beads and function is similar, but there is essential difference, the former assumes the impedance characteristics, which are impedance characteristics. Magnetic beads and the commonly used in power supply network, the filtering effect.
