Chenfei is a professional pcb board manufacturer in China.Today we introduce the production of high quality PCB board from flexible pcb manufacturer.
Printed circuit boards of any of the "long" signaling pathways can be regarded as a kind of transmission line.If the line transmission delay time is much shorter than the signal rise time, then the signal rise during the master from the reflection will be submerged, no longer appear overshoot, recoil and ringing.The current most of MOS circuits,Because of the rise time is longer than line transmission delay time,So go line length can be meters for the unit,While no signal distortion.For fast logic circuit,Especially the high-speed ECL integrated circuit, due to the edge of fast,Must shorten the length of the walk line, in order to preserve the integrity of the signal.
To ensure the high speed circuit in a relatively long online job, there are two ways without serious waveform distortion.One is a schottky diode clamping method,The impulse be restraint in a diode is lower than the ground potential level pressure drop.Another is the line impedance matching method, makes the reflection controlled, to ensure that the signal integrity.For a slower edge speed regular TTL and CMOS devices, transmission lines are not required.For high speed ECL device, with faster edge speed transmission lines are not always required.If using the transmission line, the interconnect delay, control of reflection and the advantages of the oscillation.
When making PCB is double panel or multilayer, depends on the highest working frequency, complexity and the requirement of packing density,If more than 200 MHZ clock frequency when had better choose a multilayer.Go for printed circuit board line has the following principles:
(1) all try to leave big intervals between parallel signal lines, in order to reduce crosstalk.If there are two signal lines close apart, should take a grounding line between the two lines, can rise to shielding effect.
(2) the design of signal transmission lines to avoid sharp turns, in case the reflection of the transmission line characteristic impedance discontinuity, try to design a uniform circular arc line has a certain size.
(3) the width of the printed line according to the characteristic impedance of microstrip line and stripline calculation formula.Printed circuit board on the surface characteristic impedance of the microstrip line between 50 ~ 120 Ω commonly, generally choose 68 Ω impedance value is more appropriate, so that we can achieve the best balance between the time delay and power consumption, branch within 2.5 cm must limit the length of the short term, so as not to appear a large ring.
(4) for double panel (or six layer board of four layers), circuit board to both sides of the line perpendicular to each other, in order to prevent the mutual induction to produce the main crosstalk.
(5) on the PCB if equipped with a large current device (relay, lights, horn, etc.), earth had better go to separately,In order to reduce noise on the ground.Large current device of the ground wire should be connected to the board and an independent on the backplane bus, and the earth point of the whole system must be connected.
(6) if there is a small signal amplifier board, amplified in front of the weak signal line to stay away from the strong signal lines, and to get the line as short as possible so that if possible even land line on the screen.
In a word, PCB design is one of important steps in the process of electronic products production.To design a high quality PCB circuit boards, have to put the design of every link to be thoughtful.
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