Why do we need controlled impedances?为什么我们需要进行阻抗控制

The function of a wire or trace is to transfer signal power from one device to another.Theory shows that maximum signal power is transferred when impedances are matched.

 

The impedance of the trace is extremely important, as any mismatch along the transmission path will result in a reduction in signal quality and possibly the radiation of noise.Mismatched impedance causes signals to reflect back and forth along the lines, which causes ringing at the load . The ringing reduces the dynamic range of the receiver, eats into the noise budget and can cause false triggering.

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Reflections occur whenever the impedance of the transmission line changes along its length. This can be caused by unmatched drivers/loads, layer transitions, different dielectric materials, stubs, vias, connectors and IC packages. By understanding the causes of these reflections and eliminating the source of the mismatch, a design can be engineered with reliable performance. For perfect transfer of energy and to eliminate reflections, the impedance of the source must equal the impedance of the trace,as well as the impedance of the load. When the signal path has an impedance discontinuity, place a terminator at the point of line discontinuity to equalize the impedance.

 

导线或走线的功能是将信号功率从一个设备传送到另一个设备,理论表明当阻抗匹配时,最大信号功率转移。

 

信号线的阻抗是非常重要的,因为任何不匹配的传输路径,将导致在信号质量下降,可能是噪声的辐射,不匹配的阻抗会导致信号,以反映沿线路,这会导致在负载。该铃声降低了接收器的动态范围,吃到噪音预算,并可能导致假触发。

 

反射发生时,沿其长度的传输线的阻抗变化。这可以通过无与伦比的驱动负载,造成层过渡,不同的介电材料,存根、过孔、连接器和IC封装。通过了解这些反射的原因,并消除了源的不匹配,一个设计可以设计可靠的性能。为了完美地传递能量,并消除反射,阻抗的源必须等于阻抗的跟踪,以及阻抗的负载。当信号路径的阻抗不连续,将终止在线间断均衡点的阻抗。