第5章-航天遥测系统.pptVIP

* 传输数字信号最简单的方式 只用一个电平来表示两个二进制数字。通常0是用零电平即空闲的线路状态表示。 注：脉冲的极性指其电平的正负。“单极性”得名于它的电平只有一极。 * NRZ-L A negative voltage is used to represent one binary value and a positive voltage to represent the other. The bit state determines the level of a signal. NRZ-I A transition (low to high or high to low) at the beginning of a bit time denotes a binary 1 for that bit time; no transition indicates a binary 0. Differential encoding for more reliability The bit state determines the transition of a signal between a positive and negative voltage, not the voltages themselves. * NRZ-I A transition (low to high or high to low) at the beginning of a bit time denotes a binary 1 for that bit time; no transition indicates a binary 0. Differential encoding for more reliability The bit state determines the transition of a signal between a positive and negative voltage, not the voltages themselves. * 通常与4B/5B编码结合用于100BASE-X和FDDI的光纤传输。 见教材P414 * 归零编码使用三个电平值：正，负和零。 信号变化不是发生在比特之间而是发生在每个比特内。在每个比特间隙的中段，信号将归零。 比特“1”实际上是用正电平跳变到零表示，比特“0”则用负电平跳变到零表示，而不是仅仅通过电平的正负来表示。 在每个比特内产生信号变化可以解决同步问题。但这种编码方案中每比特需要两次信号变化，从而占用了更多的带宽。 * There is a transition at the middle of each bit period. The midbit transition serves as a clocking mechanism and also a data: a low-to-high transition represents a 1, and a high-to-low transition represents a 0. * he midbit transition is used only to provide clocking. The encoding of a 0 is represented by the presence of a transition at the beginning of a bit period, and a 1 is represented by the absence of a transition at the beginning of a bit period. * 归零编码RZ Encoding 归零编码使用两组电平值： 正-零，负-零 信号变化不是发生在比特之间而是发生在每个比特内。在每个比特间隙的中段，信号将归零。 比特“1”实际上是用正电平跳变到零表示，比特“0”则用负电平跳变到零表示，而不是仅仅通过电平的正负来表示。 在每个比特内产生信号变化可以解决同步问题。但这种编码方案中每比特需要两次信号变化，从而占用了更多的带宽。 归零码：在40G传输技术中的应用 目前，绝大多数的信号均采用了非归零码（NRZ）的编码方式，这种方式可以降低信号的谱宽，但由于占空比较大，前后脉冲的间隔较小，较容易发生重叠，造成码间串扰。而归零码（RZ）的占空比通常只有普通非归零码的34％～67％，拉开了相邻脉冲的间隔，在信号平均能量不变的基础上，大大提高了峰值功率，为接收端提供了更高的光信噪比，同时也提高了对光纤中极化模色散造成的时延的抵抗能力。朗讯科技公司在其必威体育精装版的40G远距离传输技术采用了一种称为载波抑制的归零码调制技