有谁能讲讲展频是怎么一回事?

SES200666

DDR/DDR2辐射厉害,听说可以展频解决，
跪求高手讲讲如何展频！
[em14]

abc3xyz

坐着沙发等人来评论

12506

Clock modulation, also known as Spread Spectrum, began to appear in computer system designs as early as 1995. Most personal computers designed today use Spread Spectrum
technologgy to reduce EMI. Spread Spectrum provides a cost effective method to keep EMI low.
Clock modulation is a technique whereby an input reference clock at some frequency is
modulated such that the frequency of the output clock varies slightly. For example, a
40 MHz reference clock with Spread Spectrum applied can produce an output swing from
39.60 MHz to 40.40 MHz. This would represent a Spread Spectrum clock that has a 2%
bandwidth centered on the reference frequency of 40 MHz. The purpose of modulating
the frequency of a clock is to distribute the energy of a single or narrow band over a      much wider band of frequencies. This reduces the amount of peak energy at any one frequency in the spectrum. The amount of EMI reduction is affected by the modulation profile, the percentage of frequency variation (bandwidth), and the modulation rate.[br]<p align=right><font color=red>+1 RD币</font></p>

changxusu

谁能把文中说的40M的narrow band扩成39.6到40.4解释一下阿？怎么做得到的？后面说这样做降低了峰值能量。关键这种技术怎么做到的？楼上有这方面的参考资料分享一下阿
12506是你的工号吧

12506

你邮箱？那是我大学时的宿舍号码！１号公寓２５楼６号房[em12]

SES200666

是能量的分配问题，把40M的能量分配到两侧的频谱上，这样就可以把超标点的能量峰值降低.
这个要用软件还实现。

peterchen08

发表一下看法：
所谓的展频技术就是用30K~60KHz的低频信号对CLK等信号做一个微小的调制，这个调制是一种受控的调制，产生具有边带的频谱,但引入的时钟信号抖动很小,不会明显增加时钟信号的抖动.
能使窄带的时钟扩展为宽带,在基波和谐波中所包含的峰值谱能量便相应减小,但总的谐波能量不变，使EMI发射减小.如果设计合理，可将谐波的EMI降低7~20dB左右。[em10][em10]

cylbailey

应该是通过门控时钟来完成

yvnoon

展频的特色是宽频及低功率。展频讯号的安全性很高。
目前可用的展频技术为DSSS，FHSS，OFDM。
FHSS———跳频展频，这个系统的载波依据某一假乱数顺序改变频率。传送者使用这个顺序来决定传送频率，载波会在某一频道呆一段时间（Dwell Time），再用一小段时间变换频道（Hop Time）。当用完所有频道，会重新开始。