1. [local]1[/local]...................................................................................................... 5
1.1
何谓高速数字信号?.................................................................................................... 5
1.2
微带线、带状线的概念................................................................................................. 5
1.2.1
微带线(Microstrip
).......................................................................................... 5
1.2.2
带状线(Stripline
)............................................................................................. 6
1.2.3
经验数据............................................................................................................ 6
1.2.4
同轴线(coaxialcable
)...................................................................................... 6
1.2.5
双绞线(twisted-paircable
)............................................................................... 7
1.2.6
等间隔的电容负载的影响................................................................................... 7
1.3
常见高速电路.............................................................................................................. 8
1.3.1 ECL
(EmitterCoupled Logic
)电路...................................................................... 8
1.3.2 CML
(Current ModeLogic
)电路........................................................................ 9
1.3.3 GTL
(GunningTransceiver Logic
)电路............................................................ 10
1.3.4 BTL
(BackplaneTransceiver Logic
)电路........................................................... 10
1.3.5 TTL
(TransistorTransistor Logic
)电路............................................................. 11
1.3.6
模数转换电路—线接收器................................................................................. 12
1.4
常见电路匹配措施...................................................................................................... 12
1.4.1
反射................................................................................................................. 12
1.4.2
终端匹配.......................................................................................................... 13
1.4.3
始端匹配.......................................................................................................... 15
1.5
高速电路设计一般原则和调试方法............................................................................. 16
1.5.1
同步逻辑设计................................................................................................... 16
1.5.2
了解选用器件的输入、输出结构,选用恰当的匹配电路;在考虑节省功耗,电路
又能容许的情况下,可适当地引入失配。........................................................................................................ 19
1.5.3
对极高速率(300MHz
以上)的信号,一般建议选用互补逻辑,以降低对电源的要求。
19
1.5.4
了解每一根高速信号电流的流向(电流环)...................................................... 19
1.5.5信号的布线、电源和地层的分割,是否符合微带线、带状线的要求?高速信号要有回路地相配(不是屏蔽地)......................................................................................................................... 19
1.5.6
电源滤波.......................................................................................................... 19
1.5.7
对很高速度的信号要估算其走线延迟。............................................................. 19
1.5.8
在满足速度要求的前提下,尽量选用工作速率低的器件。
19
1.5.9
差分线尽量靠近走线......................................................................................... 19
1.5.10
测试方法:选择有50
Ω输入的高速示波器,一般自制一个探头,测量点应尽量靠近所观察的位置或者需要该信号的实际位置。一般不建议测输出端的信号波形,与实际使用的位置有一定差别。
19
1.5.11 ringing, crosstalk, radiated noise ——
数字系统的三种噪声
19
1.5.12
数字信号的绝大部分能量(功率谱密度)集中在fknee
之内
19
1.5.13
延时:FR4
PCB
,outertrace: 140~180 ps/inch
inner trace: 180ps/inch
20
1.5.14
集总参数与分布参数系统................................................................................... 20
1.5.15
互感、耦合电容的作用(干扰)..................................................................... 20
1.5.16 ECL
电路的上升时间、下降时间的计算........................................................... 20
1.5.17
在数字系统中,耦合电容引起的串扰比起互感引起的串扰要小。
21
1.5.18
传输通道包括器件封装、PCB
布局、连接器,至少在fknee
的范围内要有平坦的频响,以保证信号不失真,否则信号在收端可能会遇到上升时间劣化、过冲、振铃、lump
等现象。................... 21
1.5.19
阻容负载对电流变化的作用............................................................................ 21
1.5.20
噪声容限(noiseimmunity
):以10H189
器件为例............................................ 22
1.5.21
地反弹(ground
bounce
)................................................................................. 23
1.5.22
寄生电容StrayCapacitance
的影响:对于高输入阻抗电路影响尤为严重
23
1.5.23
示波器探针的电气模型................................................................................... 24
1.5.24 21:1
探针:..................................................................................................... 25
1.5.25
趋肤效应(skin effect
):在高频时导线表面附近的电流密度加大,而中心部分的电流密度减小。趋肤效应使得导线对高频信号的衰减增大。趋肤效应的频率与导体的材料有关。.................... 25
1.5.26
对低频信号,电流流经电阻最小的路径;对高频信号,回流路径的电感远比其电阻重要,高频电流流经电感最小的路径,而非电阻最小的路径。最小电感回流路径正好在信号导线的下面,以减小流出和流入电流通路间的环路面积。........................................................................................................... 25
1.5.27
负载电容对上升时间的影响............................................................................ 26
1.5.28
直流匹配和交流匹配的功耗比较..................................................................... 27
1.5.29
电源系统设计原则.......................................................................................... 27
1.5.30 TTL
和ECL
的混合系统要注意........................................................................ 27
1.5.31
电源线上的电磁辐射防护................................................................................ 28
1.5.32
旁路电容的选取和安装:................................................................................ 28
1.5.33
连接器对高速系统的影响................................................................................ 28
1.5.34
总线:............................................................................................................ 31
2
、电磁兼容性(Electromagnetic Compatibility
)....................................................................... 32
2.1
关于电磁兼容性的基本原理........................................................................................ 32
2.1.1
下面的电路布局有什么问题?........................................................................... 32
2.1.2
走线可穿过回流平面的缝隙吗?No
!............................................................... 33
2.1.3
走线的电感和电容............................................................................................ 33
2.1.4
接地的作用:................................................................................................... 34
2.1.5
信号参考点应在何处接至基底(chassis).............................................................. 35
2.1.6
周期信号......................................................................................................... 36
2.1.7 EMC
三要素..................................................................................................... 36
2.1.8
共模和差模....................................................................................................... 38
2.1.9
减小噪声的措施............................................................................................... 39
2.2
信号完整性――减小串扰和信号畸变.......................................................................... 39
2.2.1......................................................................................................................... 39
2.2.2
屏蔽................................................................................................................. 40
2.2.3
信号畸变.......................................................................................................... 41
2.3
通过滤波减小直流电源噪声........................................................................................ 41
2.3.1......................................................................................................................... 42
2.3.2 If DC power planes can’t be used, then lumped decouplingcapacitors must be sized and placed correctly.
42
2.3.3
多层PCB
、表贴电容,串联电感在何处?......................................................... 43
2.3.4 How to distribute DC power from a single supply to bothanalog and digital circuits?
43
2.4
元件放置与信号层分配............................................................................................... 44
2.5 Reducing conducted & radiated emission &susceptibility................................................. 46
2.6
电路板EMC
准则总结................................................................................................ 48
2.6.1 Component Placement........................................................................................ 48
2.6.2 DC Power Distribution....................................................................................... 48
2.6.3 Routing of Signal Output and Return Paths........................................................... 49
2.6.4 Signal Integrity – Reducing Crosstalk and Distortion.............................................. 49
2.6.5 High Frequency Transmission Lines..................................................................... 50
2.6.6 Reducing Conducted and Radiated Emissions........................................................ 50