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使用EDA分析PCB

时间:11-04 来源: 点击:

的应用有通信网路和计算机等相关领域。在通信网路方面,PCB板的工作频率已达GHz上下,迭层数就我所知有到40层之多。计算机相关应用也因为芯片的进步,无论是一般的PC或服务器(Server),板子上的最高工作频率也已经达到400MHz (如Rambus) 以上。因应这高速高密度走线需求,盲埋孔(blind/buried vias)、mircrovias及build-up制程工艺的需求也渐渐越来越多。这些设计需求都有厂商可大量生产。以下提供几本不错的技术书籍:
1、W. Johnson,"High-Speed Digital Design – A Handbook of Black Magic";
2、H. Hall,"High-Speed Digital System Design";
3、Yang,"Digital Signal Integrity";

Q我觉得信号线特性阻抗的微带线和带状线模型都是要参考地平面的,现在我想问一下,如果信号线下面的铜皮都被掏空,没有参考的地平面,该如何计算顶层的信号线的特性阻抗?另外,我看一些资料写在消除信号线上噪声方面,电源平面也可以和地平面起相同的作用,是吗?
A没有参考平面时电场与磁场的互动关系与有参考平面时不同,而这互动关系会影响到特性阻抗的值。现在绝大部分特性阻抗的计算公式都是假设有参考平面的,我还没看到这种无参考平面的特性阻抗公式。但是,可以用TDR (Time Domain Reflectometer)对实际的板子做量测来得到无参考平面的特性阻抗。信号线上的噪声产生的原因是别的线上的信号所产生的电场和磁场的能量经由mutual inductance及mutual capacitance而传到被感染的信号线上。电源平面和地平面基本上都是金属平面,所以对电场磁场都有屏蔽效应(shielding effect)。

Q我们设计的一款金属壳设备,电源接地良好(LN小于4V)电路接地端和机箱通过安装柱相连。但用户始终抱怨有麻电现象。请问你们交换机这类设备如何处理这个问题?把PCB的地和机箱的外壳隔离开来的做法是否现实可行?
A:抱歉,我没有太多这类的设计经验可以跟你讨论。 

Q:why the wien bridge can only be balanced at one frequency? even if the ratio of coupled resistors is varied
AThe operation principle of Wien bridge oscillator is positive feedback mechanism. The transfer function (or gain) of the Wien bridge oscillator (in Laplace transform) is Af(s)=A(s)/[1-A(s)B(s)], which A(s) is open loop gain of amplifier and B(s) is the gain of feedback network. To oscillate spontaneously, the Af(s) must approach to infinity which implies denominator is zero. That is, the product of A(s) and B(s) need to be equal to 1. Due to the frequency dependence of A(s)B(s), there is only one frequency can make the denominator to be zero. That is why the Wien bridge only balance at one frequency. The oscillation frequency is determined by the resistors and capacitors in the positive feedback path, f=1/[2πsqrt(R1C1R2C2)], where R1, C1, R2, C2 are the components in the positive feedback path. The components on negative feedback path are nothing to do with the oscillation frequency. The other intuitive insight to this concept of balancing at one frequency is to treat the network of positive feedback path as a frequency selector. There are a high-pass filter formed by a series capacitor with a grounded resistor and a low-pass filter formed by a series resistor with a grounded capacitor. The total effect is similar to a bandpass filter. There is a website to address this concept: http://www.interq.or.jp/japan/se-inoue/ e_ckt18_2.htm#2

Q众所周知PCB板包括很多层,但其中某些层的含义我还不是很清楚。mechanical,keepoutlayer, topoverlay, bottomoverlay, toppaste, bottompaste, topsolder, bottomsolder, drillguide, drilldrawing, multilayer这些层不知道它们的确切含义。希望您指教。
A:在EDA软件的专门术语中,有很多不是有相同定义的。以下就字面上可能的意义来解释。Mechnical: 一般多指板型机械加工尺寸标注层Keepoutlayer: 定义不能走线、打穿孔(via)或摆零件的区域。这几个限制可以独立分开定义。Topoverlay: 无法从字面得知其意义。多提供些讯息来进一步讨论。Bottomoverlay: 无法从字面得知其意义。可多提供些讯息来进一步讨论。Toppaste: 顶层需要露

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