请教一下hspice中产生随机电压信号的问题

longxianjie

hspice中可以通过如下语句来产生随机信号：
V1 ina gnd LFSR(0 1 0.01 0.05n 0.05n 7000meg 1 [5,2] rout=50)

想问一下其中[5,2]代表什么意思呢？

用一个电压源分别选择LFSR和PULSE模式，驱动一个反相器，反相器的输出端连着电容的一端，电容的另一端通过其他电容、电阻连接到其他的信号源。当用PULSE时，反相器输出端的信号很容易受到其他信号源的影响，而选择LESR的时候，输出信号却非常平稳。想问一下这是为什么呢？是不是跟表达式中的[5,2]有关？

先谢谢各位了~！

by1e11

回复 1# longxianjie


Sorry for answering in English as there is no Chinese input method supported on the computer Im using in my office...

[5,2], which indicates 5 registers in series in this LFSR, is set of taps represents the arrangement of feedback bits in this LFSR. Taps have impact on the maximum length of pseudo-random sequence that can be generated by an LFSR with certain number of registers.

For your case, the taps can be expressed as x^5+x^2+1, which means the 5th and the 2nd bits are the feedback bits (Actually the 1st bit shown as '1' in the polynomial is an inherent feedback bit) By arranging as so, you can obtain a pseudo-random sequence with maximum length of 2^5-1=31. Notice that there can be more than one maximum-length tap sequence for a given LFSR length.

Plz refer to http://users.ece.cmu.edu/~koopman/lfsr/index.html for more details about taps.

Therefore, I dont think [5,2] should be a factor that influences your experiment results.

Thanks
Bo

chensheng312

回复 1# longxianjie


   对于为什么用LFSR的时候会非常平稳，我也不能理解，  但就[5,  2]的解释我赞同上面的说法， 对于taps是可以根据自己的需求重新设置的。