hello everyone
I did problem from bode plot
please see my solution true?

 

I use alpha ,but the answer is different from my :
why?


http://www.wolframalpha.com/input/?i=bode plot ((30s^3+660s^2+3600s)/(s^3+19s^2+113s+200))

 

Here is what LTSpice shows:



Look familiar?

 

Here is what LTSpice shows:

View attachment 76955

Look familiar?

use Hz?

 

Here is what LTSpice shows:

View attachment 76955

Look familiar?

I did in alpha the answer is different from you
if you are see from 0.1 your start from 20 dB and in alpha from 0.1 start at 5.1dB

 

Begs the question: Is one in Hz and the other in radians?

 

Hello there,

Mike:
You probably want to check your Laplace function in post #3 because that seems to yield the wrong result. I'll double check this too. It could be that LTSpice leaves out the constant gain in the Laplace function.
CHECKED:
Ok, it's just that LTSpice uses frequency in Hertz instead of angular frequency w. If we plot from 0.01/2/pi to 100/2/pi and for x=f*2*pi we get the same plot as post #2 for the amplitude.

Full:
The amplitude function is:
|V|=(30*w*sqrt(w^2+100)*sqrt(w^2+144))/(sqrt(w^2+64)*sqrt(w^4+71*w^2+625))

and so the amplitude in db is:
Vdb=20*log10(|V|)

You can use that to check your results, but the plot comes out the same as your plot in post #2.
The x axis variable is 'w' not 'f in Hertz' just to clear that up, and w=2*pi*f.

Again you are probably just overlooking something simple. It's hard to read your sheets though because the writing is so faint.