# bode plot

Discussion in 'Homework Help' started by full, Dec 11, 2014.

1. ### full Thread Starter Member

May 3, 2014
225
2
hello everyone
I did problem from bode plot

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2. ### full Thread Starter Member

May 3, 2014
225
2
Last edited: Dec 12, 2014
3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Here is what LTSpice shows:

Look familiar?

Last edited: Dec 11, 2014

May 3, 2014
225
2
5. ### full Thread Starter Member

May 3, 2014
225
2
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

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Begs the question: Is one in Hz and the other in radians?

7. ### MrAl Distinguished Member

Jun 17, 2014
2,560
515
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.

Last edited: Dec 12, 2014
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