Is my bode plot inaccurate?
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Given the following transfer function
s = tf('s');
P = 125/(s*(s+1)*(s+5));
bode(P);
Why at 5 rad/s is magnitude -3.19 dB?
Should it not be -7.6 dB?
I get -7.6 dB if I compute the magnitude at 5 rad/s:
>> 20*log10(125/(5*(5+1)*(5+5)))
ans =
-7.6042
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Accepted Answer
Mischa Kim
on 18 Feb 2014
Edited: Mischa Kim
on 18 Feb 2014
Quid, the Bode plot is correct. To compute the magnitude of the P you need to replace (substitute) the complex frequency s by iw = i5.
syms s
Ps = 125/(s*(s+1)*(s+5));
Pw = subs(Ps,s,i*5);
aPw = double(20*log10(abs(Pw)))
aPw =
-3.180633349627616
pPw = (180/pi)*double(angle(Pw))
pPw =
1.463099324740202e+02
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