# Reducing fast dynamic poles in a transfer function.

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Edward Simpson Songel on 27 Feb 2021
Imagine I have a tf that looks something like this: G(s) = 1/((s+70)*(s+5))
Is there some sort of command that can return a transfer fucntion that get's rid of the fastest dynamic pole?
In the example given above my output would be something like this: G(s)=1/(s+5).

Paul on 27 Feb 2021
Edited: Paul on 27 Feb 2021
Maybe freqsep() is what you want:
>> G=tf(1,conv([1 5],[1 70]))
G =
1
----------------
s^2 + 75 s + 350
Continuous-time transfer function.
>> [Gs,Gf]=freqsep(G,10)
Gs =
0.01538
-------
s + 5
Continuous-time transfer function.
Gf =
-0.01538
--------
s + 70
Continuous-time transfer function.
>> bode(Gs,h);
The dc gain of Gs will need to be adjusted to match the dc gain of G if you intend to use Gs as a low frequency approximation to G. This doc page is a good source for starters. There are also additional pages in Robust Control Toolbox for model reduction.
##### 2 CommentsShowHide 1 older comment
Edward Simpson Songel on 28 Feb 2021
P.S. Thanks for the further resources on the subject too.

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