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## Re: Not able to add delay to transfer function

**From**: |
Sergei Steshenko |

**Subject**: |
Re: Not able to add delay to transfer function |

**Date**: |
Thu, 30 Jul 2020 01:14:41 +0300 |

**User-agent**: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 |

On 29/07/2020 22:53, shall689 wrote:

Hello Torsten,
That seems to work.
I also tried (1+s*tau)/(1-s*tau), but the step response gave me the error:
"open_gl_renderer: data values greater than float capacity error"
(1+s*tau)/(1-s*tau) was mentioned on page 2 of this document:
http://users.ece.utexas.edu/~buckman/H3.pdf
Thanks,
Stephen
--
Sent from: https://octave.1599824.n4.nabble.com/Octave-General-f1599825.html

`I do not understand why you are trying to add something to your transfer
``function (delay in this case). IIRC, in the beginning you stated you had
``frequency response of your system. I asked whether the response was
``complex (as in complex numbers) or just magnitude - I don't remember
``getting a reply; I asked the question to better understand the task at hand.
`

`If you have complex frequency response, you probably don't even need the
``corresponding transfer function. This is because having the response and
``the PID parameters you can have the resulting complex frequency response
``which can be converted by 'ifft' into the resulting impulse response,
``which also means it can be converted into step response (probably the
``step response of most interest).
`

`Anyway, having complex frequency response you can try to obtain rational
``polynomial (i.e. either B(s)/A(s) or B(z)/A(z)) using 'invfreqs' or
``'invfreqz' functions respectively. I believe what I'm writing is
``methodologically correct, though from practice I know that for "funky"
``frequency responses obtaining the resulting rational polynomial is very
``tricky, i.e. small polynomial orders are not sufficient, and high
``polynomial orders give convergence problems - do not fit the input
``complex frequency response well.
`

`If you only have magnitude response, you still need to obtain complex
``frequency response - otherwise you can't build the corrected systems. Is
``this the case and are you trying to add delay to the magnitude response
``? If that's the case, I do not think your methodology is correct. Not
``that you don't need delay - quite the opposite. But for minimum phase
``systems magnitude response and phase response are tightly related - see
``https://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relations#Magnitude_(gain)%E2%80%93phase_relation
``,
``https://en.wikipedia.org/wiki/Minimum_phase#Relationship_of_magnitude_response_to_phase_response
``. So, you really need to know complete true phase response of your system.
`
Anyway, I'm just a curious observer ...
--Sergei.