Decomposition minimal phase & all pass

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Discussion Overview

The discussion revolves around the properties of transfer functions, specifically focusing on the concepts of minimum phase and all-pass filters. Participants explore the implications of poles and zeros in the context of stability and causality, particularly in discrete-time systems.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents transfer functions G1 and G2 and expresses confusion about moving a zero to the unit circle.
  • Another participant questions whether the problem is homework-related and asks for clarification on the all-pass transfer function.
  • A participant defines the all-pass condition as having a magnitude of 1 for G2(jw).
  • Concerns are raised about G1 not being minimum phase due to having right-half plane poles and/or zeros, and the potential non-causality of G2.
  • It is suggested that deriving all-pass and minimum phase functions from G may be impossible due to the presence of right-half plane zeros.
  • A later reply proposes a different form for G(s) and discusses the necessity of having a right-half plane zero to achieve the all-pass condition.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of deriving all-pass and minimum phase functions from the given transfer functions. There is no consensus on the implications of poles and zeros in this context.

Contextual Notes

Participants note the importance of causality and stability in filter design, particularly in relation to the unit circle in discrete-time systems. There are unresolved issues regarding the definitions and implications of minimum phase and all-pass filters.

Who May Find This Useful

This discussion may be of interest to those studying control systems, digital signal processing, or filter design, particularly in understanding the properties of transfer functions and their implications for stability and causality.

Hidd
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Thread moved from the technical forums to the schoolwork forums (this includes for non-homework revision studying)
Homework Statement
I have the following transfer function, and I would like to decompose it to a minimal phase G1 & all pass G2 transfer functions:



G(s) = (1-s) / (2 + 10s)

G(s) = G1 * G2
Relevant Equations
All-pass ==> magnitude of G2(jw) =1
Minimum phase ==> Re{Zeros,poles}<0
Hey everybody!

I have put G1 = (1-s)/(2-10s) & G2 = (2-10s)/ (2 +10s)
but than I read that all poles and zeroes should be inside the unit circle, and I don't know how to move the Zero S_01 = 1 to the unit circle
 
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Is this a homework problem? Do you know what the all-pass G2 transfer function is?
 
it's when the magnitude of G2(jw) =1
 
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OK, but G1 isn't minimum phase since it has right-half plane poles and/or zeros.
Still, I doubt that G2 = (1 + s)/(1 - s) is allowed since it's not causal (RHP pole). Still it is technically correct, |G2|=1. Otherwise, I don't see how to get rid of the RHP zero in G without a RHP pole in G2.* Note that non-causal digital filters are used sometimes.

But it's been decades since I did this sort of filter design problem. It's not the sort of thing you'll ever see much in practice, IMO.

I think the unit circle comment relates to discrete time systems where z outside the unit circle is equivalent to s in the RHP.

edit: * Oops! confused the first time
 
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That means that it's impossible to derive the all-pass & min-phase functions from G ?!
 
Hidd said:
That means that it's impossible to derive the all-pass & min-phase functions from G ?!
Oops, sorry I got some of my signs confused. Let's start over.
What about the obvious choice:
$$ G(s) = \frac{(1-s)}{(2 + 10s)} = \frac{(1-s)}{(1+s)} ⋅ \frac{(1+s)}{(2 + 10s)} $$

Note that all-pass filter are never minimum phase, they can't be. You must have a RHP zero to cancel the magnitude response of the LHP pole (or the other way around if you don't care about causality & stability due to RHP poles).
 
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Thnak you DaveE for your help
 
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