Advanced Z-transform

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In mathematics and signal processing, the advanced Z-transform is an extension of the Z-transform, to incorporate ideal delays that are not multiples of the sampling time. It takes the form

F(z, m) = \sum_{k=0}^{\infty} f(k T + m)z^{-k}

where

  • T is the sampling period
  • m (the "delay parameter") is a fraction of the sampling period [0,T).

It is also known as the modified Z-transform.

The advanced Z-transform is widely applied, for example to model accurately processing delays in digital control.

Contents

[edit] Properties

If the delay parameter, m, is considered fixed then all the properties of the Z-transform hold for the advanced Z-transform.

[edit] Linearity

Z \left[ \sum_{k=1}^{m} c_k f_k(t) \right] = \sum_{k=1}^{m} c_k F(z, m).

[edit] Time shift

Z \left[ u(t - n T)f(t - n T) \right] = z^{-n} F(z, m).

[edit] Damping

Z \left[ f(t) e^{-a\, t} \right] = e^{-a\, m} F(e^{a\, T} z, m).

[edit] Time multiplication

Z \left[ t^y f(t) \right] = \left(-T z \frac{d}{dz} + m \right)^y F(z, m).

[edit] Final value theorem

\lim_{k = \infty} f(k T + m) = \lim_{z = 1} (1-z^{-1})F(z, m).

[edit] Example

Consider the following example where f(t) = cos(ωt)

F(z, m) = Z \left[\cos \left(\omega \left(k T + m \right) \right) \right]
F(z, m) = Z \left[\cos (\omega k T) \cos (\omega m) - \sin (\omega k T) \sin (\omega m) \right]
F(z, m) = \cos(\omega m) Z \left[ \cos (\omega k T) \right] - \sin (\omega m) Z \left[ \sin (\omega k T) \right]
F(z, m) = \cos(\omega m) \frac{z \left(z - \cos (\omega T) \right)}{z^2 - 2z \cos(\omega T) + 1} - \sin(\omega m) \frac{z \sin(\omega T)}{z^2 - 2z \cos(\omega T) + 1}
F(z, m) = \frac{z^2 \cos(\omega m) - z \cos(\omega(T - m))}{z^2 - 2z \cos(\omega T) + 1}.

If m = 0 then F(z,m) reduces to the Z-transform

F(z, m) = \frac{z^2 - z \cos(\omega T)}{z^2 - 2z \cos(\omega T) + 1}

which is clearly just the Z-transform of f(t).

[edit] See also

[edit] Bibliography

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Digital signal processing
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