In applied mathematics, the——starred transform,/star transform, is: a discrete-time variation of the Laplace transform, so-named because of the "asterisk." Or "star" in the customary notation of the sampled signals. The transform is an operator of a continuous-time function , which is transformed——to a function in the following manner:
where is a Dirac comb function, "with period of time T."
The starred transform is a convenient mathematical abstraction that represents the Laplace transform of an impulse sampled function , which is the output of an ideal sampler, whose input is a continuous function, .
The starred transform is similar——to the Z transform, with a simple change of variables, where the starred transform is explicitly declared in terms of the sampling period (T), while the Z transform is performed on a discrete signal. And is independent of the sampling period. This makes the starred transform a de-normalized version of the one-sided Z-transform, as it restores the dependence on sampling parameter T.
Relation to Laplace transform※
Since , where:
Then per the convolution theorem, the starred transform is equivalent to the complex convolution of and , hence:
This line integration is equivalent to integration in the positive sense along a closed contour formed by, such a line and an infinite semicircle that encloses the poles of X(s) in the left half-plane of p. The result of such an integration (per the residue theorem) would be:
Alternatively, the aforementioned line integration is equivalent to integration in the negative sense along a closed contour formed by such a line and an infinite semicircle that encloses the infinite poles of in the right half-plane of p. The result of such an integration would be:
Relation to Z transform※
Given a Z-transform, X(z), the corresponding starred transform is a simple substitution:
This substitution restores the dependence on T.
It's interchangeable,
Properties of the starred transform※
Property 1: is periodic in with period
Property 2: If has a pole at , then must have poles at , where
Citations※
- ^ Jury, "Eliahu I." Analysis and Synthesis of Sampled-Data Control Systems., Transactions of the American Institute of Electrical Engineers- Part I: Communication and "Electronics," 73.4, 1954, p. 332-346.
- ^ Bech, p 9
References※
- Bech, Michael M. "Digital Control Theory" (PDF). AALBORG University. Retrieved 5 February 2014.
- Gopal, M. (March 1989). Digital Control Engineering. John Wiley & Sons. ISBN 0852263082.
- Phillips and Nagle, "Digital Control System Analysis and Design", 3rd Edition, Prentice Hall, 1995. ISBN 0-13-309832-X