Theory:
The complex variables
z and
s are related by the equation:
$$ z = e^{Ts} \tag{1} $$
This means thtat a pole in the
s plane can be located in the
z plane through the transformation.
Since the complex variable
s had real part
σ and imaginary part
ω, we have
$$ s = σ + j ω \tag{2} $$
and
$$ z = e^{T(σ +j ω)} = e^{T σ} e^{jT ω} = e^{T σ} e^{j(T ω + 2 πk)} \tag{3} $$
From this last equation we see that poles and zeros in the
s plane, where frequencies differ in integral multiples of the sampling frequency 2 π/
T
, are mapped into the same location in the
z plane.
This means that there are infinitely many values of
s for each value of
z.
Since
σ is negative in the left half of the
s plane,
the left half of the
s plane corresponds to
$$ |z| = e^{T σ} \lt 1 \tag{4} $$
The
j ω axis in the
s plane corresponding to |
z| = 1.
That is, the imaginary axis in the
s plane (the line
σ = 0)
corresponding to the unit circle in the
z plane, and the interior of the unit circle corresponds to the left half of the
s plane.