What is Z-Transform

 In Digital Signal Processing (DSP), the Z-transform is a mathematical technique used to analyze discrete-time signals in the frequency domain. It is a generalization of the Fourier transform, which is used to analyze continuous-time signals.

The Z-transform of a discrete-time signal x(n) is defined as the infinite sum of the signal values weighted by powers of a complex variable z. 

 \(X(z) = \sum_{n=-\infty}^{\infty}x(n) z^{-n}\)

The resulting function, X(z), is a complex function of z that represents the signal x(n) in the z-domain. The Z-transform of a sequence x(n) is a mathematical operation that is defined as the summation of the sequence weighted by powers of the complex variable z. See how Z-transform is derived. It is used to convert a discrete-time signal into a complex function of the complex variable z, which is useful in analyzing the properties of the signal in the frequency domain.

The Z-transform is useful in DSP because it provides a convenient way to analyze the frequency content of a discrete-time signal. Specifically, the Z-transform allows us to:

  • Determine the frequency response of a system: By taking the Z-transform of a system's impulse response, we can derive the system's transfer function, which gives us insight into how the system affects different frequencies.
  • Solve difference equations: Many systems in DSP are modeled using difference equations, which relate the input and output of the system in discrete time. The Z-transform provides a way to solve these equations by transforming them into algebraic equations in the z-domain.
  • Analyze stability: The location of the poles of the Z-transform in the complex plane can be used to determine the stability of a system.
  • FIR and IIR Filter design:Z-Transform are used extensively in the design of FIR digital filter and IIR digital filters. See FIR and IIR filter design with Z-transform.

 Overall, the Z-transform is a powerful tool in DSP that allows us to analyze and manipulate discrete-time signals and systems in the frequency domain.

Z-Transform in Matlab

Following is matlab code to perform Z-Transform on a discrete sequence.

% Define the discrete-time signal x(n)
n = 0:10;
x = 2.^(-n);

% Compute the Z-transform of x(n)
z = linspace(-pi, pi, 1000);
X = zeros(size(z));
for k = 1:length(n)
    X = X + x(k) * z.^(-n(k));

% Plot the magnitude and phase of X(z)
title('Discrete-time signal x(n)');
xlabel('Sample index n');
axis([-1 11 0 1.1*max(x)]);
title('Z-transform of x(n)');
xlabel('Frequency (normalized)');
title('Phase of X(z)');
xlabel('Frequency (normalized)');
ylabel('Phase (radians)');

The code then plots the magnitude and phase of X(z) using the subplot command to create a two-panel figure. The magnitude and phase are plotted as functions of the normalized frequency, which ranges from -π to π in this example. The signal x(n) is a geometric sequence, which means that its Z-transform X(z) has a pole at z=1/2. This pole gives rise to a single spike in the magnitude plot. Since the phase of a Z-transform is a function of frequency, the phase plot will only show non-zero values at the frequencies where the Z-transform has poles or zeros. The reason for the phase plot showing all zeros in your code is that the Z-transform of the geometric sequence x(n) = 2^(-n) has a pole at z = 1/2.

The plot is shown below.

z-transform matlab plot

More on DSP with Matlab tutorials are below.

[1] What is Discrete Fourier Transform(DFT)

[2] DFT in Matlab without built-in function

[3] FIR filter design by frequency sampling 

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