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Waveform Generation: Time Vectors and Sinusoids

Time Vectors

Most toolbox functions require you to begin with a vector representing a time base. Consider generating data with a 1000 Hz sample frequency, for example. An appropriate time vector is

t = (0:0.001:1)';

where the MATLAB® colon operator creates a 1001-element row vector that represents time running from 0 to 1 s in steps of 1 ms. The transpose operator (') changes the row vector into a column; the semicolon (;) tells MATLAB to compute, but not display the result.

Given t, you can create a sample signal y consisting of two sinusoids, one at 50 Hz and one at 120 Hz with twice the amplitude.

y = sin(2*pi*50*t) + 2*sin(2*pi*120*t);

The new variable y, formed from vector t, is also 1001 elements long. You can add normally distributed white noise to the signal and plot the first 50 points using

rng default;
yn = y + 0.5*randn(size(t));
plot(t(1:50),yn(1:50))

Impulse, Step, and Ramp Functions

Since MATLAB is a programming language, an endless variety of different signals is possible. Here are some statements that generate several commonly used sequences, including the unit impulse, unit step, and unit ramp functions:

t = (0:0.001:1)';
imp= [1; zeros(99,1)];       % Impulse
unit_step = ones(100,1);     % Step (with 0 initial cond.)
ramp_sig= t;                 % Ramp
quad_sig=t.^2;               % Quadratic
sq_wave = square(4*pi*t);    % Square wave with period 0.5

All of these sequences are column vectors. The last three inherit their shapes from t.

Multichannel Signals

Use standard MATLAB array syntax to work with multichannel signals. For example, a multichannel signal consisting of the last three signals generated above is

z = [ramp_sig quad_sig sq_wave];

You can generate a multichannel unit sample function using the outer product operator. For example, a six-element column vector whose first element is one, and whose remaining five elements are zeros, is

a = [1 zeros(1,5)]';

To duplicate column vector a into a matrix without performing any multiplication, use the MATLAB colon operator and the ones function:

c = a(:,ones(1,3));

Common Periodic Waveforms

The toolbox provides functions for generating widely used periodic waveforms:

  • sawtooth generates a sawtooth wave with peaks at ±1 and a period of 2π. An optional width parameter specifies a fractional multiple of 2π at which the signal's maximum occurs.

  • square generates a square wave with a period of 2π. An optional parameter specifies duty cycle, the percent of the period for which the signal is positive.

To generate 1.5 s of a 50 Hz sawtooth wave with a sample rate of 10 kHz and plot 0.2 s of the generated waveform, use

fs = 10000;
t = 0:1/fs:1.5;
x = sawtooth(2*pi*50*t);
plot(t,x), axis([0 0.2 -1 1])

Common Aperiodic Waveforms

The toolbox also provides functions for generating several widely used aperiodic waveforms:

  • gauspuls generates a Gaussian-modulated sinusoidal pulse with a specified time, center frequency, and fractional bandwidth. Optional parameters return in-phase and quadrature pulses, the RF signal envelope, and the cutoff time for the trailing pulse envelope.

  • chirp generates a linear, log, or quadratic swept-frequency cosine signal. An optional parameter specifies alternative sweep methods. An optional parameter phi allows initial phase to be specified in degrees.

To compute 2 s of a linear chirp signal with a sample rate of 1 kHz, that starts at DC and crosses 150 Hz at 1 s, use

t = 0:1/1000:2;
y = chirp(t,0,1,150);

To plot the spectrogram, use

spectrogram(y,256,250,256,1000,'yaxis')

The pulstran Function

The pulstran function generates pulse trains from either continuous or sampled prototype pulses. The following example generates a pulse train consisting of the sum of multiple delayed interpolations of a Gaussian pulse. The pulse train is defined to have a sample rate of 50 kHz, a pulse train length of 10 ms, and a pulse repetition rate of 1 kHz; D specifies the delay to each pulse repetition in column 1 and an optional attenuation for each repetition in column 2. The pulse train is constructed by passing the name of the gauspuls function to pulstran, along with additional parameters that specify a 10 kHz Gaussian pulse with 50% bandwidth:

T = 0:1/50E3:10E-3;
D = [0:1/1E3:10E-3;0.8.^(0:10)]';
Y = pulstran(T,D,'gauspuls',10E3,0.5);
plot(T,Y)

The Sinc Function

The sinc function computes the mathematical sinc function for an input vector or matrix x. Viewed as a function of time, or space, the sinc function is the inverse Fourier transform of the rectangular pulse in frequency centered at zero of width 2π and height 1. The following equation defines the sinc function:

The sinc function has a value of 1 when x is equal to zero, and a value of

for all other elements of x.

To plot the sinc function for a linearly spaced vector with values ranging from -5 to 5, use the following commands:

x = linspace(-5,5);
y = sinc(x);
plot(x,y)

The Dirichlet Function

The toolbox function diric computes the Dirichlet function, sometimes called the periodic sinc or aliased sinc function, for an input vector or matrix x. The Dirichlet function, D(x) is:

where N is a user-specified positive integer. For N odd, the Dirichlet function has a period of 2π; for N even, its period is 4π. The magnitude of this function is (1/N) times the magnitude of the discrete-time Fourier transform of the N-point rectangular window.

To plot the Dirichlet function over the range 0 to 4π for N = 7 and N = 8, use

x = linspace(0,4*pi,300);
plot(x,diric(x,7)); axis tight;
plot(x,diric(x,8)); axis tight;

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