Taking DFT of Swept Sine Wave

I have been having Problem in Taking DFT of a Swept Sine Signal, my code is below, please Review the Code and Let me give some Suggestions if true amp=4;

   t1=str2num(get(handles.t1,'String'));
   t2=str2num(get(handles.t2,'String'));
   dt=str2num(get(handles.interval,'String'));
   t=t1:dt:t2;
    ValF=str2num(get(handles.startf,'String'));
    ValF1=str2num(get(handles.endf,'String'));
    f1=2*pi*ValF;
    f2=2*pi*ValF1;
    f= ((f2-f1).*t/(t2-t1)) + f1;

    % Define the time vector
y = amp .* sin(f .* t);        % Compute y(t), the sine wave
plot(t,y)                           % Plot y vs. t
xlabel('Time (seconds)')            % Label x-axis
ylabel('y(t)')                      % Label y-axis
title('Time-Frequency Plot') % Give plot a title
grid on                             % Turn on plot grid
diary off                           % Turn diary off
 guidata(hObject,handles);

    % code
function fourr(handles)
amp=4; 
t1=str2num(get(handles.t1,'String'));
   t2=str2num(get(handles.t2,'String'));
   dt=str2num(get(handles.interval,'String'));
   t=t1:dt:t2;
    ValF=str2num(get(handles.startf,'String'));
    ValF1=str2num(get(handles.endf,'String'));
    f1=2*pi*ValF;
    f2=2*pi*ValF1;
    f= ((f2-f1).*t/(t2-t1)) + f1;
y = amp .* sind(f .* t);  
z=stem(y)'
fft(z);
stem(f,fft(z));
xlabel('Time(sec)')
ylabel('FFT')

function ffft_Callback(hObject, eventdata, handles)

fourr(handles); end

Answer 1

[Greg Heath]

Please do not use the acronym 'DFT' when you are using the 'FFT'

 close all,clear all, clc, plt = 0;  

    N = 48
    T = 3*pi
    dt = T/N
    t = dt*(0:N-1);

    Fs = 1/dt
    df = Fs/N
    f = df*(0:N-1);

    A =4
    y = A * sin(f .* t);

    plt=plt+1,figure(plt)
    hold on
    plot( t, y, 'LineWidth', 2)
    plot( t, zeros(1,N), 'k--',  'LineWidth', 2 )
    xlabel('Time (seconds)')
    ylabel('y(t)')
    title( 'SWEPT FREQUENCY TIME SIGNAL' )

    Y = fftshift(fft(y))/N;
    fb = f-Fs/2;
    realY = real(Y);
    imagY = imag(Y);
    absY = abs(Y);
    phaseY = angle(Y);

    plt=plt+1,figure(plt)
    subplot(2,2,1)
    hold on
    plot( fb, realY,  'LineWidth', 2 )
    plot( fb, zeros(1,N), 'k--', 'LineWidth', 2 )
    xlim( [ -Fs/2 Fs/2 ] )
    ylabel( ' REAL(Y) ' )
    title([ blanks(85) , ' SPECTRUM OF SWEPT FREQUENCY SIGNAL ' ] )

    subplot(2,2,2)
    hold on
    plot( fb, imagY,  'LineWidth', 2 )
    plot( fb, zeros(1,N), 'k--',  'LineWidth', 2 )
    xlim( [ -Fs/2 Fs/2 ] )
    ylabel( ' IMAG(Y) ' )

    subplot(2,2,3)
    hold on
    plot( fb, absY,  'LineWidth', 2)
    xlim( [ -Fs/2 Fs/2 ] )
    ylabel( ' AMPLITUDE(Y) ' )
    xlabel( ' FREQUENCY(HZ)  ' ) ; 

    subplot(2,2,4)
    hold on
    plot(fb,phaseY, 'LineWidth', 2)
    plot( fb, zeros(1,N), 'k--', 'LineWidth', 2 )
    xlim( [ -Fs/2 Fs/2 ] )
    ylabel( ' PHASE(Y) ' )
     xlabel( ' FREQUENCY(HZ)  ' ) ;

Hope this helps.

Thank you for formally accepting my answer.

Greg