usinf fft shift i need only to display the positive part of the spectrum

Question

Mohamed Ashraf 2018-2-20

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/383830-usinf-fft-shift-i-need-only-to-display-the-positive-part-of-the-spectrum

回答： wallflower 2020-12-7

I would like to display only the positive part of the frequency domain

if true
  % 
fs=1e5; %Sampling Frequency
t=0:1/(fs):0.001; %Time at which signal displays
%First,generating a signal
f=1e4;%Signal is 10kHz
amp_signal=1;
signal=amp_signal*(sin(2*pi*f*t));
%Frequency Domain of transmitted signal
transmited_signal_fft=fft(signal);
transmited_signal_fft=fftshift(transmited_signal_fft);
f_axis=linspace(-fs/2,fs/2,length(transmited_signal_fft));
%Plotting Signal
figure;
subplot(2,1,1);
plot(t,signal);
grid on;
title('Transmitted signal in time domain')
%Display transmitted signal in frequency domain
subplot(2,1,2);
plot(f_axis,abs(transmited_signal_fft)/length(signal));
grid on;
title('Transmitted signal in frequency domain')
end

Thanks in advance

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Answer 1

Star Strider 2018-2-21

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/383830-usinf-fft-shift-i-need-only-to-display-the-positive-part-of-the-spectrum#answer_306353

在 MATLAB Online 中打开

Try this:

fs=1e5; %Sampling Frequency
t=0:1/(fs):0.001; %Time at which signal displays
%First,generating a signal
f=1e4;%Signal is 10kHz
Fn = f/2;                                                               % Nyquist Freqency (Added)
amp_signal=1;
signal=amp_signal*(sin(2*pi*f*t));
L = length(signal);                                                     % Added
%Frequency Domain of transmitted signal
transmited_signal_fft=fft(signal);
f_axis = linspace(0, 1, fix(L/2)+1)*Fn;                                 % Changed
Iv = 1:length(f_axis);                                                  % Index Vector (Added)
% % transmited_signal_fft=fftshift(transmited_signal_fft);              % (Commented-Out)
% % f_axis=linspace(-fs/2,fs/2,length(transmited_signal_fft));          % (Commented-Out)
%Plotting Signal
figure;
subplot(2,1,1);
plot(t,signal);
grid on;
title('Transmitted signal in time domain')
%Display transmitted signal in frequency domain
subplot(2,1,2);
plot(f_axis, 2*abs(transmited_signal_fft(Iv))/L);                       % Changed
grid on;
title('Transmitted signal in frequency domain')

See this version of the documentation for details: fft (link).

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Mohamed Ashraf 2018-2-21

the nyquist frequency should be double the message not half of it

Nyquist Frequency:the minimum rate at which a signal can be sampled without introducing errors, which is twice the highest frequency present in the signal.