trying to take FFT of a signal but not getting the correct result

t1=load('201710h1.lvm'); %test 1

t2=load('201710h2.lvm'); %test 2

Fs = 100000; % frequency sample

x = t1 (:,1); % time for test 1

sin = t1 (:,2); % test 1 sesnor 1

plot(sin)

title('Sensor1')

[KALYAN ACHARJYA]

Have you checked here?

https://in.mathworks.com/help/matlab/ref/fft.html

Answer 1

[Star Strider]

Perhaps calling the fft function would get you closer.

Example —

t{1}=load('201710h1.lvm');  %test 1 
t{2}=load('201710h2.lvm');  %test 2

Fs = 100000; % frequency sample

for k = 1:numel(t)
    L = numel(t{k});
    Ts = 1/Fs;
    Fn = Fs/2;
    tv = linspace(0, L, L)*Ts;
    FTt = fft(t{k})/L;
    Fv = linspace(0, 1, fix(L/2)+1)*Fn;
    Iv = 1:numel(Fv);
    
    figure
    plot(tv, t{k})
    grid
    xlabel('Time (sec)')
    ylabel('Signal (Units)')
    title(sprintf('t_{%d} Time Domain',k))
    
    figure
    plot(Fv, abs(FTt(Iv))*2)
    grid
    xlabel('Frequency (Hz)')
    ylabel('Amplitude (Units)')
    title(sprintf('t_{%d} Frequency Domain',k))
end

Note that I loaded your signals into cell arrays to make the analysis easier.

[zeshan chishti]

hey i noted i copied the wrong code the intial code was the following;

t1=load('201710h1.lvm'); %test 1

t2=load('201710h2.lvm'); %test 2

fs= 100000; % sampling frequency

x= t1 (:,1); % time in test 1

sen1 = t1 (:,2); %sensor 1 in test 1

sen2 = t2 (:,2); %sensor 1 in test 2

y = fft (sen1,fs); % fft of test 1 sensor 1 with fs

plot (y)

i will take your sugessted code hopefully it will work, thank you!

[Star Strider]

My pleasure!

You will have to modify it slightly to work with your data, since I now have some relevant information about it.

Try this:

t{1}=load('201710h1.lvm');  %test 1 
t{2}=load('201710h2.lvm');  %test 2

x = t1 (:,1);       % time in test 1
sen{1} = t1 (:,2);  %sensor 1 in test 1 
sen{2} = t2 (:,2);  %sensor 1 in test 2

Fs = 100000; % frequency sample

for k = 1:numel(sen)
    L = numel(sen{k});
    Ts = 1/Fs;
    Fn = Fs/2;
%     tv = linspace(0, L, L)*Ts;
    FTsen = fft(sen{k})/L;
    Fv = linspace(0, 1, fix(L/2)+1)*Fn;
    Iv = 1:numel(Fv);
    
    figure
    plot(x, sen{k})
    grid
    xlabel('Time (sec)')
    ylabel('Signal (Units)')
    title(sprintf('sensor_{%d} Time Domain',k))
    
    figure
    plot(Fv, abs(FTsen(Iv))*2)
    grid
    xlabel('Frequency (Hz)')
    ylabel('Amplitude (Units)')
    title(sprintf('sensor_{%d} Frequency Domain',k))
end

I cannot test this with your data since I do not have it to work with.

.

[zeshan chishti]

Thank you i appreicte your help with this code it dose not recgnoise t1 as a variable. will have to change it around for it to work.

[Star Strider]

As always, my pleasure!

Change the first two lines back to:

t1 = load('201710h1.lvm');  %test 1 
t2 = load('201710h2.lvm');  %test 2

and the rest should work. (I changed my code to match your updated code, and forgot to change the ‘t1’ and ‘t2’ variables back to what they were originally.)