# Interpreting FFT Graph with Noise Floor

조회 수: 4(최근 30일)
Raidah Zaman 2021년 5월 10일
댓글: Star Strider 2021년 5월 11일
Hello,
I am trying to analyze data that I recorded from the Force Sensitive Resistors (FSRs) coded on Arduino through data streamer on excel. I used four FSRs, so there are 4 columns of data over a set of time (1200 rows, about 15 seconds). I seperated time from the FSR data so that I can graph the FFT of the FSR data against the time.
I succesfully made a correct code and have proper results from it, however I need help interpeting the graphs.
Code:
X = importdata('tapedata.csv');
t = importdata('tapetimefix.csv');
subplot(2,1,1);
plot(t,X);
grid
xlabel('Time')
ylabel('Amplitude')
title('FSR Recording (Tape)');
L = numel(X);
Ts = mean(diff(t));
% Tsd = std(mean(diff(t)))
Fs = 1/Ts;
Fn = Fs/2;
xft=fft(X-mean(X),[],1)/L;
xabs = abs(xft);
subplot(2,1,2);
plot(t,xabs);
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
xabs = abs(xft);
subplot(2,1,2);
plot(Fv,mag2db(xabs(Iv)*2))
grid
ylim([-75 50])
xlabel('Freqency [units]')
ylabel('Power [dB]')
title('FFT Analysis Graph (Tape)');
I was wondering is that just a lot noise or if there are no deviances from -50db meaning the FSRs is constant?
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Raidah Zaman 2021년 5월 10일
Here my apologies, I have attached it.

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### 채택된 답변

Star Strider 2021년 5월 11일
Try this —
SizeX = size(X)
SizeX = 1×2
1062 4
figure
for k = 1:size(X,2)
subplot(4,1,k)
plot(t, X(:,k))
title(sprintf('Column %d',k))
end
figure
subplot(3,1,1);
plot(t,X);
grid
xlabel('Time')
ylabel('Amplitude')
title('FSR Recording (Tape)');
L = size(X,1); % <= CHANGED
Ts = mean(diff(t));
% Tsd = std(mean(diff(t)))
Fs = 1/Ts;
Fn = Fs/2;
xft=fft(X-mean(X))/L;
xabs = abs(xft);
subplot(3,1,2);
plot(linspace(-Fn,Fn,numel(t)),mag2db(fftshift(xabs)*2));
grid
ylim([-75 50])
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
xabs = abs(xft(:,3)); % <= CHANGED
subplot(3,1,3);
plot(Fv,mag2db(xabs(Iv)*2))
grid
ylim([-75 50])
xlabel('Freqency [units]')
ylabel('Power [dB]')
title('FFT Analysis Graph (Tape)');
This appears to be close to the desired result.
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Star Strider 2021년 5월 11일
As always, my pleasure!

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