how to create magnitude spectrum signal based on data from excel

dataset=xlsread('Group3.xlsx','Sheet1','A1:ALM1');

stepsize=0.001;

N=1;

t=0:stepsize:N;

x= dataset;

X=fft(x);

f=size(x,2)/2;

mag_spec=abs(x)/f;

freq=(0:499/2*f*stepsize);

plot(freq,mag_spec(1:500));

i already tried this code, but the error state :

'Error using plot

Vectors must be the same length.'

Answer 1

[Star Strider]

Your code needs a few slight improvements:

dataset=xlsread('Group3.xlsx','Sheet1','A1:ALM1');
stepsize=0.001;
N=1;
t=0:stepsize:N;
x= dataset;
X=fft(x);
f=size(x,2)/2;
mag_spec=abs(X)/(2*f);                          % Normalise By The Length Of The Original Vector
freq=(0:499*(2*f)*stepsize);                    % Use Full Vector Length To Create A Compatible Frequency Vector
figure
plot(freq,mag_spec(1:500));
xlim([0  50])

[ahmad syaiful md subri]

From the amplitude spectrum diagram of the signal (codding that you have give), how to design the lowpass filter to extract low frequency signal using Butterworth approximation

[Star Strider]

Choose the frequencies you want the filter to select, then follow the instructions in the documentation for buttord, butter, and zp2sos functions. Use filtfilt to do the actual filtering.