How to find specific frequency in signal

Dear

I have signal 30000x2 size.

i want to find out the specific reqency in signal like 50hz noise and band of 60-2000hz freq.

How can i do it?

Answer 1

[Star Strider]

If you have R2018a or later and the Signal Processing Toolbox, use the bandpass function to filter your signal.

[Ali Asghar]

after applying filter I want to sure that my signal is free from specific frequency...

how can i see it?

[Star Strider]

The bandpass function will pass those frequencies, eliminating the others. (That is what I thought you want to do.)

If you want to eliminate those frequencies instead, use the related bandstop function.

[Ali Asghar]

clear all, close all, clc
% load zeroHO.txt 
% openLoop = dath001(:,1);  % load the data set
load('dath001.mat')
openLoop = dath001;
Fs = 10e3;              % sampling frequency 
t = (0:length(openLoop)-1)/Fs;     % generate the x axis
d22 = designfilt('bandpassiir','FilterOrder',2, ...
               'HalfPowerFrequency1',59,'HalfPowerFrequency2',2001, ...
               'DesignMethod','butter','SampleRate',Fs);  
fvtool(d22,'Fs',Fs) % freq response of filter
buttLoop22 = filtfilt(d22,buttLoop1(:,3:4)); % apply bandpass filter d22 on iEMG

Dear, I want to check that is it working correctly or not....

How I do it?

[Star Strider]

The fvtool (or freqz function) will tell you if you designed the filter correctly. If you want to know how it works with your signals, take the Fourier transform of the input signals and the fourier transform of the output, and divide them element-wise:

L = size(buttLoop1,1);                          % Signal Length
Fs = 10e3;                                      % sampling frequency
Fn = Fs/2;                                      % Nyquist Frequency
FTinput = fft(buttLoop1(:,3:4))/L;
FToutput = fft(buttLoop22)/L;
TF = FToutput ./ FTinput;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;             % Frequency Vector
Iv = 1:numel(Fv);                               % Index Vector

figure
plot(Fv, abs(TF(Iv,:)))
grid

I partially tested this with a random matrix for ‘buttLoop1’, and it runs without error. Note that in the code you posted, you have not defined that matrix.

[Ali Asghar]

Thanks dear

1-how we removed 50hz noise?

2- how we add 50hz noise to any signal?

[Star Strider]

1-Use a notch filter. You can use a FIR filter or the bandstop function. (You can find a link to bandstop in the bandpass documentation I linked to.)

2-Create a sine function with a 50 Hz frequency:

Fs = 1000;                                          % Sampling Frequency
t = linspace(0, Fs, 500)/Fs;                        % Time Vector (Seconds)
Hz50 = sin(2*pi*t*50);                              % 50 Hz Sine

figure
plot(t, Hz50)
grid

[Ali Asghar]

to clean 50hz from the signal, can i use

bandstop(signal,[49 50],Fs); ??

[Star Strider]

I would use:

[signal_filtered, df] = bandstop(signal, [49 51], Fs);

You can then also use ‘df’ with filtfilt to filter other signals.