How to get equation of signal in matlab

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I want to find equation of this signal.
I plot this time series data and filter it with sSgolay filter. Now I want to get equation of this signal. Is this possible in matlab??
I have attached data of this plot with this post
  4 Comments
Greg Heath
Greg Heath on 20 Jul 2017
help NARNET
doc NARNET
help NNCORR
doc NNCORR
greg NARNET
Hope this helps.
Greg

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Accepted Answer

Star Strider
Star Strider on 15 Jul 2017
I am not certain what you intend by getting an equation for it.
You can filter out the noise to see the general trend:
fidi = fopen('data.txt','rt');
D = textscan(fidi, '%s%f', 'Delimiter','\t', 'CollectOutput',1);
t = datenum(D{1});
s = D{2};
Ts = mean(diff(t)); % Sampling Interval
Fs = 1/Ts; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
L = numel(t); % Signal Length
sm = s-mean(s); % Subtract Mean To Make Amplitudes At Frequencies>0 More Prominent
FTs = fft(sm)/L; % Fourier Transform
Fv = linspace(0, 1, fix(L/2)+1)*Fn; % Frequency Vector
Iv = 1:length(Fv); % Index Vector
Phs = angle(FTs);
figure(1)
plot(Fv, abs(FTs(Iv))*2)
grid
xlabel('Frequency (Days^{-1})')
ylabel('Amplitude')
set(gca, 'XLim',[0 0.05])
Wp = [0.0045]/Fn; % Passband Frequencies (Normalised)
Ws = [0.0055]/Fn; % Stopband Frequencies (Normalised)
Rp = 10; % Passband Ripple (dB)
Rs = 50; % Stopband Ripple (dB)
[n,Ws] = cheb2ord(Wp,Ws,Rp,Rs); % Filter Order
[z,p,k] = cheby2(n,Rs,Ws); % Filter Design
[sosbp,gbp] = zp2sos(z,p,k); % Convert To Second-Order-Section For Stability
figure(2)
freqz(sosbp, 2^16, Fs) % Filter Bode Plot
s_filt = filtfilt(sosbp,gbp, s); % Filter Signal
figure(3)
plot(t, s, '-b')
hold on
plot(t, s_filt, '-r', 'LineWidth',1.5)
hold off
xlabel('Time (Days)')
ylabel('Amplitude')
legend('Original', 'Lowpass Filtered')
  8 Comments
Star Strider
Star Strider on 24 Jul 2017
As always, my pleasure!
If my Answer helped solve your problem, please Accept it.

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