# How to get equation of signal in matlab

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Muhammad Usman Saleem on 15 Jul 2017
Commented: Star Strider on 24 Jul 2017
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
Greg Heath on 20 Jul 2017
help NARNET
doc NARNET
help NNCORR
doc NNCORR
greg NARNET
Hope this helps.
Greg

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')
Star Strider on 24 Jul 2017
As always, my pleasure!
If my Answer helped solve your problem, please Accept it.

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