Plotting an amplitude spectrum of a .txt file

I wanted to ask if someone can help me plot an amplitude spectrum (log-log-scale) of a .txt file. In the file is a time series that has a seismic signal recorded at time intervals of 0.005s.

I would be very grateful!

Answer 1

[Star Strider]

Try this —

D1 = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/647750/spec.txt');
signal = D1;
L = numel(signal);                                                      % Signal Length
Ts = 0.005;                                                             % Sampling Interval (sed)
t = linspace(0, L-1, L);                                                % Time Vector

figure
plot(t, signal)
grid
title('Time Domain')
xlabel('Time (s)')
ylabel('Amplitude (Units)')

Fs = 1/Ts;                                                              % Sampling Frequency (Hz)
Fn = Fs/2;                                                              % Nyquist Frequency (Hz)
NFFT = 2^nextpow2(L);                                                   % FFT Length
FTsignal = fft(signal,NFFT)/L;                                          % Normalised Fourier Transform
Fv = linspace(0,1,fix(L/2)+1)*Fn;                                       % Frequency Vector
Iv = 1:numel(Fv);                                                       % Index Vector

figure
plot(Fv, abs(FTsignal(Iv))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude (Umits)')
Ax = gca;
Ax.XScale = 'log';
Ax.YScale = 'log';

[maxAmp,idx] = max(abs(FTsignal(Iv))*2);
fprintf('Maximum Amplitude = \t%.4f\nFrequency = \t\t%.4f Hz',maxAmp,Fv(idx))

Maximum Amplitude = 	0.0935
Frequency = 		2.2417 Hz

.

EDIT — (9 Jun 2021 at 16:38)

Corrected typographical error:

As.XScale = 'log';

to:

Ax.XScale = 'log';

.

Answer 2

[Scott MacKenzie]

This seems to work...

% load seismic signal data
ss = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/647750/spec.txt');
t = 0.005 * (1:length(ss)); % time

% plot using log-log scale
loglog(t,ss);