Plot - representation of frequency to time dependece

Hi everyone!

I have a signal (image 1) that chages its frequency (value to time).

How I can display the dependece of frequency to time? Something like image 2. Is it real?

Thanks in advance!

Answer 1

[Star Strider]

I would use the pspectrum function with the 'spectrogram' option. (The spectrogram function is also an option,. however the units are different and may be more difficult to interpret. Both are in the Signal Processing Toolbox.)

EDIT — (18 Sep 2022 at 20:14)

Added this example

There are transition regions in this example that appear as high frequency discontinuities that may not be present in your signal. However this iillustrates the approach —

t = linspace(0, 10, 1001);
f = [ones(1,250)*70 ones(1,501)*10 ones(1,250)*70];
s = sin(2*pi*f.*t);
Fs = 1/(t(2)-t(1));

figure
plot(t, s)
grid

[sp,fp,tp] = pspectrum(s, Fs, 'spectrogram');
fr = [min(fp(:)) max(fp(:))];
tr = [min(tp(:)) max(tp(:))];

figure
pspectrum(s, Fs, 'spectrogram')

tv = get(gca, 'XLim');
fv = get(gca, 'YLim');

[r,c] = find(sp >= 0.95*max(sp(:)));
 
figure
plot(c/max(c)*diff(tv)+min(tv), r/max(r)*diff(fv)+min(fv))
grid
xlabel('t')
ylabel('f')
axis([0  10    0  100])

.

[Paul]

Where do the frequencies 50 and 17 come from? Shouldn't the plot start at 30 (70 aliased down by Fs = 100), transition to 10, then transition back to 30?

t = linspace(0, 10, 1001);
f = [ones(1,250)*70 ones(1,501)*10 ones(1,250)*70];
s = sin(2*pi*f.*t);
Fs = 1/(t(2)-t(1));
[sp,fp,tp] = pspectrum(s, Fs, 'spectrogram');
[M,I] = max(sp);
figure
plot(tp,fp(I))
xlabel('t');ylabel('f')