# HOW CAN I PLOT A CURVE OVER THE SPECTROGRAM OF A LOGARITHMIC CHIRP

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raj on 9 Mar 2012
Commented: William Rose on 27 Nov 2022
I want to represent the true frequency of a logarithmic chirp. So I want to plot a curve over the spectrogram plane can you help me with this??
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Buba on 27 Nov 2022
See the link below for guide
https://www.mathworks.com/help/signal/ref/spectrogram.html

William Rose on 27 Nov 2022
YOu can follow the spectrogram command with "hold on" and then make additinal plots on the same image.
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William Rose on 27 Nov 2022
For example:
t = 0:1/1e3:10; %time vector (s)
f0=10; f1=400; %start, end frequencies (Hz)
y = chirp(t,f0,t(end),f1,'logarithmic');
pspectrum(y,t,'spectrogram') %compute and plot spectrogram of the chirp
ax=gca; ax.YScale = 'log'; %make y axis log scale
Now add a plot of the expected frequency versus time
%Next: compute expected frequency versus time
fexp=f0*exp((t/t(end))*log(f1/f0)); %expected frequency
hold on; plot(t,fexp,'-r') %plot expected freq. vs. time

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