# 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??
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.
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 