How to obtain the frequency of a signal as function of the time?

I have a discrete signal sampled at 0.005 seconds. (a 2-columns vector, the first colum is the time the second the value of the signal)

Plotting the signal as function of the time, it is possible to observe that

a) the signal amplitude decreases with time

b) the signal frequency increases with time

Therefore, I wish to plot two different things

1) the frequency of the signal as function of the time

2) the absolute value of the signal amplitude as function of the frequency

how can I do that? I will really appreciate if you can provide an example

Thanks in advance for your help !!!!!

G

Answer 1

[Youssef Khmou]

hi, This signal seems like a decreasing linear Chirp :

1) Frequency as function of time is called ' spectrogram') try :

   doc spectrogram 
   S = spectrogram(x(:,2)) % because the second column contains the signal .

2) You can use the FFT function , many resources are available , try my submission : http://www.mathworks.com/matlabcentral/fileexchange/40002-psd-power-spectral-density-and-amplitude-spectrum-with-adjusted-fft