From a Unix machine in the math department, first type xhost +sigma
so that your machine will allow sigma to display graphics on your
screen.

Then rlogin to sigma.  Once there, type setenv DISPLAY yourmachinename:0
(making sure to put your machine name in).  This tells sigma to display
graphics on your machine.

Type matlab.

Some things to try:

help signal

help chirp

The demos under help chirp show how to make spectrograms of the signal.
Try this:

    EXAMPLE 1: Compute the spectrogram of a linear chirp.
      t=0:0.001:2;                 % 2 secs @ 1kHz sample rate
      y=chirp(t,0,1,150);          % Start @ DC, cross 150Hz at t=1sec 
      specgram(y,256,1E3,256,250); % Display the spectrogram
 
    EXAMPLE 2: Compute the spectrogram of a quadratic chirp.
      t=-2:0.001:2;                % +/-2 secs @ 1kHz sample rate
      y=chirp(t,100,1,200,'q');    % Start @ 100Hz, cross 200Hz at t=1sec 
      specgram(y,128,1E3,128,120); % Display the spectrogram
 
(You should be able to simply drop this text into matlab to run it.)