problem with my results

I am plotting a spectrogram using STFT algorithm generated by me everything works fine but my results are not what i expected please help me where I am wrong.

clear all
close all
clc
Fs = 2*max(100,300);
t = 0:1/Fs:1-1/Fs;
t1 = t(end);
x = chirp(t,300,t1,100,'linear');
w = rectwin(10)';
nw= length(w);
X = fft(x);
Total = [];
y = length(x) - nw;
for tau = 0: y
  S = 0;
  if tau == 0 %1st window
      S = X(1:nw);

    else %THE remaining windows generated from here
     for k = 0:nw-1
        for n = 0:nw-1

        S= (exp(2*pi*1j*k/nw).*(X(tau : tau + nw-1) -  x(tau)+...
                      x(nw+tau).*exp(-2*pi*1j*n/nw)));
        end

     end
    end

    W= S(1:end/2);
   Total = [Total; W];
end
%plotting
T = 1:length(x)/2;
imagesc(T,1:Fs/2,20*log10(abs(Total).^2))
axis xy; axis tight; colormap(jet);
xlabel('Time in sec');
ylabel('Frequency (Hz)');

[Walter Roberson]

The easiest solution would be to change what you are expecting.

We do not know what you are expecting, so there are no code changes we can suggest.

[raj]

i am expecting a spectrogram for a chirp signal using short time fourier transform...

[raj]

the chirp should start from 300 Hz and end at 100 Hz

[Walter Roberson]

A chirp that _starts_ at 300 Hz, and _ends_ at 100 Hz? Not 300 to 1000 ? Not 100 to 300 ?

[raj]

I mean to say at t = 0 it starts at 300 Hz and at t(end) it is at 100 Hz

[raj]

I didnt understand your question..

[Image Analyst]

Chirps increase in frequency as time goes along - the wavelength decreases. See figures in http://en.wikipedia.org/wiki/Chirp_signal .

[Walter Roberson]

Looking at that page, they say that down-chirps exist, and it appears that k (rate of increase in frequency) could be negative.

[raj]

My problem is not with chirp signal consider any signal but I am not getting the spectrogram expected please try with a signal of ur own