DSP Spectral Analysis using MATLAB

Hi everyone,
I am new with matlab and am having problems with a spectral analysis
problem i'm trying to do. I need to perform spectral analysis of a
signal consisting of 1000Hz, 1000.1Hz and 999.9Hz i.e.,
x=cos(2*pi*1000*t)+cos(2*pi*1000.1*t)+cos(2*pi*999.9*t) so that all the
three peaks are seen. This is to be done using a windowing function
like blackmann-harris/guassian.

I wrote the m-file to obtain peak for only one frequency. I'm totally
stumned with how to obtained all three frequency peaks. Here's my
m-file I'm working on. Any ideas on what to choose for the value of L,
N and Fs ???? Any help would be great, since i've no idea on how to
proceed. The m-file below gives me a peak at 999.9 Hz only and nothing
at 1000 and 1000.1 Hz.

Fs = 5000;  % sampling frequency
Ts = 1/Fs;  %  sampling period
L = 25;      % number of samples
N = 1024*4;   %  number of points for FFT
f1 = 1000     %  analog frquency in Hz
f2=1000.1
f3=999.9
A1 = 1.7;

t = (0:L-1)*Ts;     %  array of sampled time from 0 to Ts*(L-1)
x = A1*cos(2*pi*f1*t)+A1*cos(2*pi*f2*t)+A1*cos(2*pi*f3*t);  %  sampled
signal
X = fft(x,N); % take DFT of padded signal
XX = fftshift(X)   %center spectrum
bin_inc = Fs/N,     %compute the increment between discrete frequency
bins
f =[-N/2:(N/2-1)]*bin_inc;  %  create an array of frequencies
figure
plot(f, abs(XX));   %  plot the magnitude annotated with frequency
title('Rectangular')
xlabel('frequency Hz')
ylabel('magnitude')
zzR = max(abs(XX))
ampR = zzR/N*2

%  now use windows to process the data prior to performing FFT
wb  = window(@blackmanharris,L);
xb = x.*wb'; %  blackman window
XB =fftshift(fft(xb,N));

%  plot FFT
figure
plot(f, abs(XB))
title('Blackman-Harris');
xlabel('frequency Hz')
ylabel('magnitude')
zzB = max(abs(XB))
ampB = zzB/N*2

Thanks a lot in advance, waiting for reply. 
-Farah

"farah727rash@gmail.com" <farah727rash@gmail.com> wrote in 
news:1155903546.818338.17570@74g2000cwt.googlegroups.com:

> The m-file below gives me a peak at 999.9 Hz only and nothing
> at 1000 and 1000.1 Hz.
> 


Instead of 1024*4 points in your fft, try using a nice round number, like 
10,000 points.  Matlab algoriths no longer require a power of 2.
-- 
Scott
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