FFT Chirp Problems MATLAB

I am doing an FFT of a chirp function.

I am having problems with realizing the FFT! The FFT should when p =1 look
like the signal in. It doesn't Any suggestions?! Thanks...


The following is the m-code:

function s = dtFMchirp(p,TW)

% DCHIRP generate a sampled chirp signal
% usage s = dchirp(TW,p)
% s : samples of a digital "chirp" signal 
% 	exp(j(W/T)pi*t^2)  -T/2 <= t < T/2
% TW : time-bandwidth product
% p : sample at p times the Nyquist rate (W)
%p=10;
%TW=50;
%N = p*TW;
%n = 0:N;
%alpha = 1/(2*p*p*TW);
%s = exp(j*2*pi*alpha*((n-.5*N).^2));
%x=real(s);
%plot(x);
%length1=length(x);
%figure;
%fftx=fft(x,length1);
%fftx=abs(fftx);
%plot(fftx);
close all;
clear;
p=1;
TW=50;
N = p*TW;
n = 0:N;
alpha = 1/(2*p*p*TW);
s = exp(j*2*pi*alpha*((n-.5*N).^2));
figure(1); 
x=real(s);
plot(x);
length1=length(x);

fftx=fft(x,length1);
k=round(length1/2);
fftx=[fftx(k:end) fftx(1:k)];
fftx=abs(fftx);
fftx=abs(20*log10(fftx));
x=-2:4/length1:2;
figure(3)
plot(fftx);

"kyletae" <kyletae@gmail.com> wrote in message 
news:m6-dnUuLDphvsR3ZnZ2dnUVZ_vSdnZ2d@giganews.com...
>I am doing an FFT of a chirp function.
>
> I am having problems with realizing the FFT! The FFT should when p =1 look
> like the signal in. It doesn't Any suggestions?! Thanks...
<...>
It looks like your smallest phase increment is 2*pi , this will cause 
problems.

Best of Luck - Mike

"Mike Yarwood" <mpyarwood@btopenworld.com> wrote in message 
news:haednfXDS6deph3ZnZ2dnUVZ8qOdnZ2d@bt.com...
>
> "kyletae" <kyletae@gmail.com> wrote in message 
> news:m6-dnUuLDphvsR3ZnZ2dnUVZ_vSdnZ2d@giganews.com...
>>I am doing an FFT of a chirp function.
>>
>> I am having problems with realizing the FFT! The FFT should when p =1 
>> look
>> like the signal in. It doesn't Any suggestions?! Thanks...
> <...>
> It looks like your smallest phase increment is 2*pi , this will cause 
> problems.
Acchh! No it's not - I forgot the alpha..

Best of luck- Mike

kyletae wrote:
> I am doing an FFT of a chirp function.
>
> I am having problems with realizing the FFT! The FFT should when p =1 look
> like the signal in.

Where is signal in? or in where?

It doesn't Any suggestions?! Thanks...
>
>
> The following is the m-code:
>
> function s = dtFMchirp(p,TW)
>
> % DCHIRP generate a sampled chirp signal
> % usage s = dchirp(TW,p)
> % s : samples of a digital "chirp" signal
> % 	exp(j(W/T)pi*t^2)  -T/2 <= t < T/2
> % TW : time-bandwidth product
> % p : sample at p times the Nyquist rate (W)
> %p=10;
> %TW=50;
> %N = p*TW;
> %n = 0:N;
> %alpha = 1/(2*p*p*TW);
> %s = exp(j*2*pi*alpha*((n-.5*N).^2));
> %x=real(s);
> %plot(x);
> %length1=length(x);
> %figure;
> %fftx=fft(x,length1);
> %fftx=abs(fftx);
> %plot(fftx);
> close all;
> clear;
> p=1;
> TW=50;
> N = p*TW;
> n = 0:N;
> alpha = 1/(2*p*p*TW);
> s = exp(j*2*pi*alpha*((n-.5*N).^2));
> figure(1);
> x=real(s);
> plot(x);
> length1=length(x);
>
> fftx=fft(x,length1);
> k=round(length1/2);
> fftx=[fftx(k:end) fftx(1:k)];
> fftx=abs(fftx);
> fftx=abs(20*log10(fftx));
> x=-2:4/length1:2;
> figure(3)
> plot(fftx);

The signal is the real part of the following:
>> p=1;
>> TW=50;
>> N = p*TW;
>> n = 0:N;
>> alpha = 1/(2*p*p*TW);
>> s = exp(j*2*pi*alpha*((n-.5*N).^2));


it basically is creating a chirp. 

When p=1 the FFT should look like the actual signal. Now this line

>> fftx=[fftx(k:end) fftx(1:k)];
is turning the middle to the far left and the middle to the far right and

adding them together to create a function of what it is supposed to look
like.

I'm trying to make it look like this website:
http://cnx.org/content/m11718/latest/

My chrip function very similar but the FFT comes up strange! :(


----------------------old crap of the post--------

>
>kyletae wrote:
>> I am doing an FFT of a chirp function.
>>
>> I am having problems with realizing the FFT! The FFT should when p =1
look
>> like the signal in.
>
>Where is signal in? or in where?
>
>It doesn't Any suggestions?! Thanks...
>>
>>
>> The following is the m-code:
>>
>> function s = dtFMchirp(p,TW)
>>
>> % DCHIRP generate a sampled chirp signal
>> % usage s = dchirp(TW,p)
>> % s : samples of a digital "chirp" signal
>> % 	exp(j(W/T)pi*t^2)  -T/2 <= t < T/2
>> % TW : time-bandwidth product
>> % p : sample at p times the Nyquist rate (W)
>> %p=10;
>> %TW=50;
>> %N = p*TW;
>> %n = 0:N;
>> %alpha = 1/(2*p*p*TW);
>> %s = exp(j*2*pi*alpha*((n-.5*N).^2));
>> %x=real(s);
>> %plot(x);
>> %length1=length(x);
>> %figure;
>> %fftx=fft(x,length1);
>> %fftx=abs(fftx);
>> %plot(fftx);
>> close all;
>> clear;
>> p=1;
>> TW=50;
>> N = p*TW;
>> n = 0:N;
>> alpha = 1/(2*p*p*TW);
>> s = exp(j*2*pi*alpha*((n-.5*N).^2));
>> figure(1);
>> x=real(s);
>> plot(x);
>> length1=length(x);
>>
>> fftx=fft(x,length1);
>> k=round(length1/2);
>> fftx=[fftx(k:end) fftx(1:k)];
>> fftx=abs(fftx);
>> fftx=abs(20*log10(fftx));
>> x=-2:4/length1:2;
>> figure(3)
>> plot(fftx);
>
>