how to generate a broadband signal for a given bandwidth in Matlab ?

[rakeshKM]

well ,I am about to design a broadband beam former. so I am curious,is it possible to create broadband signal of continuous frequency for a particular bandwidth. If no ,what is the best way to do so ?

 

[Mityan]

If there should not be any signal parameters manipulaton, just continuous wave, then use chirp - linear FM.

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If there should not be any signal parameters manipulaton, just continuous wave, then use chirp - linear FM.

 

[rakeshKM]

well i tried this:

i need a broad band signal of band 500-1000 Hz


what i did is ...wrote code for above
and taken the idft....
and finally find fft to check ...but i am getting this


my code is :
clc;
close all;
clear all;
t=0:0.5:1000;
for i=1:length(t)
if i>=500 & i<=1000
t(i)=1;
else t(i)=0;
end
end
f=t;
plot(f)
axis([0 2000 0 2]);
[x] = idft(f,2001);
figure
plot(0:0.5:1000,x)

figure
[Xk] = dft(x,2001)
plot(0:0.5:1000,Xk)


what is going wrong in this code ...is this approach correct

 

[Mityan]

1. Nobody uses cycle to generate such vector.
t = [zeros(500,1); ones(500,1)];

2. I dont know where you got 'dft' function, in MATLAB there is 'fft' (and 'ifft').

3. About LFM.
For example take sample rate 4 kHz.
Assume any frequency in band 500-1000 Hz.
Taking 4 samples for 1 ms you may get the resolution of your spectra 1 kHz. (because 4 khz fs represent signal in band 2 kHz, 4 samples 2 for spectral components 0-1khz and 1-2 khz and the rest two are the same reflected). - or just 1 bin in band 0-1000.
Taking 8 samples (for 2 ms) you may improve spectrum resolution twice - with 500 Hz step.
Taking 16 samples (for 4 ms) you may improve spectrum resolution twice - with 250 Hz step. So you may detect what region your initial sime wave belong to - 500-750 or 750-1000 hz.
But you want to have 500 hz wideband signal.
So for the period of 2 ms your wave should change its frequency for that resolution - 250 Hz. In this case you will still observe the signal in band 500-1000 but not 500-750 or 750-1000 hz.
So the rate of your linear FM should be 250 Hz for 2 ms or 500 Hz per 4 ms. In band from 500 to 1000 Hz.
Something like that I think.
How to generate by means of matlab discover by yourself.
For generating this with the given sample rate 4 kHz each next sample must be of the frequency increased by (500/0.004/4000=31.25 hz) related to previous. Cyclic in band 500-1000. Clear?

 

[newtonoid]

Is there a reason you can't generate it in simulink? It may help you visualize the process before coding it in matlab.

 

[rakeshKM]

thank u for ur response


i finally did it with the help of Fm modulation...i think LFm will do also fine

here is what i have done have
T=2e-6;
Fs=440e6;
Fc=120e6;
B=200e6;
ft=0:1/Fs:T-1/Fs;
N=length(ft);
k=(Fc/Fs)*2*pi/max(ft);
y=modulate(ft,Fc,Fs,'fm',k); %%band pass
y_fft=fft;
y1=y_fft %(1:N/2);
figure;
subplot(2,1,1);
plot(ft,y);
xlabel('t');
ylabel('y');
title('modulated output signal(t)');
subplot(2,1,2);
plot((0:Fs/N:Fs-Fs/N),abs(y_fft));
xlabel('frequency(Hz)');
title('frequency respone');
axis([-50e07 50e+07 0 50])

x=ifft(y1); %%bandpass final

figure
plot(ft,x)

j=sqrt(-1);
x1=x'*exp(-j*2*pi*120e6*ft); %% lowpass
figure
plot((-(Fs-Fs/N)/2:Fs/NFs-Fs/N)/2),abs(fft(x1)))