Problems with the creation of a randomic signal

Hello everyone, i'm having a problem in my code, trying to create a random signal with monolateral bandwidth = 5 kHz, I wrote these lines:

fs=200000; %sampling frequency 
B = 5000;  %bandwidth
rng(1); 
x = rand(1,n);         
x = lowpass(x,B,fs);

The problem is that the signal thus obtained has a monolateral bandwidth = 20 kHz. What am i doing wrong? Anyone to suggest me what i should correct?

Below the signal i get with my lines, my goal is to get a signal which, zooming in the image, like i did in the left picture, has a bandwidth = 0.5. I really hope someone can help me out. Thanks in advance.

Answer 1

[Paul]

Hi Andrea,

How are you defining the bandwdith?

fs=200000; %sampling frequency 
B = 5000;  %bandwidth
rng(1);
n = 1e4;  % not supplied in the Question
x = rand(1,n);         

lowpass can return the digital filter used to process the signal

[xf,d] = lowpass(x,B,fs);
figure
[h,freq]=freqz(d,4096,fs);
plot(freq,abs(h)),grid
xline(5e3)

Is that not the desired amplitude response of the filter? It seems to match up well with your plot.

[Andrea]

Thank you, I just needed to add a steepness parameter to my bandpass function.