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expected spectrum of aliased BW limited white noise

Started by nombwa November 12, 2007
I'm fairly new to DSP and am trying to see the effect of undersampling on
bandwidth limited white noise in Simulink.

I took the bandwidth limited white noise source (set the BW to 1kHz), put
it through a zero order hold with Sampling rate set to initally 5kHz and
the output of this is connected to the FFT spectrum scope block (I buffer
2048 points and take a FFT of the same length).  

When I change the sampling rate of the zero order hold block to 1kHz and
lower, I expected the aliased signal to have a higher magnitude than the
unaliased spectra.  But the undersampled signal shows a flat spectrum with
a magnitude that doesn't change regardless of how low the sampling
frequency gets (1kHz, 500Hz, 250Hz). 

Can anyone tell me what I'm doing wrong?  I expected the spectra to add up
as the root sum square 
Thanks!


nombwa wrote:
> I'm fairly new to DSP and am trying to see the effect of undersampling on > bandwidth limited white noise in Simulink. > > I took the bandwidth limited white noise source (set the BW to 1kHz), put > it through a zero order hold with Sampling rate set to initally 5kHz and > the output of this is connected to the FFT spectrum scope block (I buffer > 2048 points and take a FFT of the same length). > > When I change the sampling rate of the zero order hold block to 1kHz and > lower, I expected the aliased signal to have a higher magnitude than the > unaliased spectra. But the undersampled signal shows a flat spectrum with > a magnitude that doesn't change regardless of how low the sampling > frequency gets (1kHz, 500Hz, 250Hz). > > Can anyone tell me what I'm doing wrong? I expected the spectra to add up > as the root sum square
Nothing. Aliases are parts of the original signal shifted and possibly inverted in frequency. A flat spectrum part makes a flat alias. Why do two (or more) flat noise spectra remain flat when added, while two (or more) noise signals (usually) approach a Gaussian distribution when added? That's left as an exercise. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������