expected spectrum of aliased BW limited white noise

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.
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