[sdiy] Vote for your favorite noise source

[Tim Ressel]

Wow,

Thanks for all the responses. I knew if I asked, I'd
get alot of noise back...

Something about the shift-register method you may not
know: they don't produce all frequencies! Instead, you
get a picket-fench effect. One day I was playing
around with a new RF spectrum analyzer we just
released. I wanted an interesting source, so I built
an 8-stage shift register noise source. Much to my
surprise instead of the flat line I expected
(ageraging and peak hold turned on), I saw a picket
fence. I got 255 frequency lines between 0hz and the
clock frequency. Of course, with a large chain like 32
registers, the lines are so close together, ya ain't
gonna notice it. 
One nice thing about the shift register method is: its
easy to get low freqency noise. Just slow down the
clock. The problem with filtering wideband noise is
you are selecting a small band by filtering, and
therefore getting a small portion of the energy ergo
low amplitude. 

One topic I'm surprised didn't come up was
distribution. When evaluating noise sources for signal
processing, the distribution is important. It can be
seen like this: picture doing a histogram of the
amplitudes of the noise. That histrgram shows the
distribution. Uniform distribution will have a
histogram that ramps linearly to zero. Gaussian
distribution will have what else? a Gaussian curve. 

I saw flash by a notion of of using resistor banks on
the shift register output to for an FIR filter.
Interesting idea. Has anyone done an inverse-FFT of
the pink curve yet? This will yield the weights of the
FIR filter. Query: the -3dB slope of the pinking
filter, is that per octave of per decade? And does
-3dB refer to voltage or power? I assume voltage.

--TR

************************************
TR has a cold today. No warranty on
neural activity expressed or implied
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