🚀 go-pugleaf

rja14@cl.cam.ac.uk (Ross Anderson) writes:
> ericb@hplsla.hp.com (Eric Backus) writes: 
> > Fundamentally, I think this idea works.  I have built a zener-diode
> > noise source similar to this, that I then xor'ed into a hardware
> > feedback-shift-register PRN generator.  The noise coming out of this
> > appears to be quite random.
> 
> You have to be a bit careful here. The bitstream from the feedback shift
> register will look random even if there is not a lot of noise coming out
> of the zener, or if the entropy of the noise source is being killed by a
> circuit resonance somewhere.
>
> Ross Anderson

Agreed.  The analog noise source must first be verified before hooking
it to the digital noise source.  One thing to verify is that the
high-frequency cutoff (whether due to an explicit filter or due to
bandwidth limit of your op-amp) is much higher than the frequency at
which you will sample the noise source.  Another thing to verify is
that the frequency spectrum of the noise output doesn't contain any
suspicious peaks, especially at frequencies related to the frequency
at which you will sample the noise source.
----------
					Eric Backus
					ericb@hplsla.hp.com