[Cryptography] A TRNG review per day: Turbid

Clemens Ladisch clemens at ladisch.de
Wed Oct 29 08:55:02 EDT 2014

Bill Cox wrote:
> On Tue, Oct 28, 2014 at 5:28 PM, Clemens Ladisch <clemens at ladisch.de> wrote:
> > A typical ADC chip has a delta-sigma modulator running at about 6 MHz,
> > which requires an external analog filter that reduces noise at that
> > frequency.  The modulator is followed by a digital decimation filter
> > that goes very near the Nyquist frequency of the currently used sample
> > rate.  (There also is a high-pass filter to remove any DC offset from
> > the input.)
> [...] In that case it's the decimation filter cut-off that counts.
> That's typically 2X the highest audio frequency of interest, isn't it?

Yes, if with "2X" you mean what I'd call "half".  For example, the
CS5381 datasheet says:
  Passband (-0.1 dB): from 0 Fs to 0.47 Fs
  Stopband (-95 dB): from 0.58 Fs

This filter and the HPF will attenuate extremly high and low frequencies;
what remains is usable white noise over almost the entire frequency

> There will still be significant correlation between samples.  There is
> thermal noise in a band from 9X to 10X below the sample rate

Why are you singling out this band?

> which will turn into a significant short-term correlation between
> samples 10 away from each other.

The noise in all the other bands will cancel out these correlations.


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