Hi Zonn,

At 07:29 AM 3/12/99 GMT, you wrote:
>You'll find no patterns in the thermo noise of a diode. Unless something's
wrong
>with your circuit, like outputting all ones?

Well,I may not have been clear about what I meant by pattern: Take for
example, the white noise produced across a resistor due to thermal effects.
It has a Gaussian distribution. This means that values near zero are more
likely than say, 1 volt,across the resistor! This is an extreme example,
but it also means that values or around 1uV are more likely than 5uV,etc.
So,if you were to make a one time pade which was,say,the number of
microvolts times 16,most of he numbers in your one time pad would be in the
range 0 to 16 * sigma, where sigma was sqrt(4*k*T*R*B),which might be say
4uV. So, you would get almost NO values greater than 64. Upon thinking
about it more,I must admit that this probably could be minimized by
increasing the gain,so that our range of values was well within the sigma
range. Still,I would think it better to apply a little algorithm to the
data to make higher values more likely,so that ridiculously high gain
wasn't needed.

I'm not saying that a diode is unsuitable, just warning that someone should
examine the distribution of their output to make sure that it is reasonably
random and uniform (not the same thing,since not all random processes show
no law-LIKE behavior,in this case,procuding lower voltages more than higher
ones).

>
>When I've needed truly random patterns in the past, I've used a 16 bit sound
>card and sampled the hiss between FM radio stations, and then concatenated
the
>lowest order bit of the 16 bit words, into a string of bytes.
>

Well, I think if you were to actually look at the whole 16 bits,you'd find
that very few times were the highest bits set(unless you had the volume up
very high or extra amplification),since the noise from the radio is almost
never putting 5V (or whatever was your Vref for the ADC) across the
speaker. You actually did something to fix the situation,by only using the
lowest bits,which were very close to the zero point on the Gaussian "bell",
therefore very close to a uniform (abs. level) distribution because the
slope of the Gaussian is very small near zero.

>The lower the sample rate, the better the guarantee the sampled noise will be
>truly random.  At too high a sample rate you run into the frequency
response of
>the radio, at which point consecutive samples may return the same value.
>
>A 1khz sample rate works fine.

That makes sense to me.

>
>-Zonn
>

Sean

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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