[Cryptech Tech] Alpha Avalanche Noise Block

[Benedikt Stockebrand]

Hi folks,

Fredrik Thulin <fredrik at thulin.net> writes:

>> >> Speaking of voltage required for avalanche effect, you should consider
>> >> part-to-part parameter variation. Transistors are analog components, if
>> >> you buy ten BC818 transistors, they will all have different DC current
>> >> gain. Because of this, if you have 10 identical boards, they may require
>> >> slightly different voltages for avalanche effect.> 

that depends on the circuit.  The one Fredrik and I use needs some
minimal voltage for the avalanche effect to work, but beyond that the
only effects of voltage are output speed and power consumption.

The circuit we use adjusts the voltage at the avalanche diode by itself,
so tolerances on that component simply don't matter.

>> > They _will_ require.
>> > So production boards with noise source should have possibility to tune
>> > parameters for given source diode. We made that by putting set of
>> > resistors into design and getting necessary voltage by short cutting some
>> > of them.

Manually tuning these is in my opinion a bad idea, because it implies
monitoring and recalibrating them over time and such.  That's why the
circuit Fredrik and I have used so far takes care of it automatically.

>> Emphasizing: set of fixed precise resistors. Never use any kind of variable
>> ones. Never use 5-10% accuracy stock resistors

That's what you might do with high fidelity analog circuitry, but
generally you want to compensate for component variation in your circuit
design.

> I don't get this, but I think it is because the circuit you are referring to 
> worked differently than this.

Definitely looks like.

> In my tests, avalanche effects with BC-818-40 transistor starts around 9.5v.

That sounds reasonable.  In the test series I've done I found that you
need at least a 6.8V "Zener" (i.e. avalanche) diode to get any usable
effect; lower voltage diodes work mostly through the "true" Zener
effect, which needs much more amplification, while higher voltage ones
are simply slower.  Add to that the various junction voltage drops and
such and your measurement sound quite right to me.

One more thing about increasing the voltage: It's possible as long as
you don't exceed the limits of the first transistor, but power
consumption will go up by the square of the voltage applied if I
remember correctly.

> At 10v, it seems to work but is slow.

Yes, and if you assume 10% tolerances on components there's a chance
that you'll have a bad yield on production.

> At say 13v, I would be very surprised if it didn't work.

With the ARRGH board I first used 12V and then bumped up the voltage to
about 13.5V because it would simplify the bill of materials by making
one of the resistors used the same value as another one.

> At 15v it should *always* work (famous last words).

Definitely.  There were two reasons why I didn't bother to go there: I
wanted the power consumption to be well within limits of USB (100mV
without prior negotiation) and because the speed limiting component at
that point was the ATtiny2313 I used anyway.

> So why not just pick something high and be done with it?

I hope I've cleared up some more of the rationale behind the design we
use here.

> Why bother with tuning?

Tuning is a bad thing, because it provides a major opportunity for
mistakes.

> As I said, I think your circuit required that because it worked
> differently, but I don't think this circuit will require tuning. Let
> me know if I'm missing something =).

I don't know Pavel's circuit, but yes, it surely looks like that.


Cheers,

    Benedikt

-- 
Benedikt Stockebrand,                   Stepladder IT Training+Consulting
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