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Famed hardware hacker Bunnie Huang has posted an overview of his notes on designing an open source entropy generator. His summary links to the full notes which include schematics, measurement results, as well as other key details.
The final optimized design takes <1cm2 area and draws 520uA at 3.3V when active and 12uA in standby (mostly 1.8V LDO leakage for the output stage, included in the measurement but normally provided by the system), and it passes preliminary functional tests from 2.8-4.4V and 0-80C. The output levels target a 0-1V swing, meant to be sampled using an on-chip ADC from a companion MCU, but one could add a comparator and turn it into a digital-compatible bitstream I suppose. I opted to use an actual diode instead of a NPN B-E junction, because the noise quality is empirically better and anecdotes on the Internet claim the NPN B-E junctions fail over time when operated as noise sources. I'll probably go through another iteration of tweaking before final integration, but afaik this is the smallest, lowest power open-source avalanche noise generator to date (slightly smaller than this one [PDF]).
(Score: 1, Informative) by Anonymous Coward on Saturday April 20 2019, @02:30PM (1 child)
There is a concern: the external NOISE_ON signal is directly coupled onto the input of the amplifier. Any digital noise in it might exceed the noise of the diode.
Had the author looked for the switcher frequency in the spectrum of the noise? It might be strong enough, but still invisible on the scope.
The 5.1M divider is not practical due to leaks; you'd have to put a protective ground trace and bathe the board in conformal coating. Easier to add two more resistors and feed the divider with a lower, cleaned voltage. Myself, I'd use a voltage reference as the bias source, as that voltage has to be super clean.
The +1.8V rail must be regulated (linearly) within the device and feed only these parts. The needed regulator in a small SOT package will not hurt.
(Score: 2) by RS3 on Saturday April 20 2019, @04:13PM
If there is noise in the NOISE_ON signal, it can be simply filtered with another resistor and a capacitor.
If by "switcher frequency" you mean the power supply, I agree- it could be a significant factor, and needs to be housed in a metal can (shielded), maybe 2 stages (poles) of power supply filtering used (two resistors and two capacitors), and/or located far away, and good ground-plane PC board layout techniques used.