[time-nuts] Anybody want a Thunderbolt power supply
csteinmetz at yandex.com
Thu Sep 1 03:14:34 EDT 2016
> It will show up if the value of the voltage setting bypass capacitor is reduced. Both the Johnson and excess noise in this resistor coupled with the current source flicker noise should become evident. The datasheet graphs indicate that the Johnson noise of the voltage setting resistor(33k2 ~22nV/rtHz) isn't significant in comparison with other sources.
I have found that the voltage setting bypass capacitor can be profitably
increased to ~22uF, beyond which there is only a very small gain below
1Hz. I would use 22uF unless that delayed reaching full voltage for too
long at start-up. (With 22uF, it takes ~15 seconds to reach the 3042's
maximum output voltage of 15v, correspondingly less for lower output
With 22uF, the output noise density is ~20nV/sqrtHz at 10Hz and
~80nV/sqrtHz at 1Hz. I believe most of the remaining noise (at least
above 0.1Hz) is being generated in the unity-gain amplifier -- the
reactance of the 22uF capacitor is only ~700 ohms at 10Hz.
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