[time-nuts] Anybody want a Thunderbolt power supply

Charles Steinmetz csteinmetz at yandex.com
Thu Sep 1 03:14:34 EDT 2016

Bruce wrote:

> 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.

Best regards,


More information about the time-nuts mailing list