I have always wanted one of these Keithley Nanovolt Sources, but financially they have always been out of reach, until one turned up on our favorite auction site.
While the outside leaves much to be desired , mainly the extremely yellow knobs. The rest of the outside case was actually in decent condition. There were remnants of calibration void stickers that have long since been removed.
Sourcing voltages down to 1nV requires much attention to detail for shielding ,thermocouples, and avoiding thermal related drafts(creates thermal emf's). Therefore the output terminals are housed in a shielded enclosure.
Sourcing voltage to low levels requires Low thermal emf binding posts , and if Low Johnson noise is required a Low source resistance is needed as well. With Source resistances ranging from 100-Ohms (high range) , 1-Ohm(uv/nv range). For those who noticed the binding posts , yes , they are L&N ones.
Opening up the enclosure were are greeted with a small transformer , a circuit board , and a very large shielded enclosure.
Zooming into the circuit board reveals nothing special. A 1N706 acts as the voltage regulator for the power supply , while a 1N936 works as a voltage reference. The only interesting aspect to this board is that the it is a constant current output supply , as opposed to a constant voltage.
Removing the shield reveals the Current divider , polarity switch , and the shielded voltage dividers.
The divider circuit baffled me at first , but reading the manual helped me in that regard.
The ranges are 10,1,0.1 Ohm resistors. The 1/10 ohm resistors are in a series divider with 1-10 , and 10-100 ohm resistors respectively.. The 0.1 is by itself.
---Opening up the shielded divider will wait until after dinner...