Had a very interesting talk with PTB about a project for a new secondary 10V standard they are currently working on.
As there is the permission to talk about it, I can give some details - even I do not really understand this sophisticated stuff in detail.
This project is aimed to fill the gap between todays secondary voltage standards and JJA.
The key specs are <0.14ppm/year drift, TC <0.014 ppm/K and LF noise <0.041 ppmpp (0.014-14Hz).
It is based on a standard voltage reference in conjunction with a sophisticated PLL.
This allows to calibrate/adjust the voltage with a frequency standard e.g. GPS disciplined oscillator - so this can be made on site without need for cal-lab.
The PLL consists of a OCXO as frequency reference and a voltage controlled oscillator.
The voltage is controlled by a very precision combined DAC consisting of a very precision DAC for the MSBs and an overlapping precision DAC for LSBs which is adjusted to 10V (+-0.14ppm) and maintains this value due to the PLL and the OCXO.
The standard voltage reference is boosted with the DACs to 10V (DAC output is connected to voltage reference) - they act as the voltage divider made of precision resistors in classic 10V standards.
Edit 2.4.: corrected all specs