Twenty precise compact DC voltages sources based on 6.4-V Zener diodes have been manufactured and investigated by the NPO VNIIFTRI measuring system during the year. Eight sources with voltage drift of less than 8 ppm/year and different drift signs as well as with noise levels lower than 100 nV in the 0.01-10 Hz frequency band have been selected. These sources were used for manufacturing two four-channel reference standards. One standard has been investigated by the Russian 1-V Josephson systems, and another one was examined at the NPO VNIIFTRI. The output voltage drift of the standard sources both under laboratory conditions and during transportation exhibits a linear time dependence. The spread of experimental points with respect to a linear extrapolation did not exceed 2/spl middot/10/sup -7/. The drift value of the best channel was less 0.5 ppm/year.
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