Answer 2—The voltage-to-frequency relationship can be turned around. An applied AC current frequency (f) generates a DC voltage (V_n) at the quantized values: V_n = nhf/2e, where n is an integer, and the value of 2e/h is approximately 483.6 MHz/µV. It turns out that this value is entirely independent of the materials used to construct the junction, so it can function as a frequency-to-voltage converter suitable for use as a primary reference. In other words, if you can produce frequencies of a known accuracy, you can also produce voltages with essentially the same accuracy.

The NIST first adopted the Josephson junction as the voltage standard in 1972. In 1990, the Josephson constant (K_J-90 = 483597.9 GHz/V) was adopted internationally. To produce a usable voltage, an array of a few thousand junctions is patterned on the surface of a chip and then irradiated with microwave energy at a frequency of tens of gigahertz.

Contributor: David Tweed