Towards the propagation of AC quantum voltage standards, the EMPIR ACQ-PRO ProjectRaúl Caballero, Javier Díaz de Aguilar, Ralf Behr, Jonathan Williams, Andrea Sosso, Martin Šíra and Yolanda A. SanmamedInternational Congress of Metrology, (2017) 07001DOI: https://doi.org/10.1051/metrology/201707001