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Amorphous Nb-Si has been previously demonstrated as a Josephson junction barrier material for Nb-based superconducting voltage standard circuits, including both DC programmable and AC Josephson voltage standards operating at frequencies up to 20 GHz. This material was chosen so that the junctions could be fabricated in vertical stacks, increasing the number of junctions in an array, which in turn increases the output voltage of the circuits. This barrier material may also be used to create higher-speed junctions, because the same factors that lead to improved stacks also lead to more reproducible junctions with thin, insulating barriers. Recently, a collaboration between the Physikalisch-Technische Bundesanstalt (PTB) and the National Institute of Standards and Technology (NIST) produced 1 V and 10 V programmable Josephson voltage standard chips operating at 75 GHz that use these junctions. In this paper, we demonstrate junctions with characteristic frequencies approaching 500 GHz and observed Josephson phase locking at frequencies of 400 GHz and 800 GHz. These junctions are promising for applications in high-speed superconducting digital electronics.