  Phasor Measurement Units (PMUs) are measurement devices long used in
transmission systems and today even more essential for a proper monitoring of
distribution grids. The expected massive penetration of distributed energy
resources (DERs) is slowly taking place, carrying along a new set of challenges
that put to test traditional instruments and requiring more performance and
flexibility to adapt to this evolving scenario. Cheap devices based on single
board computer (SBC) are proving to be a valid alternative to traditional PMU
architectures, able to combine together high-performance, great versatility and
low-cost. However, such devices lack a proper modeling of their measurement
errors that conversely would be extremely useful for improving their design and
evaluate their performance in accordance with the relevant standards. The paper
intends to fill this gap by discussing the error sources and their effects on
the observed signals. An analysis of error statistics is presented, in order to
give a more complete metrological characterization.
