Owing to the extension of electric distribution networks, monitoring and control issues rely on large scale distributed measurement systems, able to carry out simultaneous measurements of electrical quantities in several points on the monitored systems. In this paper the implementation of digital procedures, suitable for the evaluation of the synchronized harmonic phasors, in a flexible Phasor Measurement Unit (PMU) based on PXI modular hardware is presented. Some results of experimental tests are presented to characterize the measurement system in order to evaluate the behavior of the designed instrument in real operating conditions on three-phase electric distribution networks.