This paper presents an original method to emulate a single fiber action potential in a quasi-infinite conductive volume, suitable for reproducible testing of bio-potential recording systems. The model is accurate for reproducing the bio-potential even for a small electrode to fiber radial distances. Established current activities of axon is used and programmed in the developed FPGA-based instrument, the model takes into consideration action potential propagation properties, electrode to fiber radial distances, medium conductivity. This paper investigates differences in the action potential amplitude for two longitudinal probe positions one in front of a node of Ranvier (NOR) and one between two NOR, for a large range of radial distances. Results are reported and compared with simulation with a correlation level of 97.6 %. The model is realistic enough to help the design of new recording systems.