The use of falling weight deflectometers and portable falling weight deflectometers (PFWDs) is now common for field characterization of pavement system layers. In particular, the application of portable deflectometers in quality assurance of newly constructed granular base has become more widespread. Typically, these devices are used for an in situ assessment of the Young's modulus of the base layer. The traditional backcalculation uses an elastostatic half-space framework to relate Young's modulus of the pavement foundation to the stiffness estimates obtained from force and velocity measurements. The data interpretation method that is customarily used for stiffness estimation uses peak values of the force and displacement records in lieu of their static counterparts. The performance of a particular device, PRIMA 100, was examined with the newly developed beam verification tester (BVT) of known static stiffness. It was shown that the conventional, peak-based method of backanalysis produces incorrect estimates of the static stiffness of the BVT. An alternative, spectral-based data interpretation method was proposed. This method, based on (a) concept and measurement of the frequency response function and (b) a single-degree-of-freedom mechanical model, was employed to extract the true static stiffness from PRIMA 100 measurements. The results show a good agreement between the true static stiffness of the BVT and its PFWD estimates stemming from the modified approach. The BVT apparatus can therefore be used to assess the performance of the sensors and data interpretation of PRIMA 100-type deflectometers.