The classical radio technique used for FM detection, the frequency discriminator can also be used in optical frequency detection. In this sense, Chanin et al  proposed a lidar system that measures atmospheric wind fields by detecting Doppler-shift in the return signal in a differential way by using two Fabry-Perot interferometers or any other high resolution optical filters as frequency discriminators. This technique was also studied and used by Korb et al. They named it "edge-technique." The UPC lidar group is developing a wind lidar based on the "edge-technique." The first prototype of the wind lidar is a continuous-wave system that is able to measure the surface displacement velocity of solid targets. The transmitting laser is the seeder of a Spectra Physics GCR-190 laser, which will be used for the final wind measurements. It includes a Fairy-Perot interferometer, two APD-based optical receivers, and several auxiliary optics, electro-optic and electronic elements. Among them, there is a servo-loop, based on two acousto-optic frequency-shifters and a lock-in amplifier, which is responsible of the proper tuning between the laser and the Fabry-Perot interferometer. To our knowledge, this servo-loop has not been used before for wind lidars based on the edge-technique. The aim of this first prototype is to test the performance of the edge-technique to measure velocities and to assess the role of the servo-loop in the precision of the measurements. The study and design of the prototype, with emphasis in the servo-loop will be presented.