This work describes development, analysis, implementation and application of a new type of the time recursive FLL (frequency locked loop). This FLL of the second order is based on the measurement and processing of the input and output periods. Although this FLL can be used for different applications, the main emphasis in the paper is given to its tracking performances. Mathematical description and stability condition analyzes of FLL are performed using Z transform. The region of the parameters, which provides the stability of FLL and suits to the specific applications, are analyzed. Computer simulations of FLL functioning, enables better insight into the FLL properties and confirms the mathematical analyses too. The hardware realization of FLL is described.

Keywords: Phase locked loop, Frequency locked loops, Tracking, Digital circuits