Abstract: In this paper some practical problems occuring in the design of a phase-locked sub-carrier frequency discriminator are discussed. These problems include a) the results ob-tained from the analysis of the "loss-lock" threshold of the phase-locked loop by themethod of statistical linearization, b) static and dynamic errors of the phase-locked dis-criminator, c) the effect of carrier suppression filter upon the closed loop response.Finally the feasibility in using phase-locked loop in telemetry as a frequency discriminatoris also discussed.