Frequency estimator which based on two-order adaptive notch filter is the one frequently used for sinusoid on engineering

with a precondition that there is a small frequency bias of reference signal to input signal.With frequency bias increasing

estimated bias and variance of adaptive frequency estimator (AFE) enlarge.In this paper properties of AFE and influences of frequency bias are analyzed

and formula on estimated variance lower bound is derived

and condition which is required by the bound is got.Frequency auto-tracking adaptive frequency estimator (FATAFE) is brought forward.Simulation and theoretical results show that under the same conditions FATAFE decreases obviously estimated bias and variance relative to AFE

and under medium signal-to-noise ratio condition estimated variance of FATAFE can be declined to one percent of that of AFE.FATAFE realizes unbiased estimation and its estimated variance approaches to theoretical lower bound if only signal-to-noise ratio is not very low.