Instantaneous frequency estimation, a topic of fundamental importance in signal processing, has received considerable attention in the open literature. This paper presents a new instantaneous frequency estimator based on digital differentiation concepts. The proposed estimator, referred to as the GDF estimator, is closely related to, but computationally more efficient than the Kay estimator. A slightly modified version of the GDF estimator, referred to as the MGDF estimator, is especially attractive for software-based real time demodulation of binary FSK signals.