Routine studies are performed on JET using a new set of antennas to excite Toroidal Alfven Eigenmodes (TAE). A TAE resonances footprint is observed in the plasma response measurement when there is a noticeable variation in both the amplitude and the phase of the response with respect to the excitation. An algorithm for real-time identification of TAE resonances, based on a hardware lock-in amplifier, is presently used at JET for detecting such variations. In the paper, we revisit the problem of estimating the I-Q characteristics from a known non-stationary frequency mode, with a resonant-like phase response, embedded in a digital signal. A non-stationary linear model is used in a recursive filter implementation of a lock-in amplifier. We propose it as a viable alternative to hardware synchronous detectors such as the one in use at JET and compare its' performance with standard digital lock-in techniques in terms of bandwidth and phase response under high throughput rates requirements.