The linear stability properties of Alfvén modes are studied on JET using an active excitation technique [1,2]. The Saddle Coils drive low amplitude, | dB/B| 10-6, stable plasma modes with toroidal mode number n=0 ÷2. The diagnostic technique uses repetitive sweeps of the driving frequency in a pre-defined range, controlled in real-time. The plasma response is extracted from background noise using synchronous detection, and is used to identify in real-time the resonance corresponding to a global mode. When a resonance is found, the real-time controller locks to that frequency and tracks the mode. This provides a direct evaluation of the mode damping rate, g/w from the width of the frequency sweep. Two systems are used to measure fast fluctuation data. The KC1F system [3] is a 8-channel, 1MHz/4s continuous digitizer used to analyze magnetic and reflectometry data in the frequency range 5 f(kHz) 500. This system is particularly suitable to follow the time evolution of the instability. The CATS system [4] collects and digitizes a large number of channels generally using short time snapshots. This system is useful to determine the position of the instability using the cross-correlation between the magnetic and other radially localized measurements, such as soft X-rays, reflectometry or electron cyclotron emission.