Several different modulation schemes are employed in reflectometer systems to determine the group delay time required for density profile measurements (Laviron et al, 1996). The JET multi-channel reflectometer uses slow, narrow-band sweeps of the microwave frequency in each of the channels (Sips and Kramer, 1993). 10 channels of the original system are currently in operation, spanning the range from 18 to 70GHz, in the 0- mode. Three problems are associated with this approach: First, the frequency sweep is slow (6ms) in comparison to the time-scale of most plasma fluctuations, whether turbulent or coherent. Second, the corresponding phase change is measured by fringe counters which rely on the signals being continuously present during the sweep. Third, the exact frequency excursion depends on the characteristics of the microwave sources. These circumstances result in frequent loss of profile information, or in relatively large errors in the position of the density profiles, as well as errors in their shape, though the latter are partially corrected by making use of the phase evolution during the fixed­frequency intervals between sweeps. These limitations are significant in measurements of edge density gradients and densities, both important in the study of high-confinement (H-mode) plasmas in JET (Deliyanakis et al, 1994). On the other hand, the present system can perform simultaneous measurements of density fluctuations at 10 radial positions; such measurements were crucial in identifying the so-called outer modes, external kink modes limiting the performance of H-mode plasmas (Smeulders et al, 1995). The present system is also able to function in conjunction with the long waveguide runs needed at JET. To eliminate the problems discussed above and improve the general performance of this diagnostic, whilst retaining the advantageous aspects of the present system, a novel scheme relying on fast frequency modulation has been designed and is being implemented. This paper discusses the underlying theory and technical aspects of this novel system. These will be illustrated with examples of preliminary measurements.