  We propose to take advantage of the properties of long Josephson junctions to
realize a frequency variable Fabry-Perot Filter that operates in the range
100-500 GHz with a bandwidth below 1 GHz. In fact, we show that it is possible
to exploit the tunability of the effective impedance of the Josephson
component, that is controlled by a dc bias, to tune, up to $10\%$ of the
central frequency, the resonance of the system. An analysis of the linearized
system indicates the range of operation and the main characteristic parameters
Numerical simulations of the full nonlinear Josephson element confirm the
behavior expected from the linear approximation.
