A filter assembly is provided comprising a first filter (12), a notch filter (26, 26′), and a phase-shifter (36). The first filter has a stop-band. The filter assembly is configured to, in use, split the output of the first filter into a main signal on a first path (18, 20, 22) and a secondary signal on a parallel path (26, 36, 38), and to pass the signal on the parallel path through the notch filter (26) having a stop-band corresponding to the pass-band of the first filter and, or including, through the phase-shifter (36) so as to be phase-shifted at least substantially into anti-phase to the main signal to provide an adjustment signal Φf(ω). The filter assembly is further configured to combine (22) the adjustment signal with the main signal so as to provide the main signal attenuated in the stop-band.