The FX105A is a monolithic CMOS tone operated switch, designed for tone decoding in single and multitone signalling systems. The FX105A uses decoding techniques which allow a tone to be recognised in the presence of high noise levels or strong adjacent signals. Detection centre frequency and bandwidth can each be independently adjusted. The design is immune to high levels of harmonic and sub-harmonic interference. Excellent noise immunity and constant bandwidth are maintained over a wide range of input signal levels.