Reduction of Adjacent-Channel Interference Components from Frequency-Shift-Keyed Carriers

The abrupt changes in frequency associated with binary modulation generate sideband components which frequently interfere with services in adjacent channels. Reductions in interfering bandwidths of 10 to 1 or greater over unfiltered keying at sideband levels 80 db or more below the unmodulated carrier are possible by the use of relatively simple filters in the keying circuit of frequency-shift-keyed transmitters. The frequency spectra resulting from filtered keying waves of FSK transmitters are derived, and the results presented in graphic form. The sizeable reduction of the interfering sideband components as predicted by the analysis is found to agree very closely with experimental measurements. Characteristics of desirable keying circuit filters to obtain these reductions are specified.