Extension of FM/FM capabilities

An experimental investigation has been conducted by Sandia Corporation to determine the feasibility of using the higher frequency FM sub-carriers as carriers for a group of lower frequency FM subcarrier channels. The system tested was composed of eighteen channels being carried on three high-frequency subcarrier channels operated with a modulation index of one together with eight conventional FM/FM channels, or a total of twenty-six information channels per RF system. The maximum error due to nonlinearities in the transmission system was found to be ±1.5 per cent when a modulation index of two is used. The system tested employed only conventional FM/FM components. From this investigation it is indicated that at least forty information channels per RF system may be employed using standard RDB subcarrier frequencies and that the response of information channels may easily be doubled over their normal ratings if extreme accuracy is not required.