Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

406 FREQUENCY MODULATION, Volume Imission of sound and facsimile. In this case a 4000-cycle tone wasused for the audio frequency and a 17,000-cycle tone for the facsimilefrequency.Simultaneous application of two tones to a system having any nonlinearityresults in combination tones that are the sum and differenceof multiples of the individual tones.For example, if two tones havingfrequencies a and 6 are applied to a non-linear element, the result willbe frequencies 2a, 26, 3a, 3b, 2a — b, 2a + b, 3a — b, 3a + b, 26 — a,2b + a, 36 — a, 36 + a, etc. In general, in a non-linear system the coefficientsof the higher-order terms are much smaller than those of thelower-order terms so that the greatest amplitude of resultant combinationtones is likely to occur with a high audio frequency and a lowfacsimile frequency.However, audio frequencies much above 4000 or5000 cycles seldom have sufficient energy, even when pre-emphasis isused, to modulate the transmitter fully. Consequently interferencebetween sound and facsimile is most likely for audio frequencies in the2000 to 5000-cycle range.The test was performed by modulating a laboratory frequencymodulationsignal generator by 4000 cycles and 17,000 cycles simultaneouslyand measuring the resultant frequencies in a dummy loadreplacing the voice coil in the output of the receiver by means of awave analyzer. Throughout the series of tests the input to the receiverwas 50 microvolts and the output was held constant by means of thevolume control at 0.5 watt. In all cases, except where otherwise noted,tuning was performed by applying approximately 150-kc deviation andthe receiver tuned to symmetrical distortion of the top and bottom ofthe wave as observed on an output oscilloscope.As an initial test, the 4000-cycle tone alone was applied and theharmonics read as the deviation was varied.4000-cycleTable ImodulationDeviation