Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

INTERMEDIATE-FREQUENCY VALUES FORFREQUENCY-MODULATED-WAVERECEIVERS*!ByDudley E. Foster and John A. RankinRCA License Laboratory, New York, N. Y.Summary— The selection of an intermediate frequency for a superheterodynereceiver involves consideration of the signal frequency, thetuning range, the pass-band width, the minimizing of spurious responses,regeneration stability and frequency stability. The consideration of thefrequency-modulation broadcast band of i2 to 50 megacycles illustrates themanner in which these several factors affect the choice of intermediatefrequency.Likelihood of Spurious Responses in Frequency-Modulation ReceiversFREQUENCY-MODULATION receivers are subject to spuriousresponses as are amplitude-modulation receivers, and because ofthe probable frequency-modulation-transmitter locations, thespurious responses are likely to be worse than on the present broadcastband. The superheterodyne type of receiver is used almost universallytoday because of its great advantage in sensitivity and selectivity, butit is subject to spurious responses. The selectivity requirements in the42- to 50-megacycle frequency-modulation band are so severe that theuse of the superheterodyne is even more necessary there than in thestandard broadcast band.Stations may be assigned to alternate channels in the frequencymodulationband in a given locality. Since the channels are 200 kilocycleswide this permits assignments 400 kilocycles apart. At the meanfrequency-modulation band frequency of 46 megacycles a separationof 400 kilocycles is the same per cent separation as 10 kilocycles at1150 kilocycles so that selectivity becomes a prime consideration.Since the service area of a frequency-modulation transmitter isessentially limited by the horizon, transmitter antennas will have asgreat a height as possible.In large cities this means that the tendencywill be to place the transmitter antenna on the tallest building available.Since tall buildings are generally in the center of the city, it is likely* Decimal classification: R630.1 X R361.111.t Reprinted from Proc. I.R.E., October, 1941.245