Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

254 FREQUENCY MODULATION, Volume IThe types of signals, other than frequency-modulation stations,likely to cause interference are those where the transmitter may belocated in a populous district. These are amateurs, television, andpolice signals mainly and possibly international broadcasting.If image response from other frequency-modulation stations is tobe eliminated an intermediate frequency of at least 4 megacycles hasbeen seen to be necessary. To minimize direct intermediate-frequencyinterference, a frequency range of some 200 kilocycles free from types112 4 a a /O 12 /* '& IS TO 22 *4MT£RM£D/ATE Fff£0(/£AICV W M£0/ICYCL£SPig. 4—Probability of spurious responses due to image, two signals separatedby the intermediate-frequency and harmonic combinations of signaland oscillator for different intermediate frequencies.Image weighting factor = 1.0.Two stations separated by the intermediate-frequency weightingfactor = 0.5.Spurious responses due to Nth harmonic of oscillator weightingfactor = 1/JV 2 .Probability of spurious responses for 1.0-megacycle intermediatefrequencytaken as 100.oscillator lower.oscillator higher.of services likely to cause such interference is required. The lowestsuch range above 4 megacycles is at 4.3 megacycles to which are allocatedgovernment, general communication, and coastal-harbor transmitters.Another intermediate frequency in that general range whichappears relatively free from direct intermediate-frequency interferenceis at 5.38 megacycles, where are allocated government, fixed, and generalcommunication channels.A frequency of 8.25 megacycles has been widely used as the inter-