Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

246 FREQUENCY MODULATION, Volume Ithat many frequency-modulation transmitters will be located in closeproximity to each other near the center of population of the area.Withtransmitters so located, high field intensities will occur at many receivinglocations, a condition which favors generation of spurious responses.Types op Spurious ResponsesSpurious responses may be of several types, some of which areunique to superheterodyne receivers, others exist in any type of receiver.The types of spurious responses which may exist in frequencymodulationreceivers are 1. image response, 2. direct intermediatefrequencyresponse, 3. response from two stations separated by theintermediate frequency, 4. combination of signal and oscillator harmonics,5. half-intermediate-frequency image response, 6. cross modulation,and 7.intermediate-frequency harmonic response.Cross modulation, whether within the receiver or of the externaltype, does not depend upon the type of receiver. The other forms ofspurious responses occur only in superheterodynes. Therefore thechoice of intermediate frequency does not influence this category ofspurious response.Image ResponseImage response in frequency-modulation receivers is similar innature to that in amplitude-modulation receivers. Since the band extentis only 8 megacycles (42- to 43-megacycle educational and 43- to 50-megacycle commercial broadcasting)and intermediate-frequency over4 megacycles will prevent image response from frequency-modulationstations. Consideration must also be given to image response fromother radio services, but except for television transmitters and possiblyamateur and police transmitters, the location will make image responseunlikely.Furthermore, the greater frequency separation of such transmissionswill aid the radio-frequency attenuation of the image-frequencyresponse.Direct Intermediate-Frequency ResponseTransmission of signals having the same frequency as the intermediatefrequency through the radio-frequency system isto be a serious factor in frequency-modulation receivers.not believedVirtually anyintermediate frequency chosen will be so far different from the radiofrequencytune frequencies, that attenuation at the first-detector inputwill be considerably better than in the case of the 455 kilocycles on thestandard broadcast band. Nevertheless, all other things being equal,