Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

54 FREQUENCY MODULATION, Volume IVIII.Generalized Frequency-Modulated-WaveReceiver ConsiderationsThe radio frequency, first detector and intermediate frequencycircuits of a frequency-modulated-wave receiver are identical basicallywith those in an amplitude-modulated-wave receiver. The only importantdifference is in the width of the pass band, which is generallyseveral times greater in the frequency-modulated-wave receiver.A detector of amplitude modulation, of and by itself, is immune tovariations in the frequency of the applied wave. There can be noexact counterpart in the form of a frequency-modulation detectorimmune to amplitude variations. Zero amplitude is bound to producezero output from any detector, so the output of all detectors mustbe some function of amplitude.For this reason the second detector of a frequency-modulated-wavereceiver is usually preceded by a limiter stage whose output is constantfor wide variations in the amplitude of the applied signal.However,this is necessary only because of the inherent limitations of the detectorand thus it might properly be considered as an integral part of thedetection circuit.Another characteristic of the usual frequency-modulation detectorstill further reduces its output response to amplitude variations. Thisis occasioned by the common practice of using so-called back-to-backor "push-pull" detectors. These are so arranged that a frequencydeviation in a given direction causes an increase in the current orvoltage output of one, while simultaneously causing a decrease inoutput of the other. Both detectors, however, respond similarly tochanges in amplitude of the applied wave. Thus with the outputcircuits of the two connected in series opposition, amplitude variationscan act only to increase or decrease the "sensitivity"theof the combinationto frequency modulation, and cannot produce output in theabsence of frequency deviations.Frequency-Modulation BibliographyNotes on the Theory of Modulation—J. R. Carson. Proc. I.R.E., February1922, p. 57.The Reduction of Atmospheric Disturbances—J. R. Carson. Proc.I.R.E., July 1928, p. 967.Uber Frequenzmodulation—H. Roder. Telefunken-Zeitung, No. 53,1929, p. 48.Frequency Modulation—J. Harmon. Wireless World, January 22, 1930,p. 89Ȧ Study of the Frequency-Modulation Problem—A. Heilman. E.N.T.(in German), June 1930, p. 217.Frequency Modulation—B. Van der Pol. Proc. I.R.E.,. July 1930, p. 1104.