Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

—GENERATION AND DETECTION OF FREQUENCY-MODULATED WAVES*tByStuart Wm. Seeley.J Charles N. Kimball, Allen A. BarcoRCA License Laboratory, New York, N. Y.Summary—This paper contains a description of a frequency detectorwhich is inherently linear in operation and, consequently, adaptable for precisionmonitoring measurements. In conjunction with a mixer and heterodyneoscillator, the detector is capable of operation at signal frequenciesencountered in the usual FM transmitter. The monitor detector is discussedin the first part of the article.The availability of a detector capable of demodulating frequency-modulatedwaves with the introduction of less than 0.1 per cent distortion makespossible the investigation of other low-distortion frequency-modulateddevices as, for example, a generator of phase or frequency-modulated waves.The second part of the paper contains a description of a method of producinglow-distortion frequency-modulated signals. The system was developed,and its loiv-distortion capabilities realized, with the aid of the aforementionedlinear detector circuit, in conjunction with an audio-frequency waveanalyzer.Part ILinear Frequency-Modulated Monitor DetectorPrinciple of OperationOPERATIONIntroductionof the detector unit may best be described with theaid of the basic circuit diagram of Figure 1. e in is, in practice,a frequency-modulated signal whose carrierfrequency lies .inthe range between 100 and 300 kc. The amplitude of e in is sufficientto overswing the limits of cut-off and zero bias of the 807 tube.Duringportions of the cycle when this tube is cut off, the plate potential risesto the -]-B level. During positive grid portions of any cycle the platecurrent rises to a certain maximum value, beyond which it does notincrease with further increase in positive grid swing.curves of beam pentode tube characteristics.)(See plate familyThus, the minimum valueas well as the maximum value of the instantaneous plate potential isconstant, and the output wave is squared off on top and bottom atdefinite fixed potentials, in spite of possible variations in amplitude ofgrid swing.With a square wave of plate voltage, having a repetition rate equalto that of the input signal e in , the action is as follows : when the platepotential of the 807 reaches its peak value (equal to E B ) the smallcondenser C (25 /xfif approx.) in Figure 1 has charged through dioded xto a potential equal to E B, with the diode end of C negative. At that* Decimal classification: R630.1.t Reprinted from RCA REVIEW, January, 1942.J Now Manager, Industry Service Laboratory, RCA Laboratories Division,New York, N. Y.147