Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

THE RATIO DETECTOR*!ByStuart Wm. Seeley and Jack AvinsIndustry Service Laboratory, RCA Laboratories Division,New York, N. Y.Summary— A new circuit for f-m% detection known as the ratio detectoris coming into wide use. In this circuit two frequency-sensitivevoltages are applied to diodes and the sum of the rectified voltages heldconstant. The difference voltage then constitutes the desired a-f% signal.This means of operation makes the output insensitive to amplitude variations.The basic principles governing the operation and design of the ratiodetector are described. The ratio between the primary and secondary componentsof the frequency-sensitive voltages in a phase-shift type of ratiodetector is a function of the instantaneous signal amplitude. The a-mfrejection properties, however, are shown to depend upon the mean ratiobetween these voltages. An expression which is developed for this ratio interms of the circuit parameters provides the basis for arriving at anoptimum design. The measurements necessary in the design of a ratiodetector and in checking its performance are described.General DescriptionTamplitudes are a function of frequency, together with meansy ]|^HE principle underlying most circuits for f-m detection hasbeen the peak rectification of two i-f% voltages whose relativefor combining the rectified voltages in reversed polarity.This introducesa difference voltage, proportional to the difference of the twoapplied i-f voltages, which is a function of the instantaneous appliedfrequency. A representative circuit of this type is shown in FigureKa).While this circuit represents a considerable improvement overunbalanced f-m detectors in that it is amplitude-insensitive at thecenter frequency, it has the disadvantage that changes in amplitudeat other than the center frequency will cause the audio output tofluctuate proportionally as shown in Figure 1(b). This has beenrecognized and has resulted in the use of limiter circuits to removeextraneous amplitude variations from the applied signal.In the ratio type of f-m detector, the problem of making the de-Decimal Classification:R362 X R361.111.t Reprinted from RCA REVIEW, June, 1947.t Throughout this paper the following abbreviations are used: f-m:frequency modulation or frequency-modulated; a-m: amplitude modulationor amplitude-modulated; a-f : audio frequency; i-f: intermediate frequency.S58