Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

—PUSH-PULL FREQUENCY MODULATED CIRCUITAND ITS APPLICATION TO VIBRATORY SYSTEMS*!ByAlexis BadmaieffRCA Victor Division, Hollywood, Calif.Summary— The purpose of this paper is to describe a new push-pullfrequency modulated circuit in which the push-pull action is accomplishedby varying the resonant frequencies of both the oscillator and the discriminatorin opposite phase relation to each other. Modulation of the oscillatorand discriminator is achieved through the use of 2 capacitors with a commonplate and arranged in a push-pull circuit. Thus, if the common plate is themoving element of a vibratory system, this circuit can be used for measuringvibrations or for monitoring purposes.One of the applications of the push-pull FM circuit is to calibraterecording heads while cutting phonograph records. Using this calibrator,it is possible to measure, without mechanical or inductive coupling, theactual frequency response of the cutting head under normal load, and toobserve the input-output characteristics as well as the amount of distortion.It is also possible to use it for monitoring while recording.1Basic Push-Pull Frequency Modulated CircuitCONSIDERABLE interest isbeing shown in the development offrequency modulated circuits in which the oscillator and thediscriminator are combined in one unit. This type of circuitcan be used to convert mechanical vibrations to electrical voltage variationsand is applicable to measuring instruments, pickup devices, andother forms of transducers. 1 The purpose of this paper is to discuss anew type of FM circuit in which the advantages of push-pull action areutilized.In a single-ended FM circuit, 1 when used in conjunction with avery small capacitor as the frequency controlling element, a nonlinearrelation exists between the condenser plates spacing and the frequencywhich the condenser controls. The magnitude of nonlinearity dependson the ratio between the spacing of the plates and the amount of variationof that spacing when the plates are vibrated mechanically. Toillustrate, consider an LC circuit, in which one condenser plate ismoved away from the other plate during tuning. Progressively increasedseparation has less effect on the resonant frequency because* Decimal Classification : R148.2 X R621.375.622f Reprinted from Jour. Soc. Mot. Pic. Eng., January, 1946.452