Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

458 FREQUENCY MODULATION, Volume Iwill not be realized unless the 2 parts of the system are closely balancedwith respect to each other. To realize this, inductances of the coilsmust be alike and the mechanical construction of both sides of the pushpullcapacities must be alike so to give substantially equal capacity,equally varied inopposite phase.The balance between the 2 modulated tuned circuits was checked bymodulating the oscillator and the discriminator separately, and notingthe outputs in each case.This was done by substituting an equivalentfixed capacitor for the discriminator side of the push-pull capacitor andwith a constant modulation applied mechanically to the oscillator partof the push-pull capacitor, the output was then noted. When substitutinga fixed condenser of equal value in the oscillator circuit andmodulating the discriminator, the output was found to be of the sameCOPPERPLATES,COMMON—STEEL WIRE—mmVIBRATORY MEMBER^PCROSS-SECTION A-ABAKELITEFig.6—One form of push-pull FM transducer.amplitude as in the case when the oscillator was modulated. In otherwords, both circuits were designed to produce equal output voltagevariations.In each case, however, the exact oscillator center frequency positionon the discriminator curve was noted, and the fixed substituted capacityadjusted so that the position was the same. This is important, sinceif the position were to change, a different amount of diode voltagewould be developed for a given modulation, depending on the slope ofthe discriminator curve in the section used for discrimination. Thisposition was noted by observing a microammeter connected betweenground and the diode load resistor. The reading then was highest atthe peak and proportional to the position of the oscillator center frequencyon the discriminator curve. By thus observing the reading, theposition and the side of the discriminator curve can be duplicated ineach case.