Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FM EXCITER 175lower left. The dividers are arranged as shown, with four stages, givinga total division of 240. This places the output frequency of the lastdivider in the range of 18.75 to 25 kilocycles. The output of the lastdivider is connected directly to the two balanced modulators. Thecrystal oscillator shown at the lower right may operate at any frequencybetween 94 and 125 kilocycles. The crystal output synchronizes adivider at one-fifth the crystal frequency. This frequency is also fed tothe balanced modulators, but in this case a phase-shifting network isincluded in the lead to each modulator, adjusted to maintain a 90-degreedisplacement in-phase between the modulators, over the range of frequenciesinvolved.Each balanced modulator has a pair of 6L6 tubes biased to cut-off,and connected in push-pull. An induction motor with high-impedances«(Tin poim a, suppitJ-5U4S S-0D3/VRI50I-OC3/VRI05Pig. 12center-tapped windings on each phase is used in the plate circuit of themodulator tubes, the use of matching transformers then being unnecessary.In this way the motor receives full voltage to direct current beatfrequency.The need for this is evident when one considers that themotor must respond to a beat-frequency lower than one part in a millionat 20 kilocycles, which is .02 cycles per second or 1.2 cycles per minute.This rather phenomenal performance from an induction motor is largelymade possible by the elimination of any load on the motor. The absenceof gearing and the use of viscous damping establishes a condition in