Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FM EXCITER 171beat frequency would fall in the range where the motor had torqueenough to take control. Frequency modulation of the controlled oscillatorhad no apparent effect on the motor except to reduce its torque. Thesystem was operated with program modulation from a local broadcaststation and ran for several days without once losing control. There was,however, a constant danger that a sudden jump in oscillator frequency,especially in the presence of modulation, might cause loss of control.Some improvement could be obtained by speeding up the rate of frequencycorrection. In these first tests it had been necessary to drivethe control condenser through a gear train so that the frequencycorrection would be gradual and without overshooting, which wouldresult in hunting.The slow speed of the motor, added to the backlash and friction inthe gear train, gave rather poor performance in following minor de-Fig. 8partures in frequency. To eliminate some of these troubles, the condenserwas mounted directly on the motor shaft as illustrated inFigure 8. The tendency to hunt was eliminated by fitting a dash-potto the opposite end of the motor shaft. The friction of the tuningcondenser bearings was avoided by using a split stator condenser withthe rotor on the motor shaft and insulated therefrom. One set of statorplates was grounded and the other set was connected to the plate ofthe controlled oscillator. This procedure eliminated all friction savethat in the ball bearings of the motor. The range of operation was,thereby, extended at both high and low frequencies, the motor beingable to take control at 1,000 cycles above or below zero-beat. The oildamping gave much more rapid control action without causing troublefrom hunting. These changes yielded an enormous improvement inthe operation of the frequency control.