Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

174 FREQUENCY MODULATION, Volume ITheoretically, it should have been possible to find conditions ofmodulating frequency, and degrees of modulation, where there was notorque. Actually, these points were so critical and so sharply definedthat it required considerable patience to find them. In actual operation,with program modulation, the condition of no torque would not existlong enough to lose control.The effect of frequency division is to reduce the swing of frequencywith modulation, along with the reduction of carrier frequency. Thisis carried far enough when the frequency comparison is made at 100kilocycles so that carrier loss occurs only at frequencies of modulationbelow 100 cycles and, for 100-cycle modulation, only with full frequencyswing or 100-per-cent modulation. This effect was not serious in programmodulation, but in taking performance data on a transmitter,4.5-6 M.C.AUDIOINPUTf/240I8.75-25.KC94-125 KCPig. 11—New FM Exciterwith tone modulation, it would be readily apparent.An additional divisionby five, to 20 kilocycles, eliminates this problem except for modulationbelow 20 cycles, which appears low enough not to be a source ofcomplaint.The circuit of the current model of the exciter unit is illustrated inFigure 11, as a block diagram. The items in the top row are (fromleft to right) the reactance tubes, a modulated oscillator, a frequencytripler, a second tripler, and a power amplifier. The power amplifier isused to obtain sufficient power to feed the main transmitter througha transmission line. The output frequency will fall in the range of 40.5to 54 megacycles.Multiplication by two, in the main transmitter, givesa possible output frequency range from 81 to 108 megacycles.A lead shown at the left of Figure 11 serves to conduct a synchronizingvoltage from the modulated oscillator to the first divider at the