Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FM EXCITER 167representative figures call for a frequency multiplication of about 3,000to produce a frequency swing of plus or minus 75 kilocycles at 50megacycles.Figure 2 isa more complete block diagram of an Armstrong typeof transmitter making use of an arrangement to minimize radio carrierdrift.A second method of producing a frequency modulated signalemploys a reactance tube to produce a direct change in the frequencyof a master oscillator. The basic circuit is shown in Figure 3. Theoscillator circuit is illustrated at the right and the reactance modulatoris shown at the left. The plates of the two tubes are tied together.The grid of the reactance tube is driven from the plate circuit, but is90 degrees out-of-phase with it. The plate current of the reactancetube is therefore 90 degrees out-of-phase with respect to the platevoltage. The tube reactance becomes either capacitive or inductive innature, depending on whether the 90-degree shift in grid excitationFMOUTPUTB +REACTANCE Bt"^TUBE SCREEN OSCILLATORFig. 3—Reactance Tube.leads or lags the plate voltage.The audio voltage changes the bias ofAs thethe reactance tube grid in accordance with the signal voltage.reactance tube is a pentode, this change in bias causes a correspondingchange in mutual conductance of the tube. The net result is that thereactive current of the tube varies as the impressed audio signal. Ina high-fidelity circuit it is customary to use two reactance tubes, oneacting as a capacitive reactance and the other as an inductive reactance,in order to obtain maximum frequency stability and linearityof response. The circuit is capable of a frequency swing of more thanplus or minus 10 kilocycles at 5 megacycles so that very little frequencymultiplication is required to obtain a plus or minus 75 kilocycles swing.Because of the advantages of the reactance tube modulator, considerableeffort has been devoted towards obtaining such an exciter