Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

374 FREQUENCY MODULATION, Volume 1Fig. 17—The fraction of the total rectifiedvoltage which must be stabilized toreduce the residual balanced amplitudemodulation is a function of S/P. Forvalues of S/P greater than about unity,the a-m in the output increases if lessthan the full rectified voltage is stabilized.The a-m rejection is a matter of primary interest. To determinethe effect of S/P on the a-m rejection, amplitude modulation wasapplied for each of the conditions shown in Figure 16. As shown inFigure 17, a threshold value of S/P exists, and for values of S/Pgreater than this it is not possible to obtain good a-m rejection. Atthis critical value, which is approximately unity, the full rectifiedvoltage must be stabilized. As the value of S/P is decreased belowthis value, good a-m rejection will be obtained provided a particularfraction of the full rectified voltage is stabilized. The lower the valueof S/P, the smaller the fraction of the rectified voltage which mustbe held constant. The ratio of unloaded to loaded secondary Q is fourto one for the curve shown in Figure 17. A 6AL5 double diode was used.Stabilizing the Rectified VoltageTwo general methods for controlling the desired percentage of therectified voltage which is stabilized are shown in Figure 18. In (a),a resistor is added in series with the stabilizing condenser. In (b),the same effect is obtained by adding resistors R a and R bin serieswith the load resistance which is shunted by the stabilizing condenser.If these circuits are to be equivalent, then a change A / in the d-crectified current must result in the same change in rectified voltagefor each of the two circuits.Fig. 18—Two methods of stabilizing any desired fraction of the rectifiedoutput voltage.In each case the fraction of the output voltage stabilizedisapproximately equal to 2R/(2R + Ri).