Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

RATIO DETECTOR 383Fig. 23—When unbalance is present, theeffect is to cause the crossover or point ofzero a-m response to be at other than thecenter frequency. This type of characteristichas both a balanced component ofa-m (Figure 21) and an unbalanced component(Figure 22).may also be due to unbalance in the transformer, diodes, or circuitconstants.Where the output of a detector contains both balanced and unbalancedcomponents, it will appear in general as shown in Figure 23.As would be expected, the effect of the unbalance is to shift the crossoverfrom the center frequency.Reducing Residual Amplitude ModulationIt is convenient to consider separately the means for reducing theresidual balanced component and the residual unbalanced componentof amplitude modulation in the output. Basically, the magnitude ofthe balanced component depends upon the ratio S/P at the mean carrierlevel. This may be visualized if the effect of a change in thesecondary Q on the ratio E x/E 2 in Figure 4 is considered; not onlydoes the secondary Q and hence the phase angle of the secondary currentchange with diode loading during the a-m cycle, but the relativeamplitudes of the primary and secondary voltages also change. Thereis thus a complex relationship which governs the extent to which E 1/E 2is independent of diode loading, and this relationship is clearly afunction of S/P. Thus, it is to be expected, as shown in Figure 24,that there should exist an optimum value of S/P for which E x /E 2isFig. 24—On either side of the optimum value of S/P for which the balancedcomponent of a-m is zero, there is a reversal in the phase of the a-m in theoutput. Since, for a given transformer, the value of S/P changes withcoupling, the figure shows that for a loose coupling the output is 180 degreesout of phase with the input a-m, while the two are in phase for close coupling.