Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

382 FREQUENCY MODULATION, Volume Igrid-plate gain is high, this condition can be remedied by reducingthe primary inductance.Residual Amplitude Modulation in OutputIn general the ratio detector will not completely reject all of theapplied amplitude modulation. Where the effect of the applied amplitudemodulation is to cause the slope of the detector characteristic tovary, the output of the detector will appear as shown in Figure 21.The output amplitude modulation may be either in phase with theapplied modulation, or out of phase, i.e., the slope of the detectorcharacteristic may either increase or decrease slightly with increasingvalues of input signal, depending upon the circuit constants and thesignal level. Where the output characteristic appears as shown inFigure 21, when simultaneous amplitude and frequency modulationare applied, the output is said to be balanced in that, just as in thePig. 21—Effect of amplitude modulationon the output of a balanceddetector. If 100M = % amplitudemodulation of the input signal, thenAy = yM, and Ay/y = M. This isreferred to as a balanced a-m component.Fig. 22—Effect on the output of abalanced detector when amplitudemodulation causes a detuning ofthe secondary. This is referred toas an unbalanced a-m component.balanced discriminator, there isa center frequency at which the outputis insensitive to applied amplitude modulation. With properdesign, the amplitude modulation in the output, as indicated by theslope variation in Figure 21, will be very much less than the amplitudemodulation in the input signal. This is indicated by the oscillogramsin Figure 12.The output of a ratio detector will not appear completely balancedas shown in Figure 21, unless the circuit is compensated to removeamplitude variations which result from the fact that the effectiveinput capacitance of the diodes varies with the average diode currentand therefore varies during the a-m cycle. The detector output characteristicin Figure 22 shows the output characteristic which isobtained when an unbalance is present in the circuit; this may bedue not only to variations in the input capacitance of the diodes, but