Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

RATIO DETECTOR 363voltage makes up the decrease in diode current in order to maintainthe sum voltage across the two diode load resistors constant. Stabilizingthe rectified voltage, therefore, results in the equivalent loadresistance varying in such a way that it decreases when the input signalrises and increases when the input signal falls.This action of the stabilizing voltage in varying the effective diodeload resistance provides a convenient method for analyzing the mechanismof amplitude rejection. A first approximation to the behavior ofthe ratio detector, then, is to consider how the output is affected whenthe load resistance is varied above and below its mean operatingvalue. Figure 7 shows the result of varying R, while all other parameters,including the input signal, are held constant.Pig. 7—Effect on the output of the ratio detector when the load resistanceis varied. The curves were taken with the circuit shown in Figure 8.If the rectified outputs E x + E 2 and E x— E 2are measured, it isreadily possible to determine whether the detector constants, theprimary-secondary inductance ratio,the primary and secondary Q's,the coupling, the diode perveance, and the operating load resistanceare such as to yield good amplitude rejection.If the circuit is designedproperly, it will be found that, as R is varied, E t— E 2 will increasedecrease) at the same rate as E 1 +E 2. In other words, (2?i— E 2 )/(Ei + E 2 ) must be independent of K and hence independent of therectified current. If (E 1— E 2)/(E 1 + E 2 ) is independent of the rectifiedcurrent, then it follows that the ratio EJE 2must also be independentof the rectified current.To take a specific example, suppose R is increased so that, for afixed deviation, (E x— E 2 )/2 increases to twice its original value.(or