Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

.RATIO DETECTOR 385when the input signal is applied at the center frequeney.It is possible,however, to ground one side of the rectified output, as shown in Figure25. In this case, the voltage available for a-v-c purposes is twice asgreat as in the balanced circuit case. There is no change in the a-foutput.Where only one diode load condenser isused, as in Figure 25(b),its capacitance should be twice as large as that used in Figure 25 (a)This follows because in (a) the two condensers are effectively inparallel at the intermediate frequency, while in (b) since there isonly one condenser its value must be twice as great.This relationshipis of particular interest where the unbalanced a-m component is reducedby using a critical value for the diode load capacitance, as previouslydescribed.In connection with unbalanced circuits, it may be noted for thecircuit shown in Figure 8, it is not necessary that the center of theFig. 25—Either one or two diode load condensers may be used in the ratiedetectorload circuit. To obtain the same effective diode load capacitancein (b) as in (a), Cz must be equal to the sum of Ci and C2 .load resistor be grounded; the a-m rejection will be essentially thesame if any point along the load resistor bypassed by the stabilizingcondenser is grounded. This statement is not true, however, at frequencieswhich are so low that the stabilizing condenser does noteffectively hold the voltage across its terminals constant.Time Constant of Stabilizing VoltageAt low audio frequencies, the a-m rejection of the ratio detectorThedepends upon the time constant of the stabilized rectified voltage.same considerations which determine the time constant in an a-v-ccircuit can also be applied to the determination of the time constantof the stabilizing voltage.In general, the discharge time constant ofthe stabilizing voltage, formed by the load resistance and the stabilizingcapacitance should be about 0.2 second. Time constants larger thanthis have an undesirable effecton the tuning characteristic which issimilar to the effect of too long a time constant in the a-v-c circuit ofa receiver.