Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

RATIO DETECTOR 3752RFor circuit (a) , A E = A I•#12R + R 1For circuit (b) , AE = A / (R a + R b ) = A I R 22 = AI2R + Rt.'.At•R2Ror R 1— R 2== R 2provided R 2 < < 2R2R — R 2Both circuits provide equal diode loading since the total load resistanceis the same. When the desired variation in the rectified outputvoltage is small, then i^ = R 2as shown above.Factors Affecting S/PConsider the expression for S/P given by Equation (1). If Q/ islarge enough so that the primary Q is determined solely by the diodeloading, it is clear that« 1n 2 Q,» SandP4a QJ>This gives the useful result that for this condition the ratio S/P isindependent of the ratio between the primary and secondary inductance,and also independent of the number of turns in the tertiarywinding. For this condition, then, S/P is dependent only on the percent of critical coupling and is equal to a.When the diode damping of the primary is comparable to theprimary losses, S/P is greater than a as indicated by Equation 1.The extent to which the primary is damped by the diode rectifiers isof interest since the amount of downward amplitude modulation thatcan be handled depends, among other factors, on the primary diodedamping. The greater the amount of damping, the greater will be therejection of downward amplitude modulation because downward amplitudemodulation is accompanied by an increase in the effective loadresistance and hence an increase in the effective primary Q.increases the gain and helps to offset the drop in signal level.This actionReferring again to Equation (1), it is desirable to determine theextent to which the damping on the primary can be increased withoutdecreasing the value of S/P which must be maintained at approxi-