Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

364 FREQUENCY MODULATION, Volume IThen E x + E 2 will also double. The input signal may therefore bereduced to one half itsoriginal value and the a-f output will then bethe same as it was before the change in R and the reduction in inputsignal. In practice, when a battery or a large condenser is used acrossthe load resistors, any variation in the input signal automaticallycauses the equivalent load resistance to vary in such a way as to keepthe a-f output constant, provided of course that the detector circuitparameters are properly related.The amount of downward amplitude modulation which can berejected, that is, the extent to which the signal can fall below itsinitial value without causing the output to change, is an importantcharacteristic. The limiting factor here is that, if the input signaldrops to too low a value, the voltages across the primary and secondaryare not sufficient to cause the diodes to conduct.Effectively, the diodesare biased off by the stabilizing voltage and the detector becomesinoperative until the signal rises to a level sufficiently great to causethe diodes to conduct.It is desirable to design the ratio detector so that the signal canfall to as low a value as possible without the diodes being cut off.This is accomplished, in general, by using a high value of secondary Qand by using relatively low values of diode load resistance so that theoperating Q of the secondary is on the order of 25 per cent of itsunloaded Q. Other considerations are also involved, but the extent towhich the operating Q is smaller than the unloaded Q is the principalfactor in determining the maximum percentage of downward amplitudemodulation which can be rejected without the diodes being cut off.The limitation of the diodes being biased off by the stabilizedrectified voltage does not exist as far as upward modulation is concerned.In general, the ratio detector can reject higher percentages ofupward modulation than it can downward modulation.Ratio Detector ReceiversReceivers using the ratio detector have characteristics which differfrom limiter and locked-oscillator receivers in a number of respects.Since the rectified voltage across the long-time-constant load circuitof the ratio detector automatically adjusts itself to the input signallevel, there is no fixed threshold, and the a-f output and the stabilizingvoltage (which may be used for automatic-volume-control (a-v-c) ) areproportional to the input signal. Less i-f gain is required since theratio detector in typical designs will provide appreciable amplituderejection with as little as ten to fifty millivolts of input signal to thegrid of the ratio-detector driver tube. As a result, receivers using