Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

360 FREQUENCY MODULATION, Volume Iing a large condenser across the load resistors. If a battery is used,operation is limited to a signal at least strong enough to overcome thefixed bias created by the battery. On the other hand, if a large condenseris used, the voltage across the condenser will vary in proportionto the average signal amplitude and thus automatically adjust itselfto the optimum operating level. In this way amplitude rejection canbe secured over a wide range of input signals, the lowest signal beingdetermined by deterioration of the rectifier characteristics at lowlevels.When a" condenser is used to stabilize the rectified output voltageits capacitance must be large enough so that the rectified outputASSis(b) AT RESONANCE {C> BELOW RESONANCE (d) ABOVE RESONANCEFig. 3—Basic phase shift circuit used in frequency discriminators and ratiodetectors. The phase relations between the primary and secondary voltagesare shown, as well as the resultant voltages which are applied to each diode.voltage does not vary at audio frequency. This calls for a time constanton the order of 0.2 second and for capacitance values on theorder of several microfarads.Elementary OperationAlthough the exact analysis of a practical ratio detector is complicatedby many factors, it is not difficult to obtain a qualitativeunderstanding of the method of operation. A more detailed investigationof the circuit is made later.