Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

RATIO DETECTOR 387by the separation between the primary and secondary, but also by thecapacitance unbalance from either end of the secondary to ground.The ratio detector is essentially a balanced circuit and from itsmethod of operation it is evident that the degree of a-m rejection willdepend upon the two diodes having substantially the same characteristics.If the diodes differ materially from each other, the principa\effect will be the production of an unbalanced a-m component in theoutput. The effect on the peak separation and f-m sensitivity is usuallynegligible. If the detector is tested with an f-m signal which is alsoamplitude modulated, so that oscillograms similar to those inFigure12 are obtained, unbalanced diodes will have the effect of broadeningthe output characteristic which is indicative of an unbalanced a-mcomponent in the output.MeasurementsThe measurements necessary in the design of a ratio detector andin checking its performance are described in this section. Theseinclude the unloaded and loaded values of primary and secondary Q,the per cent critical coupling, the ratio of the secondary and tertiaryvoltages, and amplitude modulation rejection.Primary and Secondary Q'sTo measure the unloaded primary Q, the double diode is removedfrom its socket and the secondary is detuned. The primary Q is thendetermined from the selectivity of the primary tuned circuit. Theloaded primary Q is measured in the same way but with the diodes inthe circuit so that the normal diode loading is reflected across theprimary through the tertiary winding.To measure the secondary Q, the primary winding is heavily loadedby shunting a resistance across it. The secondary Q is then determinedfrom the -selectivity of the secondary winding as indicated by the rectifiedvoltage variation with frequency. When measuring the unloadedsecondary Q, the diode load resistance should be replaced with highvalues of resistance, on the order of a megohm. When measuring theloaded secondary Q, the normal load resistors should be used. Forthese measurements the centertap of the secondary should be disconnectedfrom the high side of the tertiary winding, so as to prevent thetertiary voltage from contributing to the rectified voltage.CouplingThe per cent of critical coupling can be measured by noting the