Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

370 FREQUENCY MODULATION, Volume Itant because the degree of a-m rejection and the peak separation areclosely related to this ratio.Equivalent CircuitThe effect of the diodes on the ratio detector circuit can be simulated,as shown in Figure 13, by placing a resistance R/A-q across theprimary and a resistance R/rj across the secondary- This is the samerelationship which applies to the balanced discriminator circuit.From Figure 13 (b) it is clear why some form of impedance matchingis of great importance if good sensitivity is to be obtained. It isnot uncommon for R to have values of the order of 10,000 ohms or less,since R must be low enough to reduce the secondary Q to about one-5EC0HDARILOSO'NGSECONDS G>Unloaded primary Oiniooded secondor*S.'.S C »iT o DETECTORPig. 13—The equivalent circuit for a basic phase shift ratio detector show tthat the diodes reflect an equivalent shunt resistance jR/4i? across the primaryand a resistance R/v across the secondary.fourth of its unloaded value. This would result in a load impedanceof 2500 ohms which is too low to provide good energy transfer froma pentode.A typical circuit using a tapped primary to obtain impedancematching has already been described. The equivalent circuit for thisarrangement is shown in Figures 14 and 15. This circuit differs fromFigure 13 in that the diode loading isacross the primary as a resistance w 2 times as great.across the tertiary and appearsReferring to Figures 14 and 15, it is shown in the Appendix thatSP1+- w2 Q s(l)