Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

372 FREQUENCY MODULATION, Volume Iating Q of the secondary (Q, 1 ) is small in comparison with theunloaded Q (Q s°). This assumption is usually justified and accountsfor the factor R/rj not appearing in the equation. Q p° appears in theequation because it cannot usually be assumed that the loaded primaryQ will be small in comparison with the unloaded primary Q.It is shown in the Appendix that the half-secondary voltage S isgiven byS = gm *L tQ*n ====( 2 )n 2 Q/ 1 + « 24 A/ 1+ 4aQB°orgm a,L,Q,1 n / PS = ~ (3)4 \ S ) 1 + aThe expression for S is put in this form because in general S/Pwill not be an independent variable but will be fixed by considerationswhich determine the amplitude rejection and the peak separation.Effect of Varying S/PBefore discussing the significance of these equations, it is desirableto show the dependence of peak separation and a-m rejection on thevalue of S/P. For convenience, constant primary voltage is assumed;this simplifies the analysis and provides a basis for obtaining a solutionwhen the primary voltage is not constant. In this connection,whether the primary voltage variation is due to external amplitudemodulation, or whether it is due to the primary selectivity makes noappreciable difference in the analysis.The curves in Figure 16 were taken with constant primary voltage.As indicated, the a-f output characteristic is plotted for several valuesof S/P, P being held constant at the same value for all curves. Theoutput characteristics are shown with and without the rectified voltagebeing stabilized. In addition, the variation in the rectified voltage isshown for each value of S/P. The same voltage scale is used throughoutfor the a-f output and the rectified voltage variation.From Figure 16 it is clear that as S/P is increased, the peak separationincreases. This is true whether the rectified voltage is stabilizedor not; that is, it is equally true for the balanced discriminator orfor the ratio detector. When the rectified voltage is stabilized, how-