Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

IMODULATO*160 FREQUENCY MODULATION, Volume Iaudio cycle. However, the rms distortion at 50 cycles is relativelyconstant at 3.5 per cent over a 2-volt range in bias,for 100-kc deviation,as shown in Figure 9.A large number of tube types (6J7, 6K7, 6V6, 6AC7/1852, 6AC7/1853, 6J5, 6L6) were used experimentally as modulators. The6AC7/1852, for example, has higher sensitivity (kc deviation peraudio volt) , but the dependence of distortion on bias is more critical.The 6L6 was finally chosen as a result of its lower rms distortionat 50 cycles modulating frequency, and the relative constancy of distortionwith bias.Analysis of the modulator circuit shows that distortion producedat low audio frequencies, caused by mismatch of the R pvs. E 9and vs. R curves, is due principally to insufficient curvature in the R pvs.J)ISTOffTIOH VS. FREQUENCY DEVIATION616 MQDULATO0 Tt/Se/tuDIO FP£OU£/^CY * SO cyclesAMS ' SI 7 VrffSOUEriev IXW/ITIO* k~CPig. 10E„ curve. This can be corrected, if desired, by some types of regenerativeaudio circuits, which tend to increase non-linearity in themodulator tube R pcurve, or by the use of an additional modulator tosupply 2nd harmonic variations in R pto produce additional curvature.Experimental ResultsFigure 10 shows the distortion incurred at 50 cycles modulatingfrequency vs. frequency deviation (in the 40.5 Mc carrier) taken ata value of bias for which the 2nd harmonic distortion is at a broadminimum.Figure 11 is a plot of rms distortion vs. audio frequency for twovalues of frequency deviation. Note that the distortion is essentiallyindependent of deviation except at frequencies below 200 cycles, forwhich the audio swings (approximately 20 peak-peak volts at 50 cycles)are large, causing considerable mismatch in the phase and R pcurves,which, at lower swings, are normally coincident.