Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

156 FREQUENCY MODULATION, Volume Iunlimited multiplication of the phase deviation without resorting tovery low source signal frequencies in the modulator circuit.The heterodyne system was employed in the laboratory generator,as shown in the block diagram, of Figure 7. The source of signals isa crystal oscillator operating at 100 kc. This is fed through a filterto a 6SJ7 with the tuned circuit of Figure 5 as its plate load.The grid of the 6L6 modulator tube (triode connected) is suppliedwith audio potentials, and its plate resistance constitutes the variableR of Figure 5. The resultant phase-modulated signal is multipliedsuccessively by factors of 3 and 5 to produce a 1500-kc voltage, whosephase deviation is 15 times that effected by the modulator. In aFig. 7parallel chain of multipliers, an unmodulated 100-kc signal(from thesame crystal oscillator) is multiplied by factors of 4 and 4, to producea 1600-kc voltage, with no phase deviation. The 1600-kc signal servesas a constant-frequency heterodyning source, converting the phasemodulated 1500-kc signal to the difference frequency of 100 kc, whichis taken from the mixer plate through a low-pass filter. The phasedeviation in this 100-kc signal isincreased over the modulator circuitdeviation by a factor of 15. Hence, the apparent sensitivity of themodulator (degrees phase shift/audio voltage) is increased by 15 to 1,allowing a reduction of non-linear distortion products (at low valuesof audio frequency) by a factor greater than 15 to 1.The 100-kc heterodyne signal is then fed through a subsequentchain of multipliers which raises the carrier frequency to 40.5 Mc,with a phase deviation of 405 X 15 = 6075 times that produced at themodulator. For distortion measurements this output signal is appliedto the converter of Figure 3, which feeds into the linear monitordetector.