Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

PROPAGATION CHARACTERISTICS 80C and D of Fig. 2 whose audio outputs are combined by the transformerT in push-pull or parallel combination, depending upon the switch S.With the switch in the push-pull position, frequency modulation maybe received and amplitude modulation balanced out. The balance obtainableon amplitude modulation, however, is complete only for thecondition of no frequency modulation present. As soon as the carrieris frequency deviated from its unmodulated position, the carriers fed tothe detectors are no longer balanced due to the fact that the slopingfilters have opposite slopes. Hence the balance is thrown off by anamount proportional to the frequency deviation from the unmodulatedcarrier position, and the resultant balance is an average between thebalanced and unbalanced conditions. With the switch in the push-pullposition, second harmonic square-law detector distortion is completelyFig. 4—Block diagram of receiver arrangement used in determining the phaserelation between the detected outputs from receivers connected to spacedantennas.balanced out. With the switch in the parallel position amplitude modulationmay be received and frequency modulation balanced out.The intermediate-frequency amplifier of the receiver was adaptedfrom broadcast components and had a twelve-kilocycle pass band.Consequently the maximum frequency deviation was limited to onehalf of this intermediate frequency channel or six kilocycles. An experimentalharmonic analysis of the receiver showed that it was capableof this amount of deviation without serious distortion.An amplitude limiter was arranged so that it could be switched inand out of the intermediate-frequency circuits. This limiter consistedof a separate multistage intermediate-frequency amplifier feeding atube with lowered element voltages so that the tube would be heavilyoverloaded and its output would be constant regardless of the changesof the input. Thus amplitude modulation could be removed. Thismethod of removing amplitude modulation has an advantage overthe balance obtained with the opposite sloping conversion nitersthat the amplitude modulation is completely removed in the presenceof frequency modulation as well as in the unmodulated condition.in