Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FREQUENCY MODULATION GENERATOR 105It should be noted that this device isfundamentally a generator ofphase modulation — not frequency modulation. However, any phasemodulator may be made to produce the equivalent of frequency modulationby means of a network in the audio input having a characteristicthat is inversely proportional to frequency. It will be understood thatwhenever the cathode ray modulator is referred to as a generator offrequency modulation, the use of such a network is implied.Variation of the anode voltages will cause the sensitivity of the electronstream to vary so that for constant voltages on the deflectingplates the size of the circle described on the finalanode by the electronstream will vary as the anode voltages vary, thereby producing a form\*fWtf-Pig. 4—Square wave outputs of the generator, lettered to correspond withthe scanning circles shown in Fig. 2, opposite. The phase shift results infrequency modulated waves when the signal is passed through a frequencymultiplier.of phase modulation. This means that the d-c voltages supplied to theelectron gun must be well-filtered and free from fluctuations.Auxiliary electrodes may be added to the modulator tube for controlor monitoring purposes. For example a fluorescent screen may beprovided beyond the target anode, so that the electron stream will producea pattern upon it when it passes between segments of the anodeor beyond the outer edges of the anode plates. Additional electrodes,located in the same plane as the final anode, but electrically separatefrom it, may be used for adjusting the modulator and also for indicatingovermodulation. Such an electrode, of small area, located at thegeometrical center of the anode (where the anode plates are cut away)is useful in centering the electron stream. A narrow annular ring