Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

'166 FREQUENCY MODULATION, Volume Ishown dotted in the proper phase relation. A portion of the output ofthe crystal is shifted in phase by 90 degrees, and added to the sidebands,as shown in the top right of the illustration. The effect of thesidebands is to advance and retard alternately the position of the carriervector. The whole picture rotates about the point at an averageangular velocity determined by the crystal frequency. The magnitudeof frequency modulation produced depends on the modulating freiookcXTALrlZHiHiH5LH3~G36.4 MC800KCAntennaFig. 2—Block Diagram of Armstrong Type FM Transmitter.quency, and on the amplitude or length of the sideband vectors relativeto the reintroduced carrier. It can be seen that, if the length of thesideband vectors is fixed, the frequency modulation will increase sixdecibels per octave as the modulation frequency isincreased.In order to obtain pure frequency modulation it is necessary todecrease the amplitude of the audio input six decibels per octave, withrising frequency.This is shown in the response curve, at the upper left,The audioin the modulating frequency range from 50 to 2,000 cycles.response is flat for frequencies above 2,000 cycles.This means that thefrequency-modulated output increases at six decibels per octave above2,000 cycles, to give the 75 micro-seconds standard pre-emphasis ofhigh frequencies agreed upon by the Radio Manufacturer's Association,as a measure to improve signal-to-noise ratio at the receiver. Thereceiver has a corresponding drop in response above 2,000 cycles to producea flat response for the overall system. The audio response is againflat for frequencies below 50 cycles, resulting in six decibels per octavedrop in frequency-modulated output below 50 cycles.This restrictionis imposed by the fact that the distortion in the frequency-modulatedoutput is five per cent, when the phase displacement is plus or minus28 degrees, and increases rapidly with greater phase shifts. These