Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

20 FREQUENCY MODULATION, Volume Ilow deviation ratio system would be capable of producing a betteroutput signal-noise ratio than the high deviation ratio system.The difference between the improvement thresholds of receiverswith different deviation ratios may be investigated by a determinationof the carrier-noise ratio which exists in the reference amplitude modulationreceiver when the improvement threshold exists in the frequencymodulation receiver. This carrier-noise ratio may be found by a considerationof the relative band widths of the intermediate-frequencychannels of the receivers. Thus, when the deviation ratio is unity, andthe intermediate-frequency channel of the frequency modulation receiverisof the same width as that of the amplitude modulation receiver,10the two receivers would have the same carrier-noise ratio inthe intermediate-frequency channels. When the deviation ratio isgreater than unity, and the intermediate frequency channel of the frequencymodulation receiver is broader than that of the amplitudemodulation receiver, the carrier-noise ratio in the frequency modulationreceiver is less than that in the amplitude modulation receiver.For the case of fluctuation noise, where the peak values vary as thesquare root of the ratio between the two band widths concerned, thecarrier-noise ratio in the frequency modulation intermediate-frequencychannel would be lessthan that in the. amplitude modulation intermediate-frequencychannel by a ratio equal to the square root of thedeviation ratio. Thus, when equal carrier voltage is fed to both receivers,Ca/Na — {C/N)\/YJJ~F a (fluctuation noise, peak or r-m-s values) (23)in which C a/N a = carrier-noise ratio in the amplitude modulation intermediate-frequencychannel and C/N = corresponding ratio in thefrequency modulation intermediate-frequency channel.In the case of impulse noise, where the peak values of the noise10 In order to assume that the frequency modulation receiver with a deviationratio of unity has the same intermediate-frequency channel width as thecorresponding amplitude modulation receiver, the assumption would also haveto be made that the peak frequency deviation due to the applied frequencymodulation is equal to one half the intermediate-frequency channel width. Inthe ideal receiver with a square-topped selectivity characteristic, this amount offrequency deviation would produce considerable out-of-channel interference andwould introduce distortion in the form of a reduction of the amplitudes of thehigher modulation of frequencies during the intervals of high peak frequencydeviation. However, under actual conditions, where the corners of the selectivitycharacteristic are rounded, it has been found that the frequency deviation maybe made practically equal to one half the normal selectivity used in amplitudemodulation practice without serious distortion. Receivers with high deviationratios are less susceptible to this distortion due to the natural distribution of theside bands for the high values of Fd/Fm which are encountered with such receivers.