Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

42 FREQUENCY MODULATION, Volume 1low deviation ratio systems retain their frequency modulation improvementdown to lower carrier levels.The peak voltage of fluctuation noise varies with band width in thesame manner as the root-mean-square voltage, namely, as the squareroot of the band width. The peak voltage of impulse noise varies directlyas the band width.In frequency modulation systems with adeviation ratio greater than unity, this difference in the variation withband width makes the improvement threshold occur at a higher carrierlevel with impulse noise than with fluctuation noise. Hence frequencymodulation systems with higher deviation ratios are more susceptibleto impulse noise interference.Because of a phenomenon called "frequency limiting" the peak frequencydeviations of the noise or the noise-plus-signal are Jimited sothat the peak value cannot rise above the maximum peak value of thesignal at the output of the detector. The application of audio selectivityreduces this maximum value of the noise so that fluctuation noise cannotrise to a value higher than the maximum value of the signal dividedby the square root of the deviation ratio; the corresponding value ofimpulse noise cannot rise to a value higher than the maximum peakvoltage of the signal divided by the deviation ratio. Inherent with thislimiting effect is a signal-depressing effect which causes the fluctuationnoise gradually to smother the signal as the carrier-noise ratio is loweredbelow the improvement threshold. However in the case of impulsenoise, the signal depression is not as troublesome, and a noise-suppressioneffect is created which is similar to that effected in the recent circuitsfor suppressing impulse noise which isstronger than the carrierin an amplitude modulation system. When the deviation ratio isgreater than unity, this frequency limiting is more effective thanthe corresponding amplitude modulation noise-suppressionthis iscircuits;caused by the audio selectivity reducing the maximum peakvalue of the noise so that it is less than the peak value of the signal.For carrier-noise ratios greater than unity, the application of frequencymodulation to the carrier does not increase the noise above itsvalue in the absence of applied frequency modulation.At the transmitter, a four-to-one power gain is obtained by the useof class C radio-frequency amplification for frequency modulation insteadof the customary class B amplification as isused for low levelamplitude modulation. Therefore, for the same transmitter power input,a frequency modulation system will produce at its receiver a carrierwhich is twice as strong as that produced at the receiver of anamplitude modulation system. This results in two effects: first, the