Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

:..=214 FREQUENCY MODULATION, Volume Imay still be serviceable. On the other hand, if the impulse noise is ofa continuous nature like fluctuation noise, the noise discriminatesagainst the signal below the improvement threshold and, therefore,smothers the signal in the same manner that fluctuation noise does.The formula for the distance at which the improvement thresholdoccurs may be obtained by developing a formula for the distance fromthe transmitter at which carrier-noise ratios of four and zero decibelsare obtained in the intermediate-frequency channel of the frequencymodulationreceiver for fluctuation and impulse noises respectively.First—the equation for the carrier-noise ratio in the frequency-modulationintermediate-frequency channel must be determined. This maybe derived from (4) which gives the peak carrier-noise ratio as definedfor an amplitude-modulation system. For a given radiated carrier, thecarrier-noise ratio in the frequency-modulation receiver is less thanthat defined by (4) by a factor which depends upon the ratio of theeffective band widths of the two receivers and upon the type of noise.For the case of fluctuation noise,1C e/N - C/N X (fluctuation noise) (7)(F fi/2F a ) >/'where C = carrier level in i-f amplifier of the frequency-modulationreceiver,N = noise level in i-f amplifier of the frequency-modulationreceiver,F fi— i-f band width of frequency-modulation receiver,F a— band width of audio spectrum it is desired to receive.inFor convenience let Z f= F fi/2F afor fluctuation noise. Substituting(7), we obtain:C C 1= — X- (fluctuation noise) (8)N N VZ fThe equation corresponding to (8) for impulse noise, which variesdirectly with the band width instead of as the square-root of the bandwidth, will beC C 1N_— (impulse noise) (9)N Z iwhere Z i= F fi/2F a for impulse noise.The factors \fz' fand Z itwhich are the ratios between one-half theintermediate-frequency channel width and the audio channel widthof the frequency-modulation receiver for the two types of noise, are