Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

216 FREQUENCY MODULATION, Volume Ioccurs when these carrier-noise ratios are equal to four and zerodecibels, respectively, the distance at which the improvement thresholdwill occur for a given set of transmission conditions may be determinedby equating (12) to 1.585 (four decibels) and (13) to unity (zerodecibels), and solving for the distance.Thus,0.0042 ^WahfD,»-\ 1/nDj =f )(fluctuation noise) (14)N \fK^LD.=0.0067 \JW~ahf D,r 2 \ 1/nNK&(impulse noise) (15)in which D tindicates the distance at which the improvement thresholdoccurs.A study of (14) and (15) shows the effect of a variation of thetransmission conditions. For both types of noise the distance to theimprovement threshold is directly proportional to the l/2w power ofthe watts radiated. Hence, for the higher radiation frequencies, wherethe exponent "n" is large, the improvement threshold distance increasesmore slowly with increase in power. For fluctuation noise,the improvement threshold distance is inversely proportional to thel/2w power of the product of the factor K fand the deviation ratio.Thus, as the deviation ratio is increased, the improvement thresholddistance decreases and decreases at a less rapid rate when it is beyondthe horizon and for the higher radiation frequencies where the exponent"n" is larger. The importance of making the factor K, as smallas possible, by carrying the frequency deviation out on the selectivitycurve as far as possible, is also indicated. When the noise is impulsenoise (formula 15) the improvement threshold distance is inverselyproportional to the "w"th root of the product of the factor K tand thedeviation ratio. Consequently, with this type of noise, the improvementthreshold distance decreases more rapidly as these factors areincreased.The distances given by (14) and (15) may be substituted in (5)and (6) to find the frequency-modulation signal-noise ratio existingat the improvement threshold as follows:S,/N, (at D t )= 1.585 V3/i V7^I= 2.74 \f~K~,^ (fluctuationnoise) (16)S f/N, (at Dt) =2tiK { fi= 2 K, y? (impulse noise) (17)