Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

:)SERVICE RANGE OF FREQUENCY MODULATION 211"~°0.0067 VWahf D hC/N = = S a/N a ( 4JVD»where C = peak carrier-field strength,N = peak noise-field strength as above defined,S a= peak audio-signal voltage,amplitude-modulation receiver,N a= peak audio-noise voltage, amplitude-modulation receiver.Frequency-Modulation Signal-Noise Ratio vs.DistanceThe author's previously cited paper 2 on frequency-modulation noisecharacteristics gives the improvement factors effected by a frequencymodulationsystem over an amplitude-modulation system.For the caseof fluctuation noise the factor is V3 times the deviation ratio /*. (Thedeviation ratio, p., is equal to F d /F awhere F d is the peak-frequencydeviation due to modulation and F a is the width of the audio channelof the system.) This factor holds for equal carriers fed to the tworeceivers and for the condition of a peak carrier at least 4 decibelsabove the peak noise in the frequency-modulation receiverintermediate-frequencychannel. Thus, the signal-noise ratio at the output ofthe frequency-modulation receiver may be found by multiplying thesignal-noise ratio given by (4) by the improvement factor or:0.0067 VWahf D h"~-S f/N f= x/3^N D"(fluctuation noise) (5)0.0116 m VWahf D,»-NowhereS f= peak audio-signal voltage, frequency-modulation recsiver,N f= peak audio-noise voltage, frequency-modulation receiver.When the received noise is impulse noise the improvement factoris equal to twice the deviation ratio or 2 /a. The corresponding signalnoiseratio is thenIt will0.0134 ,x VWahf L>/("- 2S f/N f= (impulse noise) (6)N D"be noted that in deriving the signal-noise ratio formulas of(5) and (6), the power gain normally effected by frequency modulationat the transmitter is automatically taken care of by the fact that Wappears in both the amplitude- and frequency-modulation formulas.In the case of amplitude modulation the value of W used would normallybe less than the corresponding value used in the frequency-