Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

SERVICE RANGE OF FREQUENCY MODULATION 227It is seen from Figures 7 and 8 for fluctuation noise that at thedistance corresponding to the improvement threshold for a deviationof 75 kilocycles, a broadcasting service using a deviation of 20 kilocyclesyields a signal-noise ratio of 42 decibels.For the same distance,a service using a 75-kilocycle deviation yields a signal-noise ratio of53 decibels. At this point it is apparent that a 20-kilocycle deviationproduces a signal-noise ratio which is comfortably above what wouldbe considered commercially satisfactory,with an attendant conservationof band width in the available portion of the frequency spectrum.However, if it were assumed that a signal-noise ratio of 53 decibelsis necessary, this ratio may be obtained with a deviation of 20 kilocyclesat a somewhat shorter distance from the transmitter.For theconditions of the curves of Figures 7 and 8 this distance would be 20per cent shorter. On the other hand, a 20-kilocycle deviation is capableof furnishing what would be considered better than acceptable serviceout to a distance which is about 10 per cent greater than the maximumservice range for a 75-kilocycle deviation.itWhen the noise is impulse noise as portrayed by Figures 9 and 10,can be seen that the general shape of the curves are similar to thosefor fluctuation noise. If the frequency of recurrence happens to behigh so that the noise is continuous, the silencing properties of thefrequency modulation cannot be taken advantage of. Instead, thenoise, if stronger than the carrier, will depress the signal so as tosmother it.For this type of noise a low-deviation service will have agreater range than a high-deviation service by an amount that isconsiderablylarger than the corresponding case for fluctuation noise.This is especially true where the transmitter power is low enough tocause the improvement threshold to occur within the horizon as isthe case with the conditions of Figure 9.If the impulse noise happens to be automobile ignition where therate of recurrence of the pulses is rather infrequent and the time durationof the impulses short, the noise-silencing properties of the frequency-modulationsystem may be taken advantage of when the noiseis stronger than the carrier. With this type of noise, the effectivenessof the silencing action increases as the deviation is increased. However,for this type of noiss, even a system with a low deviation producesa noise output which has a rather low annoyance value.It is apparent that the service obtainable where fluctuation noisepredominates, may be predicted with a fair degres of accuracy.However,owing to the highly variable character and distribution of impulsenoise, predictions are difficult regarding this type of noise. Hence, itis felt that it is highly desirable to conduct field tests to further