Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

212 FREQUENCY MODULATION, Volume Imodulation formulas by a factor equal to the frequency-modulationpower gain at the transmitter.Effect of the Improvement ThresholdThe presence of the phenomena called the "improvement threshold"places a rather definite maximum service range on a frequency-modulationsystem. The improvement threshold occurs at the point wherethe peak voltages of the noise and carrier in the intermediate-frequencychannel of the frequency-modulation receiver are equal. Theexperimental work of the author's previously cited paper 2 shows thatthe full frequency-modulation improvement for fluctuation noise is notobtained until the carrier is at least four decibels above the improvementthreshold or where the carrier-noise ratio is about four decibels.The nature of the improvement threshold is such that thesignal-noise ratio drops rapidly as the carrier falls below the fourdecibel carrier-noise ratio. For the case of fluctuation noise, whichis of a continuous nature, the noise smothers the signal in a mannerwhich has been described in detail before 2 . Consequently the distanceat which the improvement threshold occurs for fluctuationnoise may be taken as the maximum service range of the frequencymodulationsystem. However, for impulse noise of the type whichconsists of sharp impulses having a low rate of recurrence, the situationis somewhat different. It has been pointed out to the authorby V. D. Landon of RCA Manufacturing Company, that for thecondition of no modulation present, if the receiver is carefully tunedso that the incoming carrier is approximately synchronized withthe oscillation frequency of the impulse, the frequency-modulationimprovement is maintained for impulse noise which is strongerthan the carrier. This effect is shown in the experimentally determinedcurve of Figure 3 which also shows the effect of a slight detuningfrom the point of synchronism. For these curves, the peak carriernoiseratio and the peak signal-noise ratios were measured by meansof an oscilloscope coupled to the intermediate-frequency channel outputand the audio output of the receiver, respectively. Curve B was takenwith the receiver carefully tuned to the impulse noise minimum. CurveA was taken with the carrier detuned about 20 per cent of the maximumfrequency deviation of the receiver used. It will be noted thatthe improvement threshold manifests itselfsharply when the receiveris detuned. Hence, for all except the very low passages of modulationthe threshold would be present since application of frequency modulationcorresponds to a momentary detuning. However, the reductionof the noise during the idle periods of the modulation is undoubtedlyvery helpful.