Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

18 FREQUENCY MODULATION, Volume 1limiting limits frequency deviations in the same manner that amplitudelimiting limits amplitude deviations. The resulting effect is the sameas though an amplitude limiter were placed at the detector output tolimit the output so that the peak voltage of the noise or signal, or theirresultant, cannot rise above a voltage corresponding to that producedby the signal alone at full modulation.Since the frequency limiting limits the noise so that its maximumamplitude cannot rise above the maximum amplitude of the appliedmodulation, a noise suppression effect ispresent which is similar tothat effected by the recent noise suppression circuits 8 ' 9used for reducingimpulse noise which is stronger than the amplitude modulatedcarrier being received. The result of such limiting is a considerable reductionof the annoyance produced by an intermittent noise, such asignition, where the duration of the impulses is short and the rate ofrecurrence is low. With such noise, the depression of the signal for theduration of the impulse reduces the presence of the signal for only asmall percentage of the time; the resultant effect is a considerable improvementover the condition where the peaks of the noise are strongerthan the signal. On the other hand, for steady noise such as fluctuationnoise, as the carrier-noise ratio is made less than unity, the signalis depressed more and more of the time so that it is gradually smotheredin the noise.When the effect of the audio selectivity is considered in conjunctionwith the frequency limiting, it is found that the noise suppressioneffect is somewhat improved for the case of a deviation ratio greaterthan unity. The reason for this is as follows: The frequency limitingholds the peak voltage of the noise at the output of the detector so thatit cannot rise above the maximum value of the signal. However, inpassing through the audio channel, the noise is still further reducedby elimination of higher frequency components whereas the signalpasses through without reduction. Consequently the over-all limitingeffect is such that the noise is limited to a value which is less than themaximum value of the signal. The amount that it is less depends uponthe difference between the noise spectra existing at the output of thedetector and the output of the audio selectivity.Experimental determinations, which will be shown later, pointout that as unity carrier-noise ratio is approached, the frequency8Leland E, Thompson, "A detector circuit for reducing noise interferencein C.W. reception," QST, vol. 19, p. 38; April, (1935). A similar circuit for telephonyreception is described by the same author in QST, vol. 20, pp. 44-45;February, (1936).8James J. Lamb, "A noise-silencing I.F. circuit for superhet receivers,"QST, vol. 20, pp. 11-14; February, (1936).