Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

IMPULSE NOISE 237the first and second except that the noise amplitude is twice that of thesignal. For a phase angle of degrees the plate voltage limiter stilldoes a good job, but the defects of the grid-leak limiter are magnified.For a phase of 180 degrees even the plate-voltage limiter fails as itobviously must since the phase of the resultant wave is reversed duringthe central portion of the noise pulse.Noise with Carrier DetunedAs a matter of fact, these oscillograms are not as important as theyseem at first sight. As was shown in Figure 3, amplitude modulationdoes not come through the discriminator when the noise pulse and thecarrier are synchronous.Thus a limiter does no good under conditionsof no modulation if the signal carrier is tuned to the center of the passband.Actually, the limiter action may make the noise worse, as was shownin Figure 2 in which grid current was shown to detune the correspondingtuned circuit. This is also illustrated in the bottom oscillogram ofthe third column of Figure 4.It will be noted that the valley at the endof the lobe is not as deep as it should be. This is a sign of detuningdue to grid current.The only time a limiter helps to reduce the noise is when the carrierisnot in the center of the pass band because of mistuning or modulation.If the carrier wave is tuned to the edge of the pass band, there isa beating action between the noise wave train and the carrier wave.The frequency of the beat note is /„ where /„ is one-half the bandwidthof the receiver.Thus, the beat note goes through one cycle in a periodof time equal to l// c. However, the duration of the pulse itself is alsol/f (this is accurately true in a sharp cutoff filter and roughly true inany filter depending on what per cent of peak amplitude is called cutoff).Thus, the beat note goes through one complete cycle in the durationof the noise wave train. This is illustrated in Figure 5. In thefirst oscillogram of the first column, the noise wave starts out 180degrees out of phase with the carrier wave. As the noise wave grows,it gradually changes its phase relative to the carrier so that at thepeak of the pulse it is in phase. The relative phase continues to changefor the duration of the pulse so that at the end of the pulse it is again180 degrees. Since the relative phase assumes all possible values in theduration of each pulse it is difficult to assign a value to it. However, therelative phase at the peak of the pulse has a definite meaning and iton this basis that the various oscillograms of the first column arelabelled. It can be seen that as the relative phase is changed, the valleyof the beat note travels progressively across the noise pulse. The corre-is