Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

duration but it encloses unit area.of energy at all frequencies.IMPULSE NOISE 241Unit impulse has the same amountIf a unit impulse is applied to an ideal low pass filter, the theoreticalsin 2irf toutput is,which is the pulsation illustrated. The energydistribution is uniform out to the cutoff frequency.If the first derivative of this pulsation is taken, the result is thepulsation illustrated at the bottom of the figure. The energy content ofthis wave is directly proportional to the frequency up to cutoff.Pig. 8—Various types of pulsations and the corresponding curves of relativeamplitude vs. frequency.It isapparent that many of the oscillograms representing the discriminatoroutput are similar to one or the other of these pulsations.Now the bandwidth of a frequency-modulation receiver is 200 kc, sothat from a modulation standpoint the cutoff is 100 kc. The energy contentin the audio spectrum isthat represented by the shaded area ineach case. It is obvious that much less energy is present in the audiospectrum of the pulsation illustrated at the bottom of the figure. Oneway of looking at it is that the low-frequency components of the wavetend to cancel out if the area in the pulse above the axis is equal to thearea below the axis.