Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

the discriminator.IMPULSE NOISE 235The intermediate phase angles produce both typesof modulation.The fact that the 90-degree phase angle produces the equivalent offrequency modulation can be seen by referring to the vector diagramon the left. The vector a represents the carrier wave, the vector brepresents the noise wave at the instant of its peak value and the vectorc represents the resultant when a and b are added. For the first halfof the duration of the noise wave the vector b is growing. For the secondhalf it is diminishing. During the time interval when b is growing,the phase angle between a and c is growing. Since frequency isthe rate of change of angle, the frequency of the vector c is low duringthe growth of b and high while b is decreasing. If b is much smallerthan a so that the angle is equal to its sine, then the frequency deviationis proportional to the rate of change of 6.In an amplitude-modulation receiver the output isa maximum fora 0-degree or 180-degree phase angle and the output wave form followsthe envelope of the noise wave train. In a frequency-modulationreceiver the output isa maximum at a 90-degree or 270-degree phaseangle, and the output waveform is the first derivative of the envelopeof the noise wave train.It should be noted that the output of the discriminatorfor 270 degrees is inverted compared to that for 90 degrees.Columns 3 and 4 in Figure 3 repeat the same tests except that adifferent signal-to-noise ratio was used. For columns 1 and 2 thesignal amplitude was twice that of the noise.For columns 3 and 4 thesignal had half the noise amplitude. It can be seen that with thestronger relative value of noise, the noise shows up as a peak even forthe 90-degree and 135-degree phase angles.For 180 degrees the noisebucks out the carrier completely and in addition there is a lobe inwhich the r-f voltage is 180 degrees out of phase with the carrier. Inspite of these differences the output of the discriminator has verymuch the same waveform for one signal noise ratio as for the other.This may be due to the inability of the audio circuit to follow thehigher-frequency components of the true wave form. The fidelitywould probably have to be flat to several hundred kilocycles to followthe output waveform accurately.Effectiveness of LimiterOscillograms of this nature can be used to test the effectiveness ofvarious types of limiters as demonstrated by Figure 4.the first column of oscillograms isIn this figurefor a signal-to-noise ratio of 2 andshows the output of a plate voltage limiter. This type of limiter obtainsthe limiting action by using a very low plate voltage. The r-f plate