Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

232 FREQUENCY MODULATION, Volume 1output of the discriminator goes plus or minus on the oscillogramaccording to whether the selectivity curve has more area above or belowthe frequency of resonance of the discriminator secondary. OscillogramH of Figure 1 shows the result of having the low-frequency peakexceed the high-frequency peak and of attempting to compensate bydiscriminator tuning. The indication is that the frequency of thepulse varies slightly with time for this condition.The output network of the discriminator consists of two separatediode output circuits connected in series. The fidelity of these twocircuits should be the same for a good balance, but this is not usuallyLow Gain Moderate Gain High GainWaveTrainC=1Q0;u,ufDiscriminatorOutputWaveTra i nO600;ujufDiscriminatorOutputFig. 2—Oscillograms of noise wave train and output of discriminatorshowing detuning due to grid current at higher amplitudes.obtained. The output circuit is grounded on one side instead of inthe center. This usually results in more capacitance across one sidethan across the other. This can be compensated for by adding capacitanceto the other side. The effect of a capacitance unbalance of 10micromicrofarads across 100,000 ohms is shown at I in Figure 1.Change of Tuning with Signnal StrengthAn unfortunate effect is the change in tuning with signal strength.This is illustrated in Figure 2. The oscillograms of this figure weretaken in the output circuit of a limiter. Limiter action was obtainedbecause of grid current and because of the low plate voltage employed.