Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

LOCKED-IN OSCILLATOR RECEIVER 265Fig. 9—Input capacitance of discriminator.The equivalent input capacitance of the discriminator circuit shownin Figure 3 decreases rapidly with frequency near the center frequencyof the oscillator. Figure 9 shows how this capacitance falls off nearthe center frequency f. If the oscillator tank circuit is to be kept intune over the operating frequency range, the tank circuit capacitanceshould decrease with increasing frequency as shown by Figure 10. Theslope of this curve is determined by the L/C ratio of the tank circuit.The discriminator input capacitance characteristic can be designedto provide an apparent capacitance change with frequency nearly tomatch the requirements for tuning the oscillator.In Figure 11 the solid line represents the falling input capacitanceof the discriminator and the dashed line is the variation of capacitancerequired to keep the oscillator in tune as the frequency is varied. Ifthe two curves have approximately the same slope at the center frequency/ , the lock-in range will be greatly increased since only a smallamount of reactive current will shift the oscillator frequency a considerableamount.Effect of Signal VoltageIf the No. 1 grid is operated with self-bias so that the operating* z^=-Fig. 10—Oscillator-tuning capacitance.