Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

260 FREQUENCY MODULATION, Volume Itube generally used in this circuit has been an A-5581, an experimentalconverter tube, which is similar to the 6SA7 but has a higher mutualconductance. The oscillator tuned circuit is connected to the plate ofthe tube and the feedback coil is connected to the No. 3 grid. This gridis operated with self-bias. The received signal is applied to the No. 1grid of the tube through a 4300-kilocycle intermediate-frequency transformer.The No. 1 grid is likewise operated with self-bias.Description of DiscriminatorOne type of discriminator that can be used with the locked-inoscillator is shown in Figure 3. This circuit has a pair of diodes connectedwith their load resistors in opposition so the discriminator isbalanced at the center frequency. One diode has a tuned circuit inseries with it and the other has a tuned circuit across it. The discriminatoris connected across the tank circuit of the locked-in oscillatorFig. 3—Discriminator,through the coupling capacitor shown in the diagram. The audiofrequencyoutput from the discriminator is fed through a de-emphasisnetwork to the audio amplifier.Why the Oscillator Locks In with the Received SignalTheoretical and experimental evidence indicates that the locked-inoscillator circuit operates in accordance with the following theory.As previously stated, the oscillator is designed to lock in at one fifththe intermediate frequency. With an intermediate frequency of 4300kilocycles the oscillator tank circuit is tuned to 860 kilocycles. Whenno signal is being received the tube will function as a normal oscillator.The amplitude of the oscillation in a feedback oscillator is determinedby the curvature of the E g— Ip characteristic and is usually so greatthat the grid voltage swings well into the curved parts of the tubecharacteristic during the cycle. This means that a distorted output