Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

LOCKED-IN OSCILLATOR RECEIVER 267experimental A-5581 tube has been found to meet these requirements.This tube issimilar to the 6SA7 but provides increased peak currentand increased mutual conductance.Effect of Intermediate-Frequency Selectivity on Lock-In RangeThe primary effect of intermediate-frequency selectivity is toattenuate the voltage on the No. 1 grid as the signal frequency movesdown the side of the selectivity curve. Naturally the oscillator cannotlock in if the incoming signal voltage becomes too small. This meansthat the bandwidth of the intermediate-frequency amplifier willaffectthe lock-in range. Figure 12 shows the variation of lock-in range withinput voltage. The range falls off very rapidly when the applied voltagefalls below the knee of the curve. The amplifier should be designed soit is broad enough to assure sufficient voltage to lock in the oscillatorat the maximum frequency swings encountered and also to provide fordrift and mistuning.Effect of Oscillator FrequencyThe lock-in range isin general inversely proportional to the oscil-|j-*- TC DISS.Pig. 13—Tuned-feedback coil.lator tank circuit C. An increase in the intermediate frequency willresult in an increase in the lock-in range only when C is correspondinglyreduced.Effect of Feedback WindingThe lock-in range will increase somewhat with increased mutualinductance from the tank coil to the feedback winding. Fairly tightcoupling should be used for increased range.A method which can be used to increase the lock-in range is to tunethe feedback winding to the second harmonic of the oscillator as shownby Figure 13. Capacitor C is chosen to tune the grid circuit to 1720kilocycles.This builds up the second harmonic, which in turn causesan increase in the fourth and sixth harmonics because of the nonlinearityof the tube. The result is an increase in the lock-in range.