Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

LOCKED-IN OSCILLATOR RECEIVER 263fifth of the incoming signal frequency. When this happens the oscillatorwill "lock in" with the incoming signal. This means that theamplitude and phase of the plate current now remain fixed with respectto the incoming signal;with OA.vector AB now makes a constant angle CABIf the incoming signal is exactly five times the frequency of thetuned-plate circuit, the vector AB will be in phase with OA. As theincoming signal frequency is decreased, the vector AB rotates to someposition such as that shown in Figure 4. A further decrease in frequencywill rotate the vector until it is 90 degrees out of phase withrespect to OA. Since this position gives the maximum amount ofquadrature current itcorresponds to the maximum amount the oscillatorfrequency can be pulled over,the lock-in range.and thus gives the lower limit ofIf the incoming-signal frequency becomes greater than fivethe plate-circuit frequency, the conditions willtimesbe similar except thatthe vector AB will be lagging as shown by Figure 6 instead of leading.The upper limit of the lock-in range is reached when the injectedcurrent lags by 90 degrees. The lagging current tends to reduce theeffective capacitance of the circuit and thus raises the frequency.When the sixth and fourth harmonics are both present simultaneously,it can be shown that the result is a single injected current ofvariable amplitude and phase.This causes the frequency of the tunedcircuit to swing back and forth in accordance with these variations;the process isvery similar to that already explained when the fourthharmonic only is present. Usually, the fourth and sixth harmonicswill be of unequal amplitude and the effect of the weaker one is toproduce relatively small variations in the other.Lock-In Range RequirementsAs previously stated by restricting the lock-inrange of the oscillatorto frequency variations in the desired channel a material improvementin selectivity can be obtained. On the other hand, it is necessarythat the lock-in range be adequate toof the received signal and infollow the frequency variationsaddition provide for receiver mistuningand frequency drift in the transmitter and receiver.The effect of the fourth and sixth harmonics in controlling thelock-in range of the oscillator has been previously discussed. Theamount of fourth and sixth harmonics on the No. 3 grid of the oscillatoris limited, and this limits the lock-in range. When the deviationexceeds the lock-in range the oscillator breaks out and starts backtoward the center frequency since it is no longer controlled. The