Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FM EXCITER 169Data on many other circuits intended to accomplish automatic frequencycontrol have been published. A number of these make use ofthe scheme shown in Figure 6. The signal from the oscillator to becontrolled is fed to two amplitude-modulation detectors or mixers. Thesignal from the crystal-controlled reference oscillator is also fed to thedetectors, but through circuits that shift phase so that there is a 90-degree displacement of the reference frequency between the two detectors.The resulting beat frequency of the two oscillators will appearin the output of each detector, and because of the 90-degree phase shiftof one of the exciting frequencies, the beat frequencies will also bedisplaced by 90 degrees. The interesting feature of the circuit is that,Fig. 5as the frequency of the oscillator, being compared to the frequencyof the crystal, is caused to change through the synchronous or zero-beatcondition, there is a reversal in phase of one beat-frequency outputwith respect to the other.There are many ways in which to make use of this phase reversalto provide an indication as to whether the controlled oscillator frequencyis high or low. Several methods were investigated. The mostpromising idea was to make use of the fact that the two beat notesdisplaced by 90 degrees constitute a source of two-phase power and