Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

262 FREQUENCY MODULATION, Volume IFig. 5—Effect on circuit.and thus will have an angular velocity relative to OA equal to thedifference of the two angular velocities.Now consider the instantaneous condition shown in Figure 4.Theinjected current AB has a component AC in phase with OA and anothercomponent AD, 90 degrees out of phase with respect to OA. Let thisresultant current OB be applied to a tuned circuit LC as shown inFigure 5. Since the LC circuit is tuned to 860 kilocycles, it will be atresonance with respect to the current OC which isalso 860 kilocyclesand equals i a + i b. The quadrature current AD is a leading currentat the instant shown by Figure 4, and the result is the same as thoughan additional condenser C is in the circuit. The effect is to decreasethe natural frequency of the tuned circuit.Now consider the condition at a later instant as shown by Figure 6.Since the vector AB is rotating with respect to OA it has now rotatedto the new position as shown. The injected current AB now has anin-phase component AC as before, but the component AD is nowlagging instead of leading. If this current OB is now impressed onthe circuit of Figure 5,the lagging component AD will cancel part ofthe leading current through C and this will be equivalent to reducingthe capacitance C since the circuit is now drawing a smaller leadingcurrent. This will raise the resonant frequency of the tuned circuit.It is now evident that the circuit of Figure 2 behaves like a reactancetube and swings the frequency of the tuned circuit back andforth. It is easy to see that if the frequency of the incoming signalis approximately five times that of the tuned circuit, a point will bereached when the frequency of the tuned circuit becomes exactly oneFig. 6—Vector diagram.