Synchronous Carriers

This gives rise to the need for synchronizing the carrier frequencies. An analysis of the equation for B (FM) shows, given a fixed carrier ratio (fixed modulation index), an increase in carrier frequency separation is equivalent to an increase in delta f, which for FM is equivalent to an increase in detected signal amplitude (see Equation 3 below).Or, graphically, this phenomenon is shown in Figure 1 (below).

Knowing this, the advantages of frequency locking the carriers becomes obvious. By taking the limit as the difference in carrier frequency approaches zero, two things happen. First, the frequency of the detected tone approaches zero, and the equivalent FM deviation produced by that tone approaches zero. In other words, the interference disappears.

Equation 3: FM Modulation Index, Alternate Form If, given B = delta f/Fm, B is held constant, and Fm is the frequency separation of the carriers, then: delta f = B * Fm	Figure 1.(below) RELATIONSHIP OF CARRIER FREQUENCY SEPARATION TO EQUIVALENT CARRIER FREQUENCY DEVIATION (B = 1)

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