Addition and Subtraction

There is yet another method of producing an AM wave; this method makes use of the fact that two phase-modulated waves may be combined in a common load to produce a wave which is amplitude-modulated. The principle of this is not difficult to understand. Two signals which are equal in amplitude but 180 out of phase with each other, fed into a common load, will cancel, so that the total output will be zero. If these same signals are in phase, they will add together to produce a large output. By varying the phase angle between these two signals, we can vary the amplitude of the combined output. High-power transmission making use of this principle of modulation was first achieved by H. Chireix of France in the early 1930s.

Figure 64-1 shows on a vector diagram the conditions which exist during various portions of the modulation cycle. The two signals A and B, when in the relationship corresponding to carrier level, are separated in phase by an angle of 135 , as shown by the vectors A _c and B _c. The vector sum of A _c and B _c yields the carrier-level output amplitude indicated on the diagram. Now, if A is delayed in phase and B is advanced in phase, so that they are 90 apart, as shown by the vectors A _p and B _p, the resultant vector sum will have an amplitude of approximately...