injection phase locking. Because of the symmetry of the directional coupler, the reverse injection will happen with the same
possible modulation problems of signal generator F2 by F1. Therefore the isolators are used in Figure 3 to effectively
increase the isolation (or directivity) of the directional coupler. Consequently the injection loss will be the isolation of the
directional coupler plus the reverse isolation of the isolator
isolators in Figure 3 are neglected, the isolation
measurement (port P2 to port P3) determines the amount of
power from the signal generator F2 that will be injected into
the signal generator F1. As the injection level increases, it
may cause modulation of signal generator F1, or even