If isolation is high, directional couplers are
excellent for combining signals to feed a single line to a
receiver for two-tone receiver tests. In Figure 3, one signal
enters port P3 and one enters port P2, while both exit port
P1. The signal from port P3 to port P1 will experience 10
dB of loss, and the signal from port P2 to port P1 will have
0.5 dB loss. The internal load on the isolated port will
dissipate the signal losses from port P3 and port P2. If the
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