The two internal FM operating modes are Locked and Unlocked. In the Locked and Unlocked modes, frequency modulation of the output signal is accomplished by summing the modulating signal into the FM control path of the YIG phase-lock loop.
If Locked is set, the YIG phase-locked loop is used in synthesizing the FM signal. If Unlocked is set, the YIG phase-lock loop is disabled and the FM signal is obtained by applying the modulating signal to the tuning coils of the YIG-tuned oscillator.