this option improves the phase noise performance from typically -135 dbc (1 hz) down to -141 dbc (1 hz) at 1 ghz carrier frequency, 20 khz offset. furthermore the nonharmonics are lower with this option (typ. -100 dbc @ >10 khz carrier offset, f < 1500 mhz).
the option also brings additionally the analog modulation types fm / φm.