I would like to know if the AD9548 would improve the phase noise performance in
comparison with the AD9518 for my application?
The AD9548 (if driven with an 50 MHz crystal on the system clock) may or may
not have better phase noise. It will depend on the desired loop bandwidth for
the users. If the user has a relatively quiet input reference signal, then the
AD9518 will probably be better.
The AD9548 excels in low loop bandwidth applications. Referring to Figure 6 in
the datasheet, at 1 kHz loop BW, the AD9548 outperforms the AD9518. Setting the
loop bandwidth to 100Hz will actually improve the close-in phase noise of the
AD9548, and make the phase noise plot from 100 Hz to 10 kHz more of a straight
line instead of a hump. If you compare Figure 6 in the AD9548 datasheet with
your modeling results, you’ll see the difference in performance.