Ext Sync Frequency Function

How important is it to have external SYNC frequency function for BCM ref designs?

For those designs related to DSL/VDSL/ADSL chipsets, an external SYNC frequency function is very important. ADSL uses multiple bands spaced by 4kHz from 14kHz to 2.2Hz in an FDM (Frequency Division Multiplexing) arrangement. This can transmit channels of analog and digital TV with VDSL going much higher. Even with only a single line VDSL, if you have 2 unsynchronized bucks switching at f1 and f2, there will be harmonics at f1 and f2 as well as beat frequencies, f1+f2, f1-f2, and so on, which can create potential noise and interference with the sensitive DSL lines.

Here is a graph for further reference: