This read-write object selects the frequency reference source: internal 10 MHz(0), or one of five external reference options (1 MHz, 2 MHz, 5 MHz, 10 MHz, or 20 MHz). Unlike the CDD-564's internal-only reference object, this CDM-570 object supports locking to an external site reference, which matters for networks requiring tight frequency coherence across multiple co-located or geographically distributed modems sharing a common precision house reference rather than each running on its own independent internal oscillator. At a hub site with a precision 10 MHz house reference distributed to all equipment racks, an admin would set this to ext10MHz(4) so this modem locks to that shared reference rather than drifting independently on its own internal oscillator.