Comtech EF Data CDM-570A - Manual (Page 140)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Front Panel Operation Revision 2
6–36
6.3.2.8 CONFIG: Ref (Reference)
1) IMPORTANT NOTE – The modem can accept an externally supplied frequency
reference via the rear panel BNC connector. Rather than bypassing the internal
reference and substituting the external signal, the modem instead uses the
internal reference in a low-bandwidth (~ 2 Hz) phase-locked loop (PLL), so the
modem actually phase locks to the reference external signal. There are distinct
advantages to this scheme:
• This scheme permits hitless switching between the operation of internal
and external reference. There are no sudden discontinuities of frequency
and phase in the transmitted carrier.
• Due to the very low bandwidth of the PLL, this scheme permits the external
reference to employ an inferior phase noise characteristic instead of the
internal reference of the modem. The narrow loop essentially ‘cleans up’
the external signal. This is particularly important if you use the CDM/R-
570ALto supply a 10MHz reference to a BUC or LNB.
2) If Internal+O/P is your active Frequency Reference setting, and you then set
the modem to CDM-570 Compatible Mode, the modem will reset this feature
to Internal 10 MHz. Reconfigure your setting as needed. See Sect. 6.2 for
further information.
Use the ▲▼arrow keys to select Internal+O/P, Internal 10 MHz, External 01 MHz, External 02
MHz, External 05 MHz, or External 10 MHz. Press ENTER.
Examples include:
Frequency Reference:
Internal 10 MHz(▲ ▼,ENT)
Frequency Reference:
Internal+O/P (▲ ▼,ENT)