Config) Reference - Comtech EF Data CDM-570L Installation And Operation Manual

CDM-570/570L Satellite Modem with Optional IP Module
Front Panel Operation
6.11

(CONFIG) REFERENCE

IMPORTANT NOTE: The CDM-570/570L can accept an externally supplied frequency
reference, using the BNC connector on the rear panel. However, rather than bypassing the
internal reference, and substituting the external signal, the internal reference is used in a
IMPORTANT
low-bandwidth (~ 2Hz) phase-locked loop (PLL), so the CDM-570/570L actually phase
locks to the reference external signal. There are two distinct advantages to this scheme
1) It permits hitless switching between the operation of internal and external reference.
There are no sudden discontinuities of frequency and phase in the transmitted carrier.
2) Due to the very low bandwidth of the PLL, it permits the external reference to have
an inferior phase noise characteristic than the internal reference of the CDM-
570/570L. The narrow loop essentially 'cleans up' the external signal. This is
particularly important if the CDM-570L is being used to supply a 10MHz reference
to a BUC or LNB.
Edit the configuration and value of the frequency reference. The value is changed using
the
▲ ▼
External 01 MHz, External 02 MHz, External 05 MHz, External 10 MHz, and
External 20 MHz, are possible. The user should then press ENTER. Two examples are
shown below:
Frequency Reference:
Internal 10 MHz (▲ ▼,ENT)
Frequency Reference:
External 05 MHz (▲ ▼,ENT)
arrow keys. The user should then press ENTER. Values of Internal 10 MHz,
6–32
Revision 4
MN/CDM570L.IOM