Mocomtech CDM-QX - Manual (Page 102)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
Front Panel Operation MN/CDMQX.IOM
5–36
When X displays, as per this example, this means that there are more channels available
beyond Channel 6. Use right arrow key X to view more, making sure that
ENTER
is
pressed once the selection is made.
CONFIG: Interface
Æ QDI Æ 1, 2, 3, or 4 Æ LineCode
Port#X Line Code:
HDB3 AMI (◄ ► ENT)
Select HDB3 or AMI using the W Xarrow keys, then press
ENTER .
HDB3
Selects High Density Bipolar Three Zeros Substitution line coding.
AMI Selects Alternate Mark Inversion line coding
CONFIG: Interface Æ QDI Æ 1, 2, 3, or 4 Æ Alarm
Port#X Bipolar Violation:
Active Masked (◄ ► ENT)
Select Active or Masked using the W X arrow keys, then press
ENTER . The default is
Masked (i.e., no alarm is generated).
5.4.7 CONFIG: Ref
The modem can accept an externally supplied frequency reference via the REF 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 low-bandwidth (~2Hz) phase-locked loop
(PLL), so the modem actually phase locks to the reference external signal. There are two distinct
advantages to this scheme:
a. 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.
b. 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 modem. The narrow
loop essentially ‘cleans up’ the external signal. This is particularly important if the
modem is being used to supply a 10MHz reference to a BUC or LNB.
Edit the configuration and value of the frequency reference using the ST arrow keys. Press
ENTER
when done. Available selections are as follows:
• Internal 10 MHz • External 10 MHz
• External 01 MHz • External 20 MHz
• External 02 MHz • Out Int. 10 MHz
• External 05 MHz • Auto
Two configuration examples are as follows:
Frequency Reference:
Internal 10 MHz (▲ ▼,ENT)
Frequency Reference:
External 05 MHz (▲ ▼,ENT)