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CDM-Qx Satellite Modem Revision 5
Front Panel Operation MN/CDMQx.IOM
5–42
5.6.1.7 (ConfigRef) - REFERENCE OSCILLATOR
The modem 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 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. The value is changed using
the STarrow keys. The user should then press ENT. Values of Internal 10 MHz,
External 01 MHz, External 02 MHz, External 05 MHz, External 10 MHz, External
20 MHz, Out Int. 10 MHz, and Auto are possible.
The user should then press ENT. Two examples are shown below:
5.6.1.8 (ConfigMask) - MASK
Select Transmit, Receive or Reference, using the ◄ ► arrow keys, then press ENT.
Frequency Refer
e
nce:
Internal 10 MHz (▲ ▼, ENT)
Frequency Reference:
External 05 MHz (▲ ▼, ENT)
Alarm Mask: Transmit Receive
Reference BUC LNB (◄ ►,ENT)