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P300H P300 Series Modem Installation and Operating Handbook Page 91
reference in order to maintain the Rx traffic. If the modem is operating with
asymmetric data rates, then the Rx PLL converts the Tx clock to the correct rate
for the Rx data. Should the Tx clock input fail the PLL switches to the internal
reference as backup in order to maintain the Rx traffic.
INTERNAL The internal 1PPM frequency reference is used to source the clock towards the
terrestrial. Again the buffer should be set large enough to accommodate the
difference between the internal reference and the incoming satellite clock, PLUS
the Doppler shift whilst providing the required interval between slips.
STATION If the Station Clock is the same frequency as the Rx data rate, then the
Station Clock is used directly, switching to the Satellite Clock (whilst the demod
is locked), and finally the internal reference for backups. If the Station Clock is
at a different frequency to the Rx Data rate the Rx PLL converts the Station
Clock to the correct rate to drive the Rx data towards the terrestrial. Should the
Station Clock fail then the modem will switch to the internal frequency reference
in order to maintain the Rx traffic. Again the buffer should be set large enough to
accommodate the difference between the Station Clock and the incoming satellite
clock, PLUS the Doppler shift, whilst providing the required interval between slips.
Note: If the Rx is set to Station Clock AND the Tx Clocking is set to Rx ref(Stn),
then it always operates as defined in the para above for the case where the
“Station Clock is at a different frequency to the Rx Data rate” (even if the are the
same rate).
DCE CLK This option is only available if a DTE card is fitted to the modem. The DTE card
`Ext Tx Clock In`, ie the clock from the DCE (see interface card details in
appendix A for line names for other interfaces) is used directly as the Rx output
clock, should the `Ext Tx Clock In` fail the PLL sources the clock, locked first to
the Station Clock (if available) or the recovered satellite clock (while the
demodulator remains locked), and finally the internal frequency reference in order
to maintain the Rx traffic.