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P300H P300 Series Modem Installation and Operating Handbook Page 90
Rx output clock: [?]
1=Sat 2=Tx 3=Internal 4=Station 5=DCE
Change, Rx, Buffer / Clocking, Rx Clock (non-Insert) Menu
Rx output bearer clock source: [?]
1=Sat 2=Tx 3=Internal 4=Station
Change, Rx, Buffer / Clocking, Rx Clock (Insert-Loop) Menu
Rx o/p bearer backup clk : [?]
1=Sat 2=Internal 3=Station
Change, Rx, Buffer / Clocking, Rx Clock (Insert-Loop) Menu
If the external 10MHz reference fails totally then it will be automatically replaced with the internal reference.
If however it degrades (level, frequency or phase noise), then until it fails totally the modem will
continue to use it as the synthesiser reference.
6.7.5 Change, Rx, Buffer / Clocking, RX CLOCK Menu
This screen varies depending on the configuration of the Rx path
Normal (non-Insert Mux operation)
This selects the source of the clock which feeds the terrestrial output:
Insert Mux operation (generated bearer)
This selects the source of the clock for the PCM BEARER onto which the Rx data is inserted:
Insert Mux operation (looped bearer)
This selects the source of the clock for the BACKUP PCM BEARER, which is only used if the normal
bearer looped from the Drop Mux fails and the unit auto generates a backup bearer onto which to insert
the Rx data. Note that this screen does not display a Tx option, as it is the failure of the Tx bearer input which
causes this backup clock to be used in the first place.
All the options available on any of these menus for the Rx output clock are as follows:
SATELLITE The clock from the satellite. This provides `Buffer Disable` operation, the buffer
size is set to zero and the buffer size menu is no longer available. This is the
same as `Rx=Rx` clock options found on some other equipment. The Rx PLL
converts the satellite clock which may include framing and or Reed-Solomon
overheads back to the rate required at the terrestrial port. If the demodulator
cannot lock onto a carrier (ie no clock from satellite), then the PLL switches to the
internal frequency reference to provide a steady clock towards the terrestrial port.
TX CLK IN The Tx path input clock is turned around to drive the Rx data back to the
terrestrial. This is normal `Plesiochronous Mode`, and the buffer should be set
large enough to accommodate both the difference in the clocks at each end of
the link PLUS the Doppler shift, whilst providing the required interval between
slips.
If the Tx & Rx rates are equal then the Tx clock is used directly to clock out the
Rx data, should the Tx input clock fail the clock is switched to a PLL using as first
backup the Station Clock if defined as available, or if the Station Clock is defined
as not available then recovered Satellite Clock (while demodulator remains
locked). Should this first backup fail it finally falls back to the internal 1PPM