SLM-3650 Satellite Modem
Revision 3
Theory of Operation
MN/SLM3650.IOM
4–24
4.5.2 Plesiochronous/Doppler/Buffer
Data from the DEMUX section is fed into a buffer. This buffer size is user-selectable in
bit increments that correspond to the length of an IESS-308 satellite superframe. The
increments range from 1 to 32 ms. Refer to the interface specifications, 4.5.1, for a list of
valid entries for each of the selected formats. The buffer automatically centers on
resumption of service, either from the front panel or remotely. The startup buffer will
overfill when centering to match the satellite frame to the terrestrial frame with a
maximum slide of 0.5 ms.
The fill status is available as a monitor and is accurate to 1%. Overflows or underflows
are stored in the stored fault section of the M&C status registers, along with the date and
time of the incident (which are provided by the modem internal clock). These are stored
in battery-backed RAM.
A normal selection is to have the data clocked out of the buffer by an external clock. This
procedure removes the Doppler from the receive satellite data.
The operator may select from four other clock sources as a backup:
•
Insert clock (D&I mode only)
•
RX satellite
•
Internal clock source
•
TX terrestrial
Problems occurring on any of the selected clocks will cause the modem to substitute the
satellite clock and a fault will be signaled.
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