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E.3
Receive Clocking
There are three receive clocking modes in the CDM-425 Advanced Satellite Modem: Buffer
Disabled (RX Satellite), Buffer Enabled (TX=RX), and Buffer Enabled (RX< >TX). An additional
setting used only for Drop and Insert is described in further detail later in this appendix. Figure
E-2 provides a graphic depiction of these modes.
E.3.1
Buffer Disabled (RX Satellite)
When the buffer is disabled, the receive clock (Receive Timing, or RT) is derived directly from the
demodulator, and will therefore be subject to plesiochronous and Doppler offsets. This may
acceptable in certain instances. There is still a minimum buffer in use to de-jitter the effects of
removing overhead framing.
G.703: Applicable.
E.3.2
Buffer Enabled, TX=RX (TX Terrestrial or Int (SCT) Clock)
In this instance, it is required that buffer be enabled, so that the clock and data appearing on
Receive Timing and Receive Data (RT and RD, respectively) are synchronous with the transmit
clock or the internal (SCT) clock. This is a relatively simple case, as the output clock for the buffer
is derived directly from ST, TT, or the external source.
G.703: Applicable.
E.3.3
Buffer Enabled, RX<>TX (TX Terrestrial or Int (SCT) Clock)
This is an uncommon case, where the receive data rate does not equal the transmit or external
reference. The modem will generate a phase-locked buffer output clock that uses the selected
reference, regardless of its frequency in relation to the receive data rate.
G.703: Applicable.
Appendix E
CDM-425 Advanced Satellite Modem
E–4
Revision 0
MN-CDM-425
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