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PSM-500/500L/500LT SCPC Satellite Modem
4. Set the A Demodulator Multiplexer to Custom mode (or match the B setup above).
5. If not using the ESC channel for some other reason then set the ESC Overhead to
"Disabled".
6. Set the MCC Overhead Rate to match that set for Site A above e.g. 1200 bps.
[ The link should now be working again from B to A. ]
7. Set the Modulator IF AUPC to "Enabled.
8. Set the Modulator IF AUPC Eb/No to that desired, e.g. 7.0 dB.
9. Set the Modulator IF AUPC Maximum Level to that desired allowing at least several dB
above the nominal operating point for margin, e.g. -10.0 dBm.
10. Set the Modulator IF AUPC Minimum Level to that desired, e.g. -32.0 dBm.
The link should now be operating and using AUPC to set the A transmit level keeping the B
receive Eb/No constant. Alarms will be generated if the transmit level reaches either the max or
min level attempting to maintain the receive E/b/No.
The exact settings required can be further adjusted to account for the type of fading expected.
For example if in a tropical environment with sudden heavy showers you may want to allow more
power margin and possibly speeding up the response time by using a higher speed MCC
channel.
To set the other direction to use AUPC, simply repeat the above instructions reversing the A and
B site.
3.9 Demodulator Receive Data FIFO Operation
The PSM-500 modem has a built-in First In First Out (FIFO) buffer on the receive data channel
that may be enabled to compensate for cyclical variations in the receive data rate or different
systems clocks at the two link ends. A receive buffer of this type is sometimes referred to as a
Plesiochronous buffer when the intent is to absorb different clocks on the transmit and receive
end. This type of clock difference is usually uni-directional and cumulative. Cyclical variations are
most often caused by the daily movement of the satellite in its position resulting in a varying
distance from earth station locations. This movement would cause the receive data rate to
increase during a portion of the day and decrease during other periods. This type of variation is
termed Doppler variation and the buffer to absorb the variation is a Doppler Buffer. If the daily or
weekly average rate is the same then this temporal variation can be absorbed by the receive
FIFO without ever losing data (assuming the FIFO is large enough). Other data rate variations
between the transmitting and receiving stations which are not periodic (that is average to zero)
can still be buffered by the FIFO, but will eventually result in lost data.
Operation of the FIFO requires two clock sources: one that clocks the data into the FIFO, which
is always the clock recovered from the received signal; and one that clocks the data out of the
FIFO. The "out" clock can come from one of four sources:
1. Receive Clock – (Option 0) Meaning that the input and output clocks are the same,
disabling the FIFO.
2. Internal Clock – (Option 1) Uses a dedicated modem internal NCO generated data
rate clock as the output clock. Use of this clock does not require that the modulator
and demodulator data rate be identical.
PSM-500/500L/500LT - Rev. 0.91
Site A AUPC Setup
Operation
Page 3-35
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