PSM-500/500L/500LT SCPC Satellite Modem
Operation
PSM-500/500L/500LT - Rev. 0.91
Page 3-33
3.7.2 SCPC Mode
Independent –
Each station of two linked SCPC modems is considered independent. The
transmit data clock is either an input to or output from each station modulator. The other station
receive data clock maintains this synchronization. The clocking in each direction is independent
and follows the same transmit to receive synchronization.
Master/Slave –
One station of two linked SCPC modems is considered the master and the other
station is considered the slave. The master transmit data clock is either an input to or output from
the master station modulator. The slave station receive data clock maintains this synchronization.
The receive data clock is used to generate a contra–directional transmit data clock (from
modulator to DTE) of the same frequency, but not necessarily phase, as the receive data clock.
3.7.3 Transmit Interface Clock Auto Mode
The PSM-500 Modem uses a transmit clock option called “Auto”, which is now the default setting.
The new clock mode appears in the "Interface I/O" menu column under “Xmt Clock” and is not
settable. The modem measures the phase relationship between the transmit clock and data and
automatically sets the clock phase correctly. This gives improved performance on slightly longer
data cables when operating at bit rates above approximately 1.5 Mbps.
3.8 Automatic Uplink Power Control (AUPC) Operation
The PSM-500 modem has built-in logic for Automatic Uplink Power Control (AUPC). AUPC
attempts to maintain a constant E
b
/N
o
at the receive end of an SCPC link by adjusting the
transmit power at the transmit end of the link. This is especially useful when operating over a
satellite at Ku-Band frequencies in locations with high rainfall periods.
The AUPC function requires a data channel at 300 to 1200 baud in order to operate. This
data channel can either be external to the modem (that is provided by an external
multiplexer or telephone line modem) or provided by the internal IBS multiplexer when
enabled.
Note: The “Enhanced” or “Custom” Multiplexer mode MUST be selected to provide a
channel for AUPC operation from the IBS multiplexer option.
The internal data multiplexer in “Enhanced” mode provides a 300 baud service channel between
the two sites of a link permitting the modem processors to send messages and get responses
over this channel. AUPC can be set to operate on either or both directions of a link but always
requires a bi–directional channel. The AUPC functions and their descriptions are shown in the
table below:
Function
Description
AUPC ENABLE/DISABLE
Enables/Disables the AUPC to function locally.
MOD AUPC E
b
/N
o
Desired E
b
/N
o
of remote modem.
MOD AUPC MIN LVL
Sets minimum output power to be used.
MOD AUPC MAX LVL
Sets maximum output power to be used.
The basic AUPC operation is described as follows: Assume that the two modems, one at each
end of the link, are set up for AUPC operation. Only one direction is discussed, controlling the
Eb/No at Site B, but the same functions could be occurring in both directions simultaneously. This
is shown in the Figure 3-4 below. Modem “A” is transmitting to modem “B” under normal
conditions and modem “B” has a receive Eb/No of 7.5 dB. Modem “A” has been set to an AUPC
