L-Band IF Application Guide
Page L-Band - 4
2.2
Transmit Output Power Levels
The PSM-4900L has a very wide range of power levels available from the transmit output. This is
to accommodate direct connection to a standard BUC including significant cable loss without the
need for inline amplifiers or attenuators, and to accommodate transmit combiners. The PSM-
4900L can output from –35 dBm to +5 dBm in 0.1 dB steps. This 40 dB range can accommodate
a wide range of cable length and BUC gain. Assuming for example that with a BUC gain of 60 dB,
and a 4 Watt maximum output (+36 dBm) the required BUC input to achieve full output power
would be –24 dBm. The modem then could drive up to a maximum of 29 dB of cable/connection
losses. This could be a maximum of 100 to 500 feet or more depending on the size and type of
cable used. More about cable selection is provided in Section 3 below on designing and setting
up an L-Band station.
2.3
Receive Input Power Levels
The PSM-4900L has increased the range of power levels acceptable to the receive input. This is
to accommodate direct connection of a standard data grade LNB including significant cable loss
or inline splitters without the need for inline amplifiers or attenuators. The PSM-4900L can accept
a window of approximately 55 dB at any given data rate. The input level range changes with data
rate. When considering the full data rate range of 1.2 kbps (BPSK, rate ½) to 4.92 Mbps (QPSK,
rate ¾ or 7/8) this results in a total range of approximately from –20 dBm to -102 dBm. The
modem automatically adjusts the range for the data rate used and the user is warned if the level
is marginal. Of course, if the level is below the AGC capability then the modem will not acquire
signal lock. This approximate 55 dB range at any particular data rate can accommodate a wide
range of cable length and LNB gains. The LNB gain minus the cable loss should always fall within
the range of 40 dB to 70 dB of overall gain. As long as this gain is achieved, the demodulator will
function properly at all data rates from 1.2 kpbs to 4.92 Mbps requiring no further system level
engineering. For example a typical data grade LNB has a gain of approximately 60 dB. This
would allow for up to 20 dB of cable loss at any data rate. Like the transmit this allows a
maximum cable length of approximately 100 to 400 feet depending on the size and type of cable
used. The LNB gain and cable loss variations due to temperature changes are less important on
the receive side as long as the overall gain range above is met at all times.
The PSM-4900L Receive input provides a direct impedance match to a typical 75 Ohm LNB.
More about cable selection is provided in Section 3 below on setting up an L-Band station.
The user does not have to specify the input power level. The modem AGC locks to the signal and
reports the receive signal level as a front panel parameter under “DEMOD INPUT LEVEL”
2.4 New/Modified
Commands
New Commands relative to the 70 MHz modem are all directly related to L-Band operation. Each
is represented by a new “parameter entry” in the front panel matrix. All of the Modulator BUC
commands are contained within one column of the Modulator parameter matrix, and all of the
Demodulator LNB commands are contained within one column of the Demodulator parameter
matrix Two new binary packet commands are also included in the command protocols. Modified
commands have modified entry parameters from the 70 MHz modem commands.
2.4.1 New
Commands
“
MOD BUC – Power
” – Transmit BUC Voltage Enable/Disable.
“
MOD BUC – Voltage Out
” – Reading of Voltage sent to BUC when it is enabled.
“
MOD BUC – Voltage Min
” – Alarm setting for minimum voltage sent to BUC. Can be used to
warn if the voltage is below the minimum.
“
MOD BUC – Current Out
” – Reading of current sent to BUC when it is enabled.
