PSM-2100L Satellite Modem Addendum
Page L-Band - 4
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-2100L has increased the range of power levels acceptable to the receive input. This is
to accommodate direct connection (through a bias T mux) of a standard data grade LNB including
significant cable loss without the need for inline amplifiers or attenuators. The PSM-2100L can
accept a window of approximately 60 dB at any given data rate. The input level range changes
with data rate. When considering the full data rate range of 3.6 kbps (BPSK, rate ½) to 2.1 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 60 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 35 dB to 70 dB of overall gain. As long as this gain is achieved, the demodulator will
function properly at all data rates from 3.6 kpbs to 2.1 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 25 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 unimportant on the
receive side as long as the overall gain range above is met at all times.
The standard Datum Systems supplied Receive Bias T Mux provides impedance conversion from
a 75 Ohm LNB and cable to the 50 Ohms used by the demodulator. An additional cable loss of 6
dB should be added to the input level range calculation. More about cable selection is provided in
Section 3 below on designing and 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, and a new binary and ASCII
command code in the command protocols. Changed commands have modified entry parameters
from the 70 MHz modem commands.
2.4.1
New Commands
“
MOD Cnvrter LO
” - Transmit Converter LO Frequency – Input to a non-zero value allows direct
RF frequency entry (see Section 2.1)
“
MOD Spectrum
” - Transmit Spectrum Sense – May be set Normal or Inverted to correct for a
converter mixing scheme that results in a spectrum inversion.
Note: Set to “Normal” when
entering a “MOD Cnvrter LO” not equal to zero. (see Section 2.1)
“
DEMOD Cnvrter - LO
” Receive Converter LO Frequency – Input to a non-zero value allows
direct RF frequency entry (see Section 2.1)
“
DEMOD Spectrum
” - Receive Spectrum Sense – May be set Normal or Inverted to correct for a
converter mixing scheme that results in a spectrum inversion.
Note: Set to “Normal” when
entering a “DEMOD Cnvrter LO” not equal to zero. (see Section 2.1)
2.4.2
Modified Commands
