Signal Power Level Considerations - Comtech EF Data Vipersat SLM-5650A User Manual

Signal Power Level Considerations

There are many parameters to consider when planning and commissioning a site
for satellite transmission. Among the most important are selections for location,
antenna size, and High Power Amplifier. Do not forget entrance link cabling.
These pre-selections determine maximum operability of the site in freedom of
range and limitations. Uplink power gain limitations and cable losses are the
main focus of this section.
The following sub-sections describe the basic segments in adjusting uplink
gains to properly achieve a balance between minimum and maximum transmis-
sion throughputs.
The example block diagram shown in figure C-7 represents a typical remote
station with relevant signal power levels in the transmit and receive chain.
DTE
There is virtually no control over the receive signal levels short of setting the
antenna size, as the demodulator uses a sliding AGC window with an approxi-
mate AGC range of 55 dB at any given data rate.
However, the transmit levels must be carefully controlled. Most BUCs have no
internal gain control and therefore represent a fixed gain block. The output
power is thus directly proportional to the input level, up until the amplifier is
driven to its 1 dB gain compression point (P1dB), beyond which the output
level no longer increases (nonlinear). Some types of amplifiers (such as TWTs)
will actually result in a lower output level as the input is increased. What is
important, therefore, is to know the maximum input level of the BUC, or the
gain and P
A p p e n d i x C - D y n a m i c P o w e r C o n t r o l
L
M5 L-Band Modem
MODEM
A
N
Figure C-7
at the 1 dB compression point. The maximum modulator output
out
S i g n a l P o w e r L e v e l C o n s i d e r a t i o n s
-13 dBm
N
Transmit IF
F
Receive IF
-65 dBm
Signal Power Levels, Remote Site
+32 dBm
-25 dBm
UC/PA
N "BUC"
LMR-400 (200 ft)
G=57 dB
RG6 (200 ft)
-45 dBm
Example VSAT Signal Levels
L-Band Version
MAB 8/12/02
O
M
T
LNB
F
C-15