L-Band IF Application Guide
Page L-Band - 8
handling the exposed end of the transmit cable with power applied. Any BUC
voltage represents a possible shock hazard, especially at higher voltages.
•
Second, when DC power is applied extreme care should be used with test
equipment. Many spectrum analyzers and power meters could be seriously
damaged by the application of DC to their inputs. A “DC Block” is a good safety
measure on equipment input.
•
Third, many common devices can be damaged by the application of DC power. For
example attenuators, directional couplers and combiners could be destroyed, and
at least cause the BUC to not work due to loss of power. This includes attenuators,
etc. that may be used to form an external test IF loob-back
3.6
Station Gain Budgets
Below is a block and level diagram of a typical station showing some example levels of relevant
signals in the transmit and receive chain.
DTE
O
M
T
UC/PA
"BUC"
G=57 dB
LNB
F
F
N
LMR-400 (200 ft)
RG6 (200 ft)
N
Example VSAT Signal Levels
L-Band Version
MAB 8/12/02
Transmit IF
Receive IF
M5 L-Band Modem
-13 dBm
-25 dBm
-65 dBm
-45 dBm
+32 dBm
There is virtually no control over the receive signal levels short of setting the antenna size. The
demodulator uses a sliding AGC window with an approximate AGC range of 55 dB at any given
data rate. As the data rate is decreased the AGC window moves down to accommodate the
decreasing signal level.
The transmit levels must be carefully controlled however. 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. The exception is notable however. If the amplifier is driven to its 1 dB
compression point and beyond the output level no longer increases. Some types of amplifiers
(like 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 Pout at the 1 dB
compression point. The maximum modem transmit output level is then computed based on the
BUC’s Pin max minus the transmit cable loss. The levels shown above assume a transmit cable
loss of about 12 dB.
3.7 Cable
Selection
Knowing what approximate levels are required at each point in the station block diagram permits
specification of required cable size and type. Several other factors enter here:
1. The transmit cable must also carry a heavy current on the order of 1 to 5 Amps to power
the BUC/PA combination. The DC resistance and cable voltage drop must allow this gear
