User guide

Appendix C - Dynamic Power Control C-17
MN/22137, rev 1 Signal Power Level Considerations
A good note here is that, with the typical LO frequencies as shown in the exam-
ple above, transmit and receive L-band frequencies are widely separated. If the
signals were within the LNB stability/drift frequency limits, there might be a
tendency for the receive side to attempt locking to its own transmit signal.
Considering the L-band IF range is 950 MHz or more spanning close to an
octave, the variation in loss between the high and low ends of the IF range may
be significant.
A nominal design point may be to allow for 10 to 15 dB of total cable loss and
select cable that will reliably achieve this. A more accurate “rule of thumb”
would be to design for a total gain from the antenna to modem receive input of -
40 dBm. For example, if the receive LNB has a gain of 60 dB and there are no
other losses, then the cable can have a maximum loss of 20 dB (60 – 40 dB). In
formula form this is:
Loss (cable max) = Gain (LNB) – 40 – Loss (misc) in dB
Or, for the transmit side, the cable loss should not keep the input to the BUC
from reaching its Power input for 1 dB compression point. Arbitrarily allowing
a 3 dB margin in the modem output, this formula would be:
Loss (cable max) = +3 – BUC Pin (1dB) – Loss (misc) in dB
Notice that this assumes no miscellaneous losses in the transmit or receive side,
but there may be other losses such as from a splitter or output sample port, if
used.
See table 6-1 below for typical cable characteristics (note that cable prices are
estimates only).
Table C-3 Typical Coaxial Cable Characteristics
Cable Type Max. Freq.
(MHz)
O.D.
(inch)
Loss/100
feet (dB)
@1.2 GHz
Shielding
Efficiency
(dB)
Estimated
Cost/ft.
(USD)
RG58 (50Ω) 1,000 0.19 21.0 70 0.39
RG59 (75Ω) 1,000 0.25 18.0 70 0.39
RG6 (75Ω) 2.200 0.25 10.0 >90 0.89
RG11 (75Ω) 2.200 0.405 5.0 >90 0.89
Times LMR-240 (50Ω) 5,000 0.24 9.2 >90 0.47
Times LMR-300 (50Ω) 5,000 0.30 6.8 >90 0.53