Comtech EF Data CDM-570A - Manual (Page 539)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix G Revision 2
G–21
It may not always be possible to drive the powers to an optimum point. The calculations
take into account:
• The minimum and maximum power levels possible in the modem (which is affected by
the operating band – 70/140 MHz IF versus L-Band).
• If the user-programmed value that determines the maximum permitted increase in Tx
power for the terminal would be exceeded. This is a very important value, and you
must know exactly how far from compression his transmit RF chain is operating. As an
example, if, under normal clear sky conditions, you determine that the backoff from
compression is -3 dB, and under no circumstances must the RF chain ever go past -1 dB,
the maximum power increase that can be permitted is 2 dB. You enter this into the
CDM-570A/AL as a setup parameter. If this value is ignored, or incorrectly entered,
compression or saturation could occur, either during the initial re-balance, or during a
fade event. This parameter is referred to as TPLI
max
(Transmit Power Level Increase,
max).
• If the power changes calculated would put the CnC Ratio out of its working range.
• If the Rx signal level is above its minimum level – if it is not, the CnC Ratio monitor will
not be accurate, which is essential correct calculations.
• If the power change would result in the Eb/No margin on either side becoming negative
(see below).
At the end of the re-balance procedure, the modem will display the result:
• Full re-balance.
• Partial re-balance.
• Some issue was found that prevents a re-balance.
In the case of a partial or no re-balance, the modem displays the reason for this result; you
can then adjust an operating parameter and try again.
CAUTION – READ THESE IMPORTANT NOTES:
• You should be aware that after the re-balance has been carried out, it is very
likely that the CnC Ratio will not be at 0dB, and the Eb/No margins will not
be equal. This is particularly true for cases where there is an asymmetry in
antenna size, terminal G/T, symbol rate etc. This is an inherent function of
the re-balancing algorithm, and is not a cause for concern.
• You should only attempt to do a re-balance if it is known with certainty that
there are CLEAR SKY conditions at BOTH SIDES OF THE LINK.
It is not necessary for there to be a person at each end – the re-balance can
be initiated from either side, with the other side unattended.