Comtech EF Data CDM-760 - Manual (Page 105)

Front Panel Operation Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
6–19
Table 6-3 and Table 6-4 list the permitted ModCod, modulation, and code rate combinations.
These tables have some roundoff of the last number in the Data Rate Range columns.
(CONFIG: Tx) ACM ConfigUnlock-Action
When distant-end demodulator loses lock:
Go to min Tx ModCod (Maintain,Min) ()
The modem sends and receives ACM messages roughly every 250 milliseconds. This Unlock-
Action occurs when the remote demodulator loses lock and the modem misses three
consecutive ACM messages.
Use the  arrow keys to select an action. Press ENTER.
Comtech recommends that you use “Min” as the go-to option. This is important,
because the ACM system depends on the feedback of the SNR metric from the
remote demodulator to find the optimum ModCod.
(CONFIG: Tx) ACM ConfigTarget-EsNo-Margin
Target Es/No Margin = 1.0 dB (0.0 – 4.5)
()
Use the  arrow keys to select a margin value. Press ENTER. Valid settings are:
Range
Step Size Default
Minimum Maximum
0.0 dB 4.5 dB 0.1 dB 1.0 dB
The ACM system is designed to switch based on thresholds that correspond to a QEF of 1E -7 PER
for each ModCod. The Target Es/No Margin value is used as a tool to compensate for fade. This
margin is ADDED to the QEF published specification to all ModCods. The QEF specification for a
ModCod plus the Target Es/No margin plus the Modulation Degradation for a modulation type
sets the “shift up” and “shift down” points for the ACM operation.
Using this table as a guide for calculating the Target Es/No Margin:
Symbol Rate
Range (Msps)
Messaging/
Processing
(ms)
GEO Round
Trip Time (ms)
MEO Round
Trip Time (ms)
Total GEO
Comms (ms)
Total MEO
Comms (ms)
1.0 1016 560 150 1756 1166
5.0 303 560 150 863 453
15.0 267.8 560 150 827.8 417.8
This table gives the total time, in milliseconds, needed to calculate the Target Es/No Margin.
Using the correct symbol rate range, multiply the MAXIMUM FADE in dB/sec by the time needed
to process an ACM message.