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<SR<5.0 1016 560 150 1756 1166
5.0<SR<15.0 303 560 150 863 453
15.0<AR 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.