Comtech EF Data CDM-760 - Manual (Page 355)

Appendix G Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
G–21
Step Task
6 Configure ACM (see Sect. 6.2.2.2.5 (CONFIG: Tx) ACM):
A. Min/Max-ModCod: Enter the lowest and highest ModCods desired for system operation (default
is ModCod-4 QPSK ½ rate is the lowest and ModCod-28 32-APSK 9/10 is the highest).
B. Unlock Action: This decides the action taken if the demodulator at the distant end loses lock.
Select “Min” (recommended) or “Maintain.” If the distant end demodulator unlocks the modulator
will either go to the minimum ModCod allowable as sent in step (a) or maintain the last known
ModCod before the distant end was lost.
C. Es/No Target Margin: This allows the addition of margin to overcome the degradation that
accompanies real links. The Es/No Target Margin is added the specified Es/No for all ModCods
to allow the link to maintain error rates. The range is 0 to 4.5 dB (the default value is 0.0 dB).
D. Modulation Impairment: The amount of degradation for each modulation type is selected (0 dB
is the default). This selection recognizes that impairments rise as modulation order increases. It
provides some help combating compression:
QPSK: 0.2 dB 16APSK: 0.4 dB
8PSK: 0.3 dB 32APSK: 0.6 dB
Note: The impairment for QPSK ≤ 8PSK ≤ 16APSK ≤ 32APSK
7 Once you complete Steps 1 through 6, the modem then determines the optimal set of ModCods and
prunes non-optimal ModCods.
G.5.2.2 Monitor ACM Performance and Operation
Use the MONITOR: ACM front panel menu to view the current Tx and Rx ModCod, as well as the
Local and Remote Es/No.
Tx Modcod=8PSK 8/9 RemEsNo:+20.0 db
Rx Modcod=8PSK 8/9 LocEsNo:+19.9 db
This read-only screen provides the active ACM Mode information:
Parameter Description
Tx Modcod Displays the Tx ModCod.
RemEsNo Displays the EsNo reported by the remote modem.
Rx Modcod Displays the Rx ModCod.
LocEsNo Displays the EsNo of the local unit.
The Es/No display values between –3.0 dB and +23.0 dB, with a resolution of 0.1 dB. If either the
local or remote demodulator is unlocked, the Es/No will display ‘No Sync’.
Note that you may often see that the RX ModCod is “00”. This is normal when there is no real
data being received. DVB-S2 states that when no data is available, the modulator should insert a
Dummy PLFRAME. These Dummy PLFRAMEs are used to keep the demod locked, but contain no
user data and have a ModCod value of “00”.