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ES10/ES26
Page 41/104
Document status: released
Operating Manual
rev. 01, 03.11.08
The built-in RF power meter controls the rms input level and indicates its status at the INPUT
LED. There are two RF modes, which can be selected:
5.3.2.1 RF Mode “AGC-on”
This mode is the recommended mode for standard operation!
As long as the input level is within the working range of the transmitter (specified by the table
above), the transmitter will itself adjust the optimum gain. Then, the specified total rms
modulation index OMI
totrms
, will be automatically obtained by the AGC.
If the input power is lower or higher than required (outside the specified range) the input LED
lights yellow and a warning is generated. If the input power is missing or very high, the input LED
lights red and an alarm is generated.
The AGC always tries to maintain the requested optical modulation index as described. This
index should be high in order to result in good noise behavior. Yet, too high modulation index
causes bit errors, which come up due to overmodulation (clipping) of the transmitter. Therefore,
an optimum optical modulation index exists, which is given in the table above. The ES10/ES26 is
factory adjusted to achieve a BER of max. 10
-9
with most frequency plans using the built in AGC
function.
Note:
The subsequent diagram shows the relationship between OMI
totrms
and the bit error rate (BER)
measured for 64QAM transmission. Obviously, OMI
totrms
should be below about 20% in order to
obtain BERs better than 10
-6
.
The BER also depends on the mix of AM, FM and QAM channels. If the QAM load is very small
compared to the AM and FM load the OMI
totrms
might be chosen about 1 dB higher while still
obtaining the BER as given in the diagram.
1.00E-10
1.00E-08
1.00E-06
1.00E-04
1.00E-02
1.00E+00
15.0
17.5
20.0
22.5
25.0
direct modulated
externally modulated
OMI
totrms
in %