Olson Technology OT-1000-HH Rev.X4 - Manual (Page 17)

025-000573 Rev. X4 - 17 - www.olsontech.com
AGC-Off Mode for Narrowcast and Return Applications
In this mode, the user has the flexibility to change the gain of the internal RF amplifier according to special
requirements to manually adjust the optical modulation index OMI. The total OMI
totrms
is measured for the
applied input signal at the selected RF gain, and can be monitored.
In the case of broadcast applications, this AGC-Off mode is for experienced users only. It should only be used
with great care since the automatic protection against over modulation is lost! (See AGC-On mode below)
AGC-On Mode for Broadcast Application
In the AGC-On mode, the optical transmitter adjusts the optimum optical modulation index OMI by auto-
matically adjusting the RF gain. The AGC-On mode is the recommended mode for standard broadcasting ap-
plications, where CATV signals (analog and digital video carriers, audio carriers) are transmitted.
Typically, about 5% is the optimum OMI for analog video carriers in broadcasting applications. In AGC-On
mode, the transmitter will find the best modulation index by itself. The transmitter will also keep the best
modulation index even if the RF input level varies. To do so, the transmitter makes use of its available RF
gain range (RF Gain = -17 … +7dB). If, for example 5% is indeed the optimum OMI – the RF input level of
the analog video carrier can be in the broad level range of +13 to +37 dBmV. To have some operating safety
margin, we recommend choosing an RF level in the range of 16 to 34 dBmV for 5%. Typical broadcasting
applications will run with analog video carriers having input levels in this range.
Please note that the correct adjustment of RF input level is monitored by the trans-
mitter!
When input power is lower or higher than required (out of required level range), the
“RF” LED lights Red and an alarm is generated.
When input power is out of the recommended input level range (no safety margin
any more) the “RF” LED turns Yellow and a warning is generated.
The AGC-On mode is also the recommended mode for test applications, where transmitter performance is
measured. Most tests (CNR / CSO / CTB) are performed with un-modulated carriers using e.g. a Matrix gen-
erator or a CATV headend, where the video modulation is switched off. The OMI
totrms
should be set to the
factory setting (0dB). It is important to note that for AM-VSB TV channels the channel power level with
modulation decreases by about 4dB (however, depending on the picture content). This decrease in input level
has to be compensated by the AGC for optimum signal transmission (adjustment of optimum optical modula-
tion index OMI).
The AGC-On mode function of the OT-1000-HH is factory adjusted to achieve a BER of 10
-9
with most fre-
quency plans. Experienced users are free to change the factory preadjustment by up to ±3dB and/or to use the
OT-1000-HH in AGC-Off mode. This gives the flexibility to optimize the total system CNR or CSO/CTB
performance, but it is recommended for very experienced users only.