Autosetup, Automatic Gain Control, and Manual Gain Control
PWRLink II 4800T Optical Transmitter Product Manual Rev A (2/02)
4–5
Figure 4-2. RF drive level as a function of channel loading
Even if you plan to add more video channels in the future, Harmonic
recommends that you use 16 dBmV/ch to ensure that the system is easy to
upgrade.
When should CW Mode or Video Mode be used?
It depends on the loading.
Example
In the case of a transmitter with the following setup:
•
Loading: 76 PAL B/G channels from 45 to 870 MHz and 100 MHz digital
loading –10 dBc down.
•
RF input level: +18 dBmV/ch.
•
Factory optimized pad value at 16 dBmV/ch: 5.5 dB.
In the case of CW unmodulated carriers:
Harmonic recommends running
Autosetup in CW Mode. In the factory, the transmitter is optimized using
unmodulated carriers. Therefore, running Autosetup in CW Mode with the
same channel load as the factory will bring the pad level back close to the
optimal value. In this case, if you use Autosetup
⊕
CW Mode
⇒
Pad value
will be 5.5 + (18-16) = 7.5 dB.
In the case of modulated carriers:
There are typically two different targets
when setting up the transmitter with modulated carriers:
Target 1:
•
Achieve the maximum possible modulation for the transmitter with better
CNR, typically 3 dB higher than for unmodulated carriers
•
The average CSO and CTB will be the same as the unmodulated case
•
Preferably no digital loading on top of the video carriers
15.0
16.0
17.0
18.0
19.0
20.0
35
45
55
65
75
85
Number of PAL B/G channels
RF drive level (dBmV/channel)