Transmitter Setup
4–6
Copyright © 2002 Harmonic Inc.
This target can be achieved by setting up the transmitter with modulated
carriers in the CW Mode. In this case, if you use Autosetup
⊕
CW Mode
⇒
Pad value will be 5.5 + (18-16) = 7.5 dB.
Target 2:
•
Achieve the same CNR as you would with unmodulated carriers
•
The average CSO and CTB will be better than they would be with
unmodulated carriers
•
Digital loading
This target can be achieved by setting up the transmitter with modulated
carriers in the Video Mode. In this case, if you use Autosetup
⊕
Video Mode
⇒
Pad value will be 5.5 + (18-16) + 3 = 10.5 dB.
4.3.2
Automatic Gain Control (AGC) and Manual Gain
Control (MGC)
The AGC function operates by measuring the input RF level to the transmitter
and continuously adjusting the RF pad to maintain constant drive level to the
laser. When you select AGC, the input level to the transmitter laser remains at
its previously established value.
When you use AGC mode with the PWL 4800T, Harmonic recommends that
you drive the transmitters 3 dB harder so that they can better compensate for
the increased variation in the RF input signal. For example, for 76 PAL
B/G channels, use an RF input level of 19 dBmV/channel.
In MGC mode, the internal RF pad value of the transmitter is kept at a fixed
value regardless of the input RF level variation.
NOTE:
You can adjust the drive level of the laser in either AGC or MGC
mode by changing the RF pad. For instructions, see Section
on page 4-3. After you have adjusted the pad,
the transmitter will continue to operate in the same mode (AGC or
MGC).
